Spotted knapweed (Centaurea maculosa L.) seed longevity, chemical control and seed morphology
by Edward Stuart Davis
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in
Agronomy
Montana State University
© Copyright by Edward Stuart Davis (1990)
Abstract:
Spotted knapweed (Centaurea maculosa Lam.) is an introduced perennial plant that has spread rapidly
to infest an estimated 4.7 million acres of grazeable rangeland, woodland and pastureland in Montana.
Spotted knapweed is a primary invader that capitalizes on soil disturbances and once established is
extremely competitive. Carrying capacity of rangeland is severely reduced as spotted knapweed
displaces productive grasses and forbs.
Seed longevity is an important survival characteristic of spotted knapweed. Over 50% of a population
of spotted knapweed seeds remained viable after 5 years of burial. In a natural seedbank, spotted
knapweed seed populations declined by 95% during the first 3 years following termination of seed
production. However, approximately 390,000 viable seeds remained per hectare 7 years after seed
production had been terminated. In laboratory tests 10 to 40% of freshly harvested spotted knapweed
seed required light to germinate.
Field trials were established to determine the long-term control of spotted knapweed achieved by a
single application of low rates of picloram. All rates significantly reduced spotted knapweed densities
60 and 84 months after treatment at two locations. Grass forage production increased 200 to 700%
when spotted knapweed was controlled. Spotted knapweed plants gradually reinfested the treated areas
as picloram soil residues declined causing reduced grass production.
Spotted knapweed seeds rapidly imbibe water and germinate within 18 hours. Water uptake occurs
primarily through the hilum and to a lesser degree through the micropyle of the seed. A thick, durable
seed coat provides effective protection which permits dormant seed to resist decomposition in soil. SPOTTED KNAPWEED (Centaurea maculosa L.> SEED LONGEVITY
CHEMICAL CONTROL AND SEED MORPHOLOGY
by
Edward Stuart Davis
A thesis submitted in partial fulfillment
of the requirements for the degree
of
Master of Science
in
Agronomy
MONTANA STATE UNIVERSITY
Bozeman, Montana
September 1990
/VJ92
ii
D Z cIZ 3
APPROVAL
of a thesis submitted by
Edward Stuart Davis
This thesis has been read by each member of the thesis
committee and has been found to be satisfactory regarding
content, English usage, format, citations, bibliographic
style, and consistency, and is ready for submission to the
College of Graduate Studies.
9 - / 2- - 9 £>
Date
Chairperson, Graduat
Committee
Approved for the Major Department
Date
Head, Major
Approved for the College of Graduate Studies
Date
Graduate Dean
STATEMENT OF PERMISSION TO USE
In presenting this thesis in partial fulfillment of the
requirements for ai master's degree at Montana State
University, I agree that the Library shall make it available
to borrowers under rules of the Library.
Brief quotations
from this thesis are allowable without special permission,
provided that accurate acknowledgement of source is made.
Permission for extensive quotation from or reproduction
of this thesis may be granted by my major professor, or in
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of either, the proposed use of the material is for scholarly
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Any copying or use of the material in this thesis
for financial gain shall not be allowed without my written
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Signature
Date
ACKNOWLEDGEMENTS
I
would like to thank my advisor and friend Dr. Pete Fay
for his guidance, support, and encouragement throughout my.
tenure at Montana State University.
/
I would also like to express my thanks to Dr. William
Dyer and Dr. Sharon Eversman for their input and advice
while serving on my committee.
A special thanks and acknowledgement goes out to Tim
Chicoine and Celestine Lacey for the early research they
conducted on spotted knapweed.
Their dedication made the
continuation of these valuable long-term studies possible.
The hours of assistance provided by Toni Moorman, Shaun
Townsend and Jackie Stockwell made it possible to complete
this research.
The scanning electron micrographs were made possible
through the expertise of John Blixt, Veterinary Research EM
Laboratory Supervisor.
I would like to thank my parents for their guidance
throughout my life, and my children for their love and
understanding.
I would especially like to thank my wife Mett for her
patience and sacrifice throughout this endeavor, and most of
all for her love.
vi
T&BLE OF COMTEMTS
Page
APPROVAL..... ......................................
ii
STATEMENT OF PERMISSION TO USE.... .................
iii
VITA...........................................
ACKNOWLEDGEMENTS .....
TABLE OF CONTENTS........................
iv
V
vi
LIST OF TABLES... .......................;..........
viii
LIST OF FIGURES.......................... ......... .
ix
ABSTRACT..........................................
xi
Chapter
I.
LITERATURE REVIEW......... ...............
Introduction........................
Seed Germination and
Seedling Establishment.......
Plant Development.............
Seed Production and Dissemination....
Allelopathy.... ................
Cultural Control..........
Plowing.... ........
Mowing.......
Burning..........
Grazing.......... ........ ....... .
Producer Experiences.........
Economic Impact....................
Biological Control...........
Chemical Control.....................
I
I
2
10
12
14
19
19
19
19
20
21
23
26
35
\
vii
TABLE OF CONTENTS -.CONTINUED
^
Chapter
2.
Page
SPOTTED KNAPWEED SEED LONGEVITY........
Introduction............
Materials and Methods.... .........
Seed Burial Study.................
Decline of Viability from a Natural
Seed Population in Soil............
Seed Germination Study......
Results and Discussion...............
Seed Burial Study.............
Viability of Natural Seed
Populations......
Germination Behavior............
3.
LONG-TERM CONTROL OF SPOTTED KNAPWEED
WITH REDUCED RATES OF PICLORAM.........
Introduction...............
Materials and Methods..............
Results and Discussion..............
4.
5.
39
40
40
40
42
44
45
45
50
53
58
58
60
63
SEED MORPHOLOGY AND GERMINATION
CHARACTERISTICS OF SPOTTED KNAPWEED.......
70
Introduction.........................
Materials and Methods..........
Scanning Electron Micrographs......
Imbibition Curves...............
Point of Water Uptake..........
Results and Discussion...........
Scanning Electron Micrographs.....
Water Imbibition.....
Point of Water Uptake.... .
70
71
71
71
72
73
73
83
84
SUMMARY.............
BIBLIOGRAPHY.................
89
91
viii
LIST OF TABLES
Table
Page
1
Recovery of Buried Spotted Knapweed Seed..
45
2
Viability of Intact, Ungerminated Spotted
Knapweed Seed Recovered Over a 60 Month
Period at Bozemanand Three Forks M T ......
48
Changes in the Seed Reserve in Soil 12
Months After Treatments were Applied in
May, 1985 in an Attempt to Stimulate
Germination of Spotted Knapweed Seed at
Harlowton and Ovando.....................
51
Number of Mature Spotted Knapweed Plants
per m2 in July, 1983 Through 1989
Following Herbicide Application in June,
1982 at Ovando...........................
63
Number of Mature Spotted Knapweed Plants
per m2 in July, 1983 Through 1989
Following Herbicide Application in June,
1982 at Harlowton..............
63
Spotted Knapweed Herbage Production 2, 14,
26, and 84 Months After Herbicide
Application at Harlowton......
65
Spotted Knapweed Herbage Production 2, 14,
26, and 84 Months After Herbicide
Application at Ovando............
65
Perennial Grass Production 2, 14, 26 and
84 Months After Herbicide Application at
Harlowton...... ..........................
669
Perennial Grass Production 2, 14, 26 and
84 Months After Herbicide Application at
Ovando..............
66
3
4
5
6
7
8
9
ix
LIST OF FIGURES
Figure
I
2
3
4
5
6
7
8
9
10
11
12
Page
Percent Germination of in situ Spotted
Knapweed Seeds Recovered Annually From
Buried Containers at Bozeman and Three
Forks....................................
46
The Number of Viable Spotted Knapweed
Seeds Recovered During a 60 Month Burial
Period at Three Forks and Bozeman........
49
The Decline in the Spotted Knapweed Seed
Population Following Termination of Seed
Production in 1982 at Ovando and
Harlowton MT................
52
Effect of Spotted Knapweed Seed Placement
on the Soil Surface or Buried 0.6 cm Deep
on Germination of Seed Collected at 7
Locations.... ......
54
Germination of Spotted Knapweed Seed in
Light or Complete Darkness..............
55
Schematic Representation of a Spotted
Knapweed Achene (Cypsela) in Crosssection..................................
74
Proximal End of Spotted Knapweed Achene
showing Hilum and Epidermal Hairs (XlOO)..
75
Cross Section of Hilum Showing "Honeycomb"
Cellular Structure (X500)................
76
Cross Section of Proximal End of Achene
With Funicular Attachment (XlOO).......
77
Hilum Surface Showing Waxy Texture with
Numerous Pores (X800)....................
77
Distal End of Achene Showing Multiserial
Bristles (X50)...........................
78
Bristles on Outer Whorl of Pappus (X300)
79
X
LIST OF FIGURES - CONTINUED
Figure
13
Page
Pappus Showing Inner and Outer Whorls of
Bristles and Disc (X50)........... .......
80
14
Disc (Nectary) and Micropyle (X300)......
80
15
Fractured Surface of a Fresh Achene
Exposing the Testa and Pericarp (XlOO) ....
82
Fractured Surface of Achene After 5 Years
of Burial Exposing the Testa and Pericarp
(X200)...........
82
Spotted Knapweed Seed Imbibition Curve
Measured over an 18 Hour Period
Culminating with Germination.............
83
Germination of Spotted Knapweed Seeds with
Paraffin Covering the Hilum, the pappus,
the Entire Seed, and Non-coated Seeds...
84
Schematic Representation of Cross-section
of Spotted Knapweed Seeds Showing Staining
Patterns Following Imbibition in a 0.1%
Methylene Blue Solution for 0, 2, 4, 6, 8,
and 12 hours.......
86
16
17
18
19
xi
ABSTRACT
Spotted knapweed (Centaurea maculosa Lam.) is an
introduced perennial plant that has spread rapidly to infest
an estimated 4.7 million acres of grazeable rangeland,
woodland and pastureland in Montana. Spotted knapweed is a
primary invader that capitalizes on soil disturbances and
once established is extremely competitive. Carrying
capacity of rangeland is severely reduced as spotted
knapweed displaces productive grasses and forbs.
Seed longevity is an important survival characteristic
of spotted knapweed. Over 50% of a population of spotted
knapweed seeds remained viable after 5 years of burial. In
a natural seedbank, spotted knapweed seed populations
declined by 95% during the first 3 years following
termination of seed production. However, approximately
390,000 viable seeds remained per hectare 7 years after seed
production had been terminated. In laboratory tests 10 to
40% of freshly harvested spotted knapweed seed required
light to germinate.
Field trials were established to determine the long-term
control of spotted knapweed achieved by a single application
of low rates of picloram. All rates significantly reduced
spotted knapweed densities 60 and 84 months after treatment
at two locations. Grass forage production increased 200 to
700% when spotted knapweed was controlled. Spotted knapweed
plants gradually reinfested the treated areas as picloram
soil residues declined causing reduced grass production.
Spotted knapweed seeds rapidly imbibe water and
germinate within 18 hours. Water uptake occurs primarily
through the hilum and to a lesser degree through the
micropyle of the seed. A thick, durable seed coat provides
effective protection which permits dormant seed to resist
decomposition in soil.
CHAPTER I
LITERATURE REVIEW
Introduction
Spotted knapweed (Centaurea maculosa Lam.) is a
pernicious noxious weed in Montana infesting an estimated
4.7 million acres of range and grazeable woodland (Lacey,
1987).
An additional 34 million acres may be vulnerable to
spotted knapweed invasion (Chicoine, Fay, and Nielson,
.1986)..
The annual forage loss due to displacement of native
forbs and grasses by spotted knapweed is estimated at $4.5
million (French and Lacey, 1983).
The perennial growth habit, profuse seed production and
aggressiveness of spotted knapweed result in rapid
establishment and spread.
Initial infestations occur in
disturbed areas such as roadsides, trails, construction
sites, overgrazed land and waterways (Watson and Renney,
1974).
Once established it is very competitive, displacing
native grasses and forbs, resulting in near-monoculture
stands of spotted knapweed.
Another factor contributing to
its success in North America is the lack of natural enemies.
In Europe, its center of origin, spotted knapweed evolved
with host-specific insects and pathogens which keep the
2
plant frequency and density at low levels.
In North America
most of these agents are not present.
Seed Germination and Seedling Establishment
Spotted and diffuse knapweed (Centaurea diffusa Lam.)
seed is disseminated as a dry, indehiscent single seeded
fruit called an achene.
Members of the Compqsitae have
epigynous flowers (flowers with an inferior ovary).
The
flower parts, calyx, corolla, and stamens originate above
the inferior ovary.
are termed cypselas.
Achenes produced from epigynous flowers
Throughout the text of this thesis,
the term seed will be synonymous with achene or cypsela.
Knapweed seeds germinate in the fall and spring when
soil moisture conditions are most favorable.
Seedlings that
germinate in the fall overwinter as rosettes and bolt the
following June.
Seedlings established in the spring usually
flower the following season (Schirman, 1981).
Knapweed seeds imbibe water and germinate within 18
hours under optimum conditions (Chicoine, 1984).
Spears et
al. (1980) and Eddleman and Romo (1986) examined canopy
cover, depth of emergence of seedlings, temperature and
moisture and determined the optimum conditions for
germination.
Canopy cover had no effect on germination of
buried seed.
Maximum germination occured at soil moisture
levels of 118 to 127% of field capacity in the soil mixture
used at soil temperatures ranging from 10 to 28° C.
3
Seedlings did not emerge from depths of 3 and 5 cm for
diffuse and spotted knapweed, respectively.
The highest
percent germination occurred when seeds were left on the
soil surface.
Watson and Renney (1974) reported germination levels of
freshly harvested diffuse and spotted knapweed seeds
increased from 40 to 68% and from 20 to 80%, respectively,
following 25 days of dry storage at room temperature.
Their
findings suggest that knapweed seeds possess a primary
(innate) dormancy which is lost during dry after-ripening.
Nolan and Upadhyaya (1988) describe three types of
germination behavior in freshly harvested diffuse and
spotted knapweed seed:
I) nondormant seeds that germinate
in darkness, 2) light-sensitive dormant seeds that germinate
in response to red light (R), and 3) light-insensitive
dormant seeds that fail to germinate after 5 days of
continuous R.
Seeds of all three germination types were
found on individual plants of both species.
Less than 5%
were nondormant, 50 to 60% were light-sensitive primary
dormant, and 35 to 45% were light-insensitive primary
dormant for diffuse and spotted knapweed seeds.
Seeds
collected from different sites, and from individual plants
within sites, had different germination levels in darkness
and following exposure to two minutes of R.
stimulated germination in each case.
Exposure to R
4
The mean levels of primary dormancy over all diffuse and
spotted knapweed sites were 78 and 84%, respectively.
The
germination behavior of seeds collected from individual
plants was similar to samples collected from different sites
for both species.
Freshly harvested seeds from individual
spotted knapweed plants generally possessed higher levels of
primary dormancy (X = 93%) than those from individual
diffuse knapweed plants (X = 71%).
Phytochrome is a cytoplasmic protein with a covalently
attached linear tetrapyrole chromophore that regulates
flowering and seed germination.
The phytochrome molecule
exists in two different conformations depending on the
wavelength of light of the last exposure.
The R absorbing
form, Pr, has maximum absorbance in the 600-680 nm range and
is the inactive form.
Upon exposure to R phytochrome
undergoes a conformational change to the active or Pfr form
which absorbs light in the far-red (FR) region with maximum
absorbance in the 720-740 nm range.
Phytochrome in the Pfr
form undergoes dark reversion to the inactive Pr form (Song,
1984).
The demonstration of R/FR reversibility implicates
involvement of the phytochrome system in light-sensitive
seed germination (Smith, 1982).
Nolan and Upadhyaya (1988)
showed that light-sensitive dormant diffuse and spotted
knapweed seed germinated in response to R and that exposure
to FR negated this effect.
Exposure to FR did not establish
5
a light requirement in imbibed seeds that were initially
non-dormant, as germination following FR was not
significantly different from dark controls.
The level of germination following any sequence of light
exposures for light-sensitive knapweed seeds depends on the
light quality of the last exposure.
Exposure to R for just
2 minutes was adequate to stimulate the germination of all
dormant seeds from some individual plants, however longer
exposures were required for maximum germination of seeds
collected from other individual plants.
When the duration
of R exposure was increased from 2 minutes to I day,
germination increased from 15 to 53% for diffuse knapweed,
and from 15 to 63% for spotted knapweed.
Further increases
in R duration from I to 5 days did not increase germination.
Watson and Renney (1974) reported no difference in
germination of diffuse and spotted knapweed seed when
exposed to alternating light and dark versus dark
incubation.
In fact continuous light significantly reduced
)
the germination of both species.
Nolan and Upadhyaya
explained this discrepancy by noting that they observed a
loss of the light requirement in several samples of seeds
used in preliminary studies following several months storage
at room temperature.
An increase in germination following
dry after-ripening was reported by Watson and Renney (1974).
Storage of seeds at -200 C arrested after-ripening in the
seeds.used by Nolan and Upadhyaya (1988) and maintained a
6
high level of primary dormancy.
Alternatively, chance
collection from plants bearing mostly nondormant seeds may
explain the lack of light-sensitivity of spotted and diffuse
knapweed seed reported by Watson and Renney (1974).
The effect of gibberellic acid (GA3) and excision on
germination of diffuse and spotted knapweed seed was also
reported (Nolan and Upadhyaya, 1988).
Germination in
darkness increased with increasing GA3 concentration.
There
was no synergism between light and GA3 on knapweed seed
germination.
Germination of diffuse and spotted knapweed
seed was 100% following excision of the distal end of the
seed.
Polymorphic germination behavior is a common phenomenon
among weed species which insures seed germination for an
extended period of time (Bewley and Black, 1982).
Germination polymorphism occurs in both diffuse and spotted
knapweed which exhibit three distinct types of germination
behavior.
Knapweed seeds germinate in the spring and fall
(Watson and Renney, 1974).
Nondormant and light-sensitive
seeds that don't become buried or shaded by other plants
would likely germinate immediately following dispersal in
the fall if temperature and moisture levels were adequate.
Delay of germination until the spring would be facilitated
by dormancy, retention of seeds in the capitula over the
winter, or seed dispersal late in the season when low
temperatures prevent germination.
Seed dormancy would
7
further distribute germination over time by allowing seeds
to become incorporated into a persistent seed bank in the
soil (Fenner, 1985).
Differences in amounts of non-dormant seeds (seeds
germinating in the dark) among individual plants within
sites could be due to genetic variability between plants or
differences in environmental conditions experienced by the
maternal plant during seed development.
The asynchronous
flowering behavior of the knapweeds increases the
possibility that variability in environmental conditions
prior to seed dispersal leads to germination polymorphism.
Variability in the rate or duration of the after-ripening
period following seed maturation is one possible explanation
for differences in dark germination (Fenner, 1985).
The higher average levels of primary dormancy in spotted
knapweed compared to diffuse knapweed might be the 'result of
the inherent differences in seed dispersal in the two
species.
Diffuse knapweed seeds are retained on the plant
for prolonged periods because the capitula, except for a
small distal opening, remain closed (Watson and Renney,
1974).
Conversely, spotted knapweed seed heads open widely
when mature and the seeds are loosely held in the capitula
so they are dispersed quickly.
Consequently, spotted
knapweed seeds would undergo less after-ripening in the
capitulum than seeds of diffuse knapweed.
If seed
germination behavior of the two species is comparable at the
8
time of maturation, and primary dormancy is lost during
after-ripening, the capability of diffuse knapweed to retain
seeds in the capitula for longer periods would favor after­
ripening resulting in lower levels of primary dormancy in
field populations of this species.
Seed germination characteristics may influence the
geographic range of adaptation of a species (Bewley and
Black, 1982).
Knapweed infestations are generally found in
areas experiencing soil disturbance, overgrazing, or
seasonal drought (Harris and Cranson, 1979; Popova, 1960;
Watson and Renney., 1974) , and are uncommon in irrigated,
shaded, or moist areas (Meyers and Berube, 1983; Watson and
Renney, 1974).
All of the above factors affect the
abundance of vegetative cover, which in turn influences the
quality of light incident upon seeds present on the
underlying soil surface.
The absorption of the red
wavelengths of energy in sunlight by chlorophyll in a plant
canopy causes a decrease in the RzFR (660:730 nm) ratio as
the leaf index increases (Frankland, 1981).
Rangelands
dominated by bunchgrasses provide favorable conditions for
knapweed seed germination due to bare areas between plants
that allows R to reach the soil surface.
The effects of stratification, temperature, and water
stress on spotted knapweed seed germination were
investigated by Eddleman and Romo (1986).
Optimum
temperature range for germination was 15 to 25° C.
9
Temperatures above or below this range resulted in reduced
germination suggesting an adaptation that minimizes
germination in late summer or early fall when seedling
mortality may be high under conditions of transient
moisture.
Low temperature-induced dormancy may be an
adaptation that restricts germination in late fall, when
slow growth rates could lead to seedling mortality.
High
mortality of spotted knapweed seedlings has been noted
during severe winters (Watson and Renney, 1974) when extreme
low temperatures and low snow accumulation occur
simultaneously.
Widespread winterkill of spotted knapweed
was noted for the winter of 1988-89 in western Montana
(personal observation).
However, the observed winterkill
reduced plant density only slightly.
Immediately after harvest, stratification for 30 or 60
days alleviated a portion of the dormancy in spotted
knapweed seeds incubated in the dark (Eddleman and Romo,
1988).
Seeds of spotted knapweed under field conditions are
subjected to cold-dry and cold-wet conditions during the
fall and winter.
Stratification increased spotted knapweed
seed germination at both supraoptimal and suboptimaI
temperatures (Eddleman and Romo, 1988).
Loss of dormancy at
colder temperatures would enhance early emergence of
seedlings in the spring.
This would maximize the period of
available soil moisture for newly developing plants and
avoid competition from later emerging species.
Such
10
adaptations such as this that place seedlings in a favorable
time-moisture relationship are critical for establishment of
seedlings on grassland sites (Harris, 1967).
Dry after­
ripening and stratification increase seed germination of
diffuse and spotted knapweed seed.
Plant Development
Spotted and diffuse knapweed bolt in May after
overwintering as rosettes.
Diffuse knapweed produces a
single branched stem, two year old spotted knapweed plants
produce one to six stems per plant, and older plants
typically produce more than a dozen branches (Watson and
Renney, 1974).
Rosettes begin to bolt and immature flowers
are first observed in mid-June.
Stems and branches elongate
and flower heads continue to appear on the end of each
branch throughout the summer.
Flowering begins in mid-July,
about 2 weeks earlier for spotted knapweed than for diffuse
knapweed.
Individual flowers remain open for 2 to 6 days.
The Centaurea species are cross-pollinated by insects and
mature seeds are produced 18 to 26 days after fertilization
(Watson and Renney, 1974).
Diffuse and spotted knapweed are adapted to a wide range
of soil types.
Susceptibility to invasion is directly
related to the degree of disturbance of the soil surface.
However, these two species are not adapted to cultivated
land or land under irrigation.
Spotted and diffuse knapweed
11
thrive in areas where an arid period occurs during the
summer months (Harris and Cranston, 1979).
Open habitats
are preferred, although spotted knapweed will invade
disturbed forest soils at relatively high altitudes.
Spotted knapweed reproduces vegetatively and is
classified as a short-lived perennial (Watson and Renney,
1974).
Lateral shoots emerge 2.5 to 7.5 cm from the mother
plant and form rosettes.
Plants normally survive for 3 to 5
years which accounts in part for the dense stands formed by
spotted knapweed.
Diffuse knapweed is classified as a
biennial or sometimes as a triannual (Watson and Renney,
1974) .
Boggs and Story (1987) studied the population age
structure of spotted knapweed in Montana.
Life span and
mean age of spotted knapweed plants were determined by
counting annual rings.
Regardless of environmental
conditions of the season, one ring of secondary xylem was
added to the taproot of known-age spotted knapweed plants
annually.
Root ring counts at six study sites indicate
spotted knapweed plants can live for at least 9 years.
The
average age of spotted knapweed plants, excluding plants
less than I year old, ranged from 3.4 years in 1984 to 5.0
years in 1985.
Based upon these data and the definition of
a perennial as any plant living longer than 2 years
(Hitchcock and Cronquist, 1978), Boggs and Story suggest
12
that spotted knapweed should be classified as a perennial
rather than either a biennial or short-lived perennial.
Seed Production and Dissemination
Massive seed production is a major competitive device of
the knapweeds.
Watson and Renney (1974) reported that
diffuse knapweed produced 925 seeds per plant when grown on
rangeland and 18,248 seeds when grown under irrigation.
Spotted knapweed produced 436 and 25,263 seeds per plant
when grown under dryland and irrigated conditions,
respectively.
Schirman (1981) reported that diffuse knapweed is
adapted to drier climates than spotted knapweed.
In a 3-
year study, which included prolonged drought conditions,
diffuse knapweed seed production was more stable than
spotted knapweed.
Diffuse and spotted knapweed produced
approximately 30,000 and 48,000 seeds/m2, respectively.
The
higher seed production by spotted knapweed was due to
multiple flower stem production.
While there was no
correlation between spotted knapweed height and the number
of flower shoots or flower heads produced per plant, there
was a correlation found between the number of flower heads
and shoot production per plant (Story, 1976).
Spotted knapweed plants produced 32 seeds per head and
29 heads per plant (approximately 1,000 seeds per plant)
(Story, 1976).
If 80% seed survival is assumed (Schirman,
/
13
1981; Watson and Renney, 1974), the soil reservoir of
spotted knapweed seed increases exponentially each year.
The longevity of spotted knapweed seed viability has not
been determined.
However, preliminary results by Schirman
(1984), Chicoine (1984), and Lacey (1985) indicate that,
buried spotted knapweed seed remains viable in the soil for
more than 5 years.
Each Centaurea species has a unique method of seed
dispersal.
Diffuse knapweed plants are commonly globe-
shaped at maturity and have an abcission layer on the stem
near the soil surface enabling them to break loose and
tumble (Watson and Renney, 1974).
The capitulum of diffuse
knapweed is more constricted than spotted knapweed which
favors seed retention prior to tumbling.
as the plant rolls along the ground.
Seeds are released
Spotted knapweed does
not disseminate its seeds over long distances since the
flower stems do not tumble and the pappus is too small to
permit wind dissemination of the seed.
The bracts enclosing
the flower heads begin to open approximately 3 weeks after
maturity (Watson and Renney, 1974).
As the relative
humidity fluctuates the bracts open and close which loosens
the seeds causing them to rise to the top of the capitulum
(Watson and Renney, 1974).
Seeds are disseminated by a
flicking motion caused when plants are moved abruptly which
scatters the seed for distances of up to I meter (Strang et
al., 1979).
14
Spotted knapweed is moved from one locality to another
by man and animals.
The bristles of the pappus enable the
seed to loosely adhere to animal hair or fur allowing
transport over short distances (personal observation).
Motorized vehicles are primarily responsible for the rapid
long distance spread of spotted knapweed seed throughout
North America (Mass, 1985; Watson and Renney, 1974).
Spotted knapweed seed floats on water due to its waxy
pericarp and bristly pappus which tends to trap air
(personal observation) and so waterways also serve as
transportation avenues for seed.
Allelopathy
There is some evidence that Centaurea species utilize
allelopathy to maintain stand densities.
Fletcher and
Renney (1963) partially characterized the suspected
allelochemical as an indole.
The highest concentration of
allelochemical was found in the leaves.
Russian knapweed
(Centaurea repens L.). had the highest concentration of the
allelochemical, followed by diffuse and spotted knapweed.
Soils from Russian knapweed infestations retarded growth of
tomato (Lycopersicon esculentum) and barley (Hordeum vulgare
L.).
Kelsey and Locken (1987) used chloroform extraction to
obtain compounds from spotted knapweed tissue.
Column
chromatography of extracts provided 17 fractions that were
15
tested for toxicity using a bioassay.
All fractions showed
some inhibition of lettuce germination and growth.
A
crystalline compound was isolated from the most inhibitory
fraction and identified as cnicin, a sesquiterpene lactone.
CO —
CHOH--CH OH
CNICIN
Seeds of several native plant species were incubated in
varying concentrations of cnicin to determine the effect on
germination and growth.
Germination of crested wheatgrass
(Agropyron cristatum (L.) Gaertn.), bluebunch wheatgrass
(Agropyron spicatum (Pursh)Scribn. & Smith), and rough
fescue (Festuca scabrella Torr.) was unaffected by up to 2
mg of cnicin per 9 cm diameter petri dish.
Concentrations
of 4 mg or greater reduced germination of crested and
bluebunch wheatgrass 50 and 30% respectively.
Germination
of rough fescue seed was reduced 50% by concentrations of 8
and 10 mg per petri dish.
Root growth was more sensitive to
cnicin since there was some inhibition at nearly all levels
tested.
Of the two conifer species tested, western larch
(Larix occidentalis Nutt.), and lodgepole pine (Pinus
contorta Dougl. ex Loud.), western larch was more sensitive
to cnicin.
Lodgepole pine germination was not affected by
the highest cnicin concentrations tested.
Spotted knapweed
16
seed germination was unaffected by cnicin, but seedling
growth was strongly retarded at all concentrations.
Under
the conditions of these experiments, spotted knapweed is as
sensitive or more sensitive to cnicin than the other species
tested.
While there is little published information on the mode
of action of sesquiterpene lactones in plant tissue, there
is substantial data from other organisms.
In animal cells,
the lactones inhibit DNA synthesis, RNA synthesis,
respiration, and enzyme function resulting in impaired
growth and development (Rodriguez, Towers, and Mitchell,
1976).
Two types of inhibitors were isolated from diffuse
knapweed.
Polar and nonpolar inhibitors were extracted from
both aboveground tissue and roots (Muir and Majak, 1983).
A
ryegrass (LoH u m multiflorum L.) bioassay of fractionated
chloroform extracts from leaves and stems led to the
isolation of cnicin.
Impure cnicin, and the remaining
column fraction it was isolated from, inhibited ryegrass
seedling growth.
Purified cnicin at a concentration of 4
mg/ml did not inhibit germination or growth and therefore
was rejected as the cause of inhibition.
polar inhibitor was not identified.
The remaining
Although cnicin was the
most abundant inhibitor found in knapweed tissue, numerous
other inhibitory compounds were reported (Muir and Majak,
1983).
The alleged allelopathic properties of the knapweeds
17
may be a result of additive effects of cnicin and other
uncharacterized inhibitory agents.
Germinating diffuse and spotted knapweed seeds did not
produce or exude chemicals that inhibit germination of
barley, crested wheatgrass, or ryegrass (Muir and Majak,
1983).
Locken and Kelsey (1987) described the distribution of
cnicin in spotted knapweed.
Cnicin was found in the
glandular trichomes that protrude from the epidermis.
Knapweed trichome density and cnicin concentrations were
highest in leaf tissue.
Leaves attached to the branches of
the inflorescence contain the most cnicin, followed by those
on the main stem.
Rosette leaves contained the least.
Cnicin was not detected in roots or flower heads of spotted
knapweed.
Cnicin concentrations were highest in September
and October; it comprised 2.1% and 2.3% of the total dry
weight of spotted knapweed tissue respectively.
Only
negligible quantities (0.7 ppm) of cnicin were found in soil
infested with spotted knapweed (Locken and Kelsey, 1987).
In order for cnicin to function as an allelopathic
compound it must enter the environment in sufficient
concentrations to be toxic.
Levels of cnicin found in soil
were well below the concentrations needed to inhibit plant
growth in bioassays (Kelsey and Locken, 1986).
Crystalline cnicin has limited water solubility.
It is
stored in trichomes surrounded by a hydrophobic cuticular
18
sac, therefore leaching of cnicin from leaves to the soil by
rain is. unlikely.
Even if cnicin leaches from spotted
knapweed tissue, leaching would occur gradually and large
quantities of cnicin would be required to accumulate in the
soil to reach toxic levels.
The four modes of release of allelochemicals from plants
to the environment include volatilization, exudation from
roots, leaching, and decomposition of residues (Rice 1974,
1984).
Since cnicin is not volatile, it does not occur in
roots, and leaching from foliar tissue is slow (Locken and
Kelsey, 1987), decomposition of litter is the most likely
route for releasing cnicin (Kelsey and Bedunah, 1989).
An
important factor in decomposition is the rate of litter
deposition on the soil surface, which is relatively slow and
variable with spotted knapweed.
Therefore, Locken and
Kelsey (1989) concluded that cnicin probably does not
routinely reach toxic concentrations in the soil.
In greenhouse studies, germination and growth of western
larch and ponderosa pine (Pinus ponderosa Dougl.) were
inhibited when dried spotted knapweed tissue was mixed in
the rooting media, but only at levels 2.5, 5, and 10 times
higher than levels normally found in spotted knapweed
infested soil (Bedunah, 1988).
Similar studies with
lodgepole pine, Douglas fir (Pseudotsuga menziesii (Mirbel)
Grenco), and western larch showed no phytotoxic effect when
spotted knapweed tissue was incorporated in soil (Bedunah,
19
1989).
Under natural conditions, spotted knapweed litter is
probably not phytotoxic to conifers or grasses since grasses
were less sensitive than forbs to cnicin (Kelsey and Locken,
1987) .
Cultural Control
Plowing
Plowing deeper than 5 cm will control diffuse and
spotted knapweed (Spears, et al., 1980).
Eddleman and Romo
(1986) showed that seedlings of diffuse and spotted knapweed
did not emerge from depths of 3 and 5 cm, respectively.
The
land should be reseeded immediately with a vigorous grass or
legume to suppress reinfestation (Harris and Cranston,
1979) .
Mowing
Popova (1960) reported that diffuse knapweed density
increased when mowed, contrary to the findings of Watson and
Renney (1974).
Watson and Renney (1974) measured a
reduction in seed production when plants were mowed in the
flowering stage.
The continued production of low growing
flowering branches permitted some flowers to "escape"
mowing.
Burning
Knapweed plants retain their leaves, stems and
flowerheads at senescence.
These dry structures persist for
20
years and burn readily (Carpenter, 1986).
However, the use
of fire is generally discouraged since it can damage bluebunch wheatgrass and is hard to control on open rangeland.
Burning can increase competitiveness of some grasses.
Popova (1960) reported that diffuse knapweed was almost
entirely replaced by grass species 2 years after burning.
Burning eliminates seeds retained in the capitulum and may
kill seeds on the soil surface (personal observation).
Zednai (1979) found the germination of spotted knapweed was
reduced from 68 to 3% after a burn.
Strang et al. (1979)
indicated that, in spite of its invasiveness, spotted
knapweed rarely invades burned areas.
However, the degree
of soil disturbance associated with the burn and proximity
of a seed source can influence establishment.
Burning does
not appear to reduce the soil seedbank of a natural
population of spotted knapweed seed (Chicoine, 1984).
Controlled burning may be an economical method of reducing
spotted and diffuse knapweed densities on low value land.
Grazing
Grazing of spotted knapweed in spring and early summer
by sheep, goats and cattle has been reported in western
Montana (Cox, 1983 & 1989; Robertson, 1989; Spoon et al.,
1979).
Kelsey and Mihalovich (1987) determined that the
nutrient content of spotted knapweed was comparable to
native forage plants and adequate to meet livestock needs
21
during spring and early summer when stems were succulent and
actively growing.
Cnicin in spotted knapweed leaves imparts a bitter taste
and may decrease palatability (Kelsey and Mihalovich, 1987;
Watson and Renney, 1974).
Developing seed heads contain
only trace amounts of cnicin and are selectively eaten by
livestock and wildlife (Bucher, 1984; Cox, 1989; Spoon et
al., 1983; Watson and Renney, 1974 ) which could be
exploited to reduce seed production of spotted knapweed.
Spotted knapweed silage contained 50% less cnicin than fresh
plant material and was comparable in chemical composition to
alfalfa and hay silage (Kelsey and Mihalovich, 1987).
)
Producer Experiences
John Robbins manages the Double Fork Ranch in Ravalli
county, MT which is heavily infested with spotted knapweed.
He has developed a grazing management program with cattle to
utilize spotted knapweed after many attempts to eliminate it
(Robertson, 1989).
He uses high intensity - short duration
grazing and monitors cattle feeding habits and performance.
After 3 years he still has knapweed and does not expect to
eradicate it.
Robbins feels that knapweed is high in nutritive value,
their cattle graze it readily, especially in spring, and the
cattle have consistently performed well.
Results from the
monitoring program indicated mixed results but Robbins feels
that the program will succeed in the long run.
Robbins
22
states that when cattle stocking density is increased, they
graze differently than under conventional stocking rates,
including a significant change in grazing preference.
Allan Savory, founder of the Center for Holistic
Resource Management, visited,the Double Fork Ranch in
September, 1988 and concluded: "As I looked at the
successional levels, water and mineral cycles and energy
flow on the Robbins Ranch, I could only come to one
conclusion - that the knapweed, far from being a problem, is
actually an asset.
In my opinion, attempts to eradicate it
are unfortunate and unnecessary" (Robertson, 1989).
At the Knapweed Symposium held in Bozeman, MT in April,
1989, James Cox (1989) reported that sheep readily graze
spotted knapweed on his 80 acre river bottom property west
of Missoula.
Cox measured palatability of spotted knapweed
versus annual brome grasses and forage legumes by counting
plants grazed along a transect within an enclosure.
Measurements taken on July 2 and 3, 1987 indicate sheep
selectively grazed spotted knapweed over the annual grasses,
birdsfoot trefoil (Lotus corniculatus L.) and sanfoin
(Onobrychis viciaefolia).
Grazing spotted knapweed on
July I was more effective in controlling flowering than
on June I.
Intensive grazing of spotted knapweed by sheep
would be most effective when spotted knapweed is in the
flower stage, well after grasses have set seed.
This
approach would stress spotted knapweed when carbohydrate
23
levels are lowest and when grasses are summer dormant, thus
reducing knapweed propagation without affecting grass
reproduction.
Fred Mass, from Thompson Falls and Charles Hahnkamp,
from Melstone have been instrumental in the formation of
cooperative weed management groups involving ranchers,
private land owners, weed boards, sportsman groups and
county, state and federal personnel (Petersen, Hahnkamp and
Dodge, 1989).
These and numerous other locally controlled
weed cooperatives have successfully incorporated an
integrated control approach to weed management including
development of educational programs and the petitioning for
legislative support.
Fred Mass (1989) has been a strong
proponent of increased public awareness of the "knapweed
invasion" and an advocate of strict control of spotted
knapweed along roadways and other transportation avenues.
Economic Impact
Spotted knapweed threatens the long-term productivity of
native rangelands.
Spotted knapweed has spread at an annual
rate of 27% since 1920 to infest approximately 4.7 million
acres in Montana (French and Lacey, 1983; Lacey, 1987).
Chicoine, Fay and Nielsen (1985) matched the soil type,
elevation, annual precipitation, evapotranspiration, frostfree season and maximum July temperature of 116 spotted
knapweed infestations with maps developed from satellite
24
images.
They estimate that about 50% of Montana (46.5
million acres) could support spotted knapweed infestations.
When cultivated lands are subtracted from this total> nearly
34 million acres of range and grazeable woodland are
vulnerable.
The annual loss of income in Montana due to
forage reduction caused by knapweed is estimated at $4.5
million (French and Lacey, 1983).
If all vulnerable land
became infested, loss estimates would be about $1.7 billion,
almost three times the current annual gross income from
cattle and sheep (Bucher, 1984).
The impact of spotted knapweed on elk winter range is a
growing concern in Montana since the plant is known to
displace desirable forage species (Harris and Cranston,
1979; Maddox, 1979; Morris and Bedunah, 1984; Watson and
Renney, 1974).
Species diversity is inversely related to
spotted knapweed stem density indicating that spotted
knapweed invasion can alter plant community composition
(Tyser and Key, 1988).
In forest clearcut sites spotted knapweed invasion was
found to reduce tree seedling survival and wildlife forage
(Willard et al., 1988).
Many studies indicate that elk have
a strong preference for grass during the winter and spring.
Mooers (1986) reported that knapweed grew on every habitat
type in western Montana, but was best adapted to drier
grassland types, many of which are important sites for
overwintering elk.
Tyser and Key (1988) found that spotted
25
knapweed was capable of expanding within Glacier National
Park's fescue grasslands which serve as important elk winter
range.
Spoon et al., (.1983) referring to the noxious weed
infestation problems of the Lolo National Forest in Montana
stated:
nThe forage production lost on big game winter
ranges could theoretically result in a loss of 220
elk annually by the year 1998.
The resulting loss
of big game populations has an adverse effect on
hunter success and the incomes of outfitters and
the state's tourist industry."
A study designed to measure the plant community response
of three elk winter ranges to the control of spotted
knapweed following herbicide application was conducted by
Bedunah and Carpenter (1989).
Two of the sites were
dominated by bunchgrasses and the other site.by bluegrasses.
The level of spotted knapweed invasion varied by site.
Two
herbicides were investigated, picloram (TordonR) and
clopyralid (Stinger*) at three rates of application, .14,
.28 and .42 kg a.i./hectare.
All treatments provided
complete control of spotted knapweed the year of spraying.
Two seasons after application, the two highest picloram
treatments continued to provide excellent control of spotted
knapweed.
Grass production increased two to three-fold 24
months following application.
Native perennial forb
densities were not adversely affected by herbicide
26
application primarily because the spotted knapweed infested
land had fewer perennial forks than the herbicide treated
(knapweed-free) plots.
The authors concluded that spotted
knapweed had a greater impact on decreasing species
diversity than the herbicides used to control spotted
knapweed.
The low productivity of knapweed-infested winter range
sites will not improve until the spotted knapweed is
eliminated for a sufficient amount of time to allow the
native grasses and forks to become reestablished.
This
recovery occurs very rapidly following treatment with
picloram or clopyralid.
Spotted knapweed will begin to
reinfest herbicide treated areas within 2 to 3 years due to
the presence of dormant seeds that persist longer than the
soil residual properties of the. herbicides (Chapters 2 and
3) .
A retreatment of the area within 6 to 1,0 years may be
necessary to maintain the vigor and competitive advantage of
the grass community.
Biological Control
Spotted knapweed was introduced to North America around
1900 as a contaminate in Turkistan alfalfa seed (Groh,
1940).
Since its introduction spotted knapweed has spread
to infest over 7 million acres throughout at least 9 states
and 2 Canadian provinces (Lacey, 1989).
27
A major factor contributing to the tremendous success of
spotted knapweed in North America has been the lack of
predators.
In southeastern Europe, where knapweeds are
native, a complex of insects exists which has evolved with
Centaurea species and feed exclusively on them.
As a
result, knapweeds in Europe exist in small, widely scattered
patches along disturbances and roadways.
There are no vast
infested acreages as there are in North America (Schroeder,
1985).
Surveys of potential biological control agents
started in 1961 and eventually developed into an aggressive
biocontrol effort led by the Canadian and U.S. governments.
At the Knapweed Symposium held in Bozeman, Montana in
June 1989, Story (1989) summarized the status of the
biological control effort on spotted and diffuse knapweed.
The first biological control agent to be employed against
the knapweeds was a small European seed head fly, Urophora
affinis, that was introduced to British Columbia in 1970
(Harris, 1980).
In 1973 the same species was released in
Montana and Oregon (Story and Anderson, 1978'; Maddox, 1982) .
A second seed head fly, Urophora quadrifasciata, was
released in British Columbia in 1972 (Harris, 1980).
Although the second seed head fly was never released in the
United States, it entered the U.S. in the early 1980's and
is now widely self-distributed throughout the knapweedinfested areas of the Pacific Northwest.
Both Tephritidae
flies attack spotted and diffuse knapweed seed heads.
The
28
larvae induce gall formation in the receptacle tissue of the
flower buds.
The galls act as metabolic sinks and sequester
nutrients and sugars resulting in reduced seed production
(Story, 1978).
Harris (1980). reported up to 95% reduction
of seed production in British Columbia where both species
were present.
In Montana seed reduction caused by the
larvae ranges from 50% to 75% (Story, 1984).
Considerable research has been conducted on the Urophora
species in the Pacific Northwest in the 20 years since their
introduction.
Important information has been obtained on
the biology, life history, establishment, dispersal, impact
on spotted and diffuse knapweed, interspecific competition,
within-head distribution procedures, feeding strategy, and
numerical sampling requirements of both flies (Berube, 1980;
Gillespie, 1983; Harris, 1980a; Harris, 1980b; Harris, 1988;
McCaffrey and Callihan, 1988; McCaffrey et al., 1987;
Story
and Anderson, 1978; Story, 1984; Story and Nowierski, 1984;
Story et al., 1987; Story et al., 1988).
A seed head moth, Metzneria paucipunctella, was released
in British Columbia in 1973 and numerous collections of the
moth have been made at the original release site for
subsequent release in the U.S.
It is now established in
Montana, Idaho, Oregon, and Washington.
Larvae of this moth
feed on the seed, each larvae attacking about six seeds per
seed head.
Although several eggs may be laid, only one M.
paucipunctella larva survives per seed head.
The moth
29
larvae are predatory and can destroy up to 63% of U. affinis
and U. quadrxfasciata larvae when they are encountered in
infested seed heads (Story, et al., 1989)
Despite this
interspecific competition, the combined effect of the two
seed head flies and the seed head moth significantly reduced
seed production in attacked seed heads below the level
caused by the two fly species alone (Story and Nowierski,
1984; Story, et al.).
Although the seed head insects have proven to be very
effective at reducing seed production, their combined effect
has not reduced population density of the knapweeds (Harris,
1980).
As a result, a survey of potential rosette and root-
feeders was conducted.
Based on this survey four root­
feeding species, including three moths, Agapeta zoegana (L.)
(Lep.: Cochylidae), Pelochrista medullana Stgr. (Lep.:
Tortricidae) and Pterolonche inspersa Stgr.
(Lep.:Pterolonchidae) and one weevil, (Cyphocleonus achates
(Fahr.)
(Col.: Curculionidae) were selected for introduction
into North America between 1979 and 1983 (Muller et al.,
1988; Muller, 1989; Muller et al., 1989).
The root weevil was released in British Columbia in 1987
where a small population is established.
The larvae of this
weevil feed on the core of large knapweed roots causing
stress on the plant.
The first release of the weevil in the
U.S. was made in Montana in 1988 (Story, 1989).
30
The root moth Pterolonche inspersa was introduced into
British Columbia in 1986 where a small caged population is
now established.
The larvae of this moth feed on the core
of knapweed roots.
While this moth has been released in
Montana, Idaho, Oregon and Washington, its status is not
known at this time.
The larvae of the root moth Pelochrlsta medullana feed
in the cortex and epidermal tissues of knapweed roots.
small population is established in British Columbia.
A
Adults
received in 1984 in Montana failed to produce fertile eggs.
Inability to locate the moth in Eurasia since 1984 has
prevented subsequent introduction during the past four
years.
The root moth Agapeta zoegana is the most promising
insect introduced on spotted knapweed to date.
Larvae of
the moth feed on the cortex and epidermal tissues of
knapweed roots.
Heavy attack by the larvae can cause death
to small plants (Muller et al., 1988).
This moth is now
established in British Columbia, Montana, and Washington.
The root beetle Sphenoptera jugoslavica was initially
established in British Columbia and has been redistributed
to Idaho, Washington and Oregon.
The larvae feed on the
core of diffuse knapweed roots, and under favorable
conditions, can reduce diffuse knapweed density (Powell and
Meyers, 1988).
31
Three additional insects are being screened for
potential use on spotted knapweed: I) a seed head fly,
Terellia virens; 2) a seed head fly, Chaetorellia sp.; and
3) a seed head weevil, Larinus obtusus.
Larvae of T. virens
and L. obtusus feed on the soft, developing seeds while
Chaetorellia sp. larvae feed on the ovaries.
These three
species should complement the effects of the two established
gall flies, Urophora spp., because the three insects attack
the flower buds at a later date and, in the case of
Chaetorellia sp., are more adept at attacking isolated
plants (Harris, 1988).
Two insect species are currently undergoing host
specificity screening for use on diffuse knapweed.
The
larvae of a seed head weevil, Bangasternus fausti, feed in
the seed head receptacle.
Recent concern has been expressed
about this weevil since it reportedly destroys all other
insects in the seed head, especially U. affinis.
is the seed head weevil, Larinus minutus.
The second
Larvae of this
insect feed on the soft, developing seeds.
Of the remaining insects known to be associated with
spotted knapweed in Eurasia, the insect with the most
potential is the. small seed head gall wasp, Isocolus
minutus.
This wasp forms a woody gall in immature flower
buds.
Harris and Cranston (1979) suggest that the
establishment of a complex of at least six natural enemy
32
species would be necessary for biological control to be
effective against diffuse and spotted knapweed.
This
complex must include significant feeders attacking different
plant parts, such as seed head feeders and root-crown
feeders.
If screening tests proceed satisfactorily, a total
of 11 and 10 insect species will have been introduced
against spotted and diffuse knapweed respectively by 1995
(Story, 1989).
Other natural enemies such as plant pathogens have been
investigated for their potential as biocontrol agents for
the knapweeds.
Puccinia spp. have been isolated from
spotted and diffuse knapweed in Eurasia (Watson and Clement,
1986).
Several of these rust pathogens are excellent
candidates for biological control because they have
restricted host ranges, are autoecious, and are capable of
producing abundant quantities of uredospores which
facilitate their spread.
Unfortunately, host specificity
tests conducted in controlled environment facilities
indicate that safflower (Carthamus tinctorius L.) is a
potential host of the rust pathogen Puccinia jaceae which
attacks diffuse knapweed and Puccinia centaureae which
attacks spotted knapweed (Watson and Clement, 1986).
f
Another fungus, Sclerotinia sclerotiorum (Lib) de Bary,
was isolated on diffuse knapweed and identified by Watson,
Copeman and Renney (1974).
Inoculation of spotted knapweed
33
with S. sclerotiorum resulted in the production of symptoms
identical with those observed on diffuse knapweed.
Sclerotinia sclerotiorum is a well-known plant pathogen
with a very broad host range (Purdy, 1979).
In order for
this naturally-occurring fungus to be utilized as a
biocontrol agent against spotted and diffuse knapweed it
would have to be contained so as to negate any threat to
non-targeted crop plants.
Research was begun in the early
19801s at Montana State University to produce stable clones
of S. sclerotiorum with limited ability to spread or survive
for more than one season (Ford, 1989).
Two types of mutants
were obtained through ultra-violet light mutagenesis.
first mutant is auxotrophic for cytosine.
The
The second mutant
has limited ability to overwinter because it lacks the
ability to produce sclerotia, survival structures comprised
of dense, compacted aggregates of hyphae.
They are
resistant to unfavorable environmental conditions and are
capable of remaining dormant for long periods of time.
Also, sclerotia are precursors to ascospore production, the
main means of dissemination of wild type S. sclerotiorum.
A critical requirement for infection is the presence of
a readily available nutrient source.
Numerous studies have
shown that Sclerotinia ascospores are unable to infect
healthy hosts plants unless they first contact and colonize
a suitable organic nutrient source*
Furthermore, cool wet
weather or irrigation seems to be necessary for infection to
\
34
occur.
In field studies inoculum placed on grain, such as
millet or oats, provided a nutrient base allowing the
Sclerotinia hyphae to grow and form appressoria that readily
penetrate the host plant tissue, causing disease when
adequate moisture was present.
Stierle et. al. (1988) discovered the black leaf blight
fungus Alternaria alternata Lam. on spotted knapweed.
They
isolated the phytotoxin maculosin from the fungus and
synthesized the compound in the laboratory.
Although the
specific mode of action and field efficacy of maculosin is
not known, it appears that both the naturally-occurring and
synthetic forms of the compound are highly phytotoxic only
to spotted knapweed.
However, due to genetic diversity of
spotted knapweed populations, some plants and seeds survive
maculosin treatment.).
Cuda, Sindelar ,and Cardellina
(1989) proposed that established populations of Agapeta
zoegana L., a root mining moth, would control these
surviving plants.
Overseeding the treated site with
aggressive forage grasses would inhibit spotted knapweed
seedling establishment and replace knapweed plants weakened
from combined effects of maculosin and A. zoegana.
Maculosin has not been field tested and therefore
performance outside of the laboratory has not been
demonstrated.
It appears that maculosin would need to be
applied in a similar manner as conventional herbicides, but
35
uptake, translocation and persistence information is
lacking.
Biological control will contribute greatly to the
longterm management of the knapweeds in North America;
however, the ultimate benefits will not be evident for many
decades.
The implementation of effective biological control
programs must include sound land resource management which
favors desirable competing plant species.
Muller and
Schroeder (1989) concluded that establishment of a
competitive grass cover is most effective for both the
reduction of knapweed density and the long term
stabilization of its population by refilling its niche once
the knapweed population has declined.
Chemical Control
Herbicides are an effective means of controlling spotted
and diffuse knapweed.
Two,4-D ((2,4-dichlorophenoxy)acetic
acid) applied from the bolt to early flowering stage of
growth provides adequate control of established plants and
excellent control of seedlings (Lacey et al., 1986).
Annual
applications of 2,4-D at rates of 1.12 to 2.24 kg a.i./ha
halted seed production of spotted knapweed (Belles, et al.,
1980; Wattenbarger, et al., 1980).
Reapplications are
necessary to control regrowth of older established plants
since translocation of 2,4-D to the root system of spotted
36
knapweed is minimal (Lacey, 1985).
The ester formulations
are more effective than amines (Belles, et al., 1978).
Dicamba (3,6-dichloro-o-anisic acid) has soil residual
properties that make it more effective than 2,4-D, giving 2
to 3 years of spotted knapweed control depending on the
level of plant competition following treatment (Fay, et al.,
1989).
Application rates of 1.1 kg a.i./ha provided
excellent control when applied in the rosette stage (Lacey
et al., 1986).
Picloram (4-amino-3,5,6-trichloropicolinic acid) is the
most effective herbicide for long term control of spotted
and diffuse knapweed (Lacey, 1985; Penney and Hughes, 1969;
Wattenbarger et al., 1980) .
Picloram applied at a rate of
0.28 kg a.i./ha has provided up to 5 years of complete
control on certain sites (Belles et al., 1980; Fay et al.,
1989).
This extended period of control is due to soil
persistence of picloram and plant competition after
treatment which discourages reinfestation (Chicoine, 1984;
Penney and Hughes, 1969).
Several factors influence the degradation and movement
of picloram in soil.
The decomposition rate increases in
the presence.of plant roots, high soil organic matter
levels, elevated moisture and soil temperatures, and acidic
soils (Goring and Haymaker, 1971).
Picloram is susceptible to leaching because of its
relatively high water solubility and low adsorption to soil
37
colloids (Bovey and Scifresz 1971).
is correlated with adsorption.
Resistance to leaching
Leaching and sorption
characteristics are influenced by soil properties and
seasonal precipitation.
Picloram has a pKa of 3.6,
therefore the proportion of ionized to non-ionized picloram
decreases with decreasing soil pH.
Maximum soil adsorption
occurs in low pH soils with high organic matter and hydrated
iron and aluminum oxides.
Adsorption is minimal in neutral
or alkaline sandy loam soils low in organic matter.
Spotted
knapweed thrives on neutral to high pH, well-drained sandy
or gravelly soils in Montana which may account for the
shorter.duration of control achieved with picloram at these
sites.
Picloram is susceptible to rapid photodecomposition on
plant and soil surfaces (Hall et al. 1 9 6 8 ; Rice, 1989).
Rice (1989) applied picloram at 0.28 kg a.i./ha and measured
68 and 42% loss from vegetation and soil surfaces
respectively after 7 days without precipitation during June
in Montana.
Although 90% of the picloram applied may
intercept the vegetative canopy, only I to 2% of the foliar
applied picloram is. actually absorbed into the plant (Bovey,
Davis and Merkle, 1967).
Therefore, picloram is vulnerable
to photodecomposition until the first rainfall occurs and
moves it into the soil.
Spotted knapweed seedlings begin to reestablish when
picloram concentrations in soil drop below 0.012 ppm
38
(Watson, Rice and Monnig, 1989).
Picloram provides long­
term control of spotted knapweed when site conditions and
timing of precipitation following application are optimum
(Cranston, 1984).
Precipitation is necessary for
incorporating most of the picloram into the soil where it is
available for root uptake and protected from decomposition
by sunlight.
Clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) is a
pyridine herbicide closely related to picloram.
Clopyralid
is more selective than picloram and has a shorter soil
residual period.
Rates of 0.28 kg a.i./ha provided 100%
control of spotted knapweed I year following application at
two sites in Montana without altering the native forb
density or diversity (Lacey, McKone, and Bedunah, 1989).
Seedling reestablishment occurs by the second year after
application so reapplication is necessary to prevent seed
production.
The narrow weed control spectrum of clopyralid
makes it a valuable herbicide for spotted and diffuse
knapweed control in environmentally sensitive areas where
the broad spectrum herbicide picloram can't be used.
Spotted knapweed displaces native vegetation and often
results in near monoculture infestations.
In these
situations chemical treatment with picloram or clopyralid at
a rate of 0.28 kg a.i./ha results in greater plant community
diversity than exists in untreated, knapweed dominated sites
(Bedunah and Carpenter, 1989; Lacey, McKone and Bedunah,
39
1989).
Reseeding infested areas following herbicide
treatment increases forage production and augments control
of spotted and diffuse knapweed by suppressing seedling
reestablishment (Fagerlie, 1989; Hubbard, 1975).
40
CHAPTER 2
SPOTTED KNAPWEED SEED LONGEVITY
Introduction
Spotted knapweed currently infests over 4.7 million
acres of grazeable woodland, rangeland, and pastureland in
Montana (Lacey, 1987).
The plant reproduces by seed, and
individual plants in a natural' setting produce an average of
1,000 seeds (Story, 1976).
Approximately 90% of the seed
produced by spotted knapweed is viable (Schirman, 1981).
The longevity of viability of spotted knapweed seed has
not been determined.
Previous research showed that 40% of
buried spotted knapweed seed remained viable in soil for at
least 5 years (Shirman, 1984).
The objective of this study
was to determine the long-term viability of seed in soil.
Materials and Methods
Seed Burial Study
This study was established at two locations in Montana
by Chicoine (1982).
Site One was located at the Research
Farm near Bozeman in a 46 cm rainfall area on a Bozeman
silty clay loam soil (fine-silty, mixed, Frigid Argic Pachic
CryoborolI).
Site Two was located.on rangeland at the Dale
Folkvord farm near Three Forks in a 31 cm rainfall area on a
41
Amesha loam soil (coarse-loamy, mixed, Borollic
Calciorthid).
Both soils are deep and well drained.
Burial rings were constructed by cutting 7.6 cm diameter
polyvinyl-chloride (PVC) pipe into 2.5 cm sections.
Nylon
window screen (16 mesh) was cemented to the bottom of each
ring.
The rings were half filled with dry soil obtained
from the respective burial sites.
One hundred spotted
knapweed seeds were placed on the soil surface in each
burial ring and covered with soil.
Nylon window screen was
cemented to the top of each ring.
The PVC rings were buried
flush with the soil surface at both locations in a
randomized complete block design on September 22, 1982.
The
seed was collected on August 30, 1982 from spotted knapweed
plants located 4 miles west of the Bozeman site.
Initial
percent germination of the freshly harvested seed was not
determined.
Following 2 months of storage at room
temperature germination was 100% (Chicoine, 1984).
Five seed containers were recovered at each location on
November 11, 1982, April 7, 1983, May 16, 1983, June 2,
1983, and October 10, 1983.
The number of seeds that had
germinated while buried and the percentage of viable dormant
seed was determined for each collection date.
Because of
low in situ germination and obvious seed dormancy, the study
was revised so that data could be collected for a longer
period of time.
Three containers were recovered in October
of each year at both locations from 1984 through 1987.
Seed
42
was recovered by sifting the soil contained in each burial
ring through a 10 nun2 mesh screen followed by a 3 mm2 mesh
screen.
The remaining seed and soil mixture was placed on a
0.6 mm2 screen and gently washed with running water for 3
minutes to remove fine soil particles.
Seeds were removed
from the remaining material by hand.
The number of seeds that germinated during burial was
determined by counting empty seed hulls.
Germination was
measured daily on 25 seeds by floating them on distilled
water for 7 days at 21° C.
Decline of Viability from a Natural
Seed Population in Soil
The longevity of viability of spotted knapweed seed in a
natural population in soil was studied at two locations.
The first site was located on the Blackfoot Clearwater Game
Refuge near Ovando.
The second site was located on the
Jones ranch 15 miles south of Harlowton on the east side of
Highway 191.
Both sites were heavily infested with spotted
knapweed when the study was initiated in October, 1982.
Five treatments were applied in 1982 by Chicoine (1984)
to plots at each site to determine their effect on the
longevity of seed viability.
Each treatment was an attempt
t o 'increase the rate of germination to hasten the
elimination of the spotted knapweed seed reserve in soil.
Treatments included rolling, harrowing, burning, mowing,
2,4-D application, and an untreated control.
Plots were
43
arranged in a randomized complete block design with six 6.1
X 12.2 m plots per block, and three replications at each
site.
All plots except the untreated control received an
annual application of 2.2 kg a.i./ha of 2,4-D amine in midJune to prevent spotted knapweed seed production.
One plot
per replication was left as an unsprayed control treatment
to permit annual seed production.
A second set of treatments was applied to the plots on
May 22, 1985 to determine the germination response of
spotted knapweed seed.
Treatments performed on whole plots
included fertilization with 86 kg 26-13-0 per ha,
application of glyphosate at 2.2 kg a.i./ha, application of
2,4-D amine at 2.2 kg a.i./ha, rototilling 7.6 cm deep, and
no treatment.
Seed production was prevented with an annual
application of 2.2 kg a.i./ha 2,4-D.
An additional
treatment, the original untreated control, was left
untreated to permit annual seed production.
The seed population remaining in soil was measured by
taking six 5.6 cm diameter soil cores 7.6 cm deep within
each plot with a tapered tulip bulb planter in April and
October from 1982 until 1989.
Seed was separated from soil
through a series of screening, washing, and air cleaning
steps (Chicoine, 1984).
Seed from each core was floated on
distilled water at 21° C for 7 days and the number of
germinated seed were counted daily.
44
Seed Germination Study
Mature spotted knapweed seed was hand collected from
Drummond, Bozeman, Anaconda, Bonner, Butte, Hyalite and
Florence, Montana in the fall of 1988 and stored at -260 C.
A greenhouse study was established to determine the
effect of burial on spotted knapweed seed germination.
One
hundred spotted knapweed seeds from each collection site
were placed on the soil surface or buried 0.6 cm deep in 11
cm diameter by 8 cm deep plastic containers filled with a
1:1 mixture of sand and soil (Bozeman silty clay loam).
The
experiment was arranged in a randomized complete block
design with five replications.
Temperature was maintained
at 24°.C .±.2° C day (14 hours light) and 18° C ± 2° C night.
Uniform watering was maintained with an overhead automatic
mist system which cycled every 3 hours delivering a total of
5.2 cm of water daily.
Seedlings were counted and removed
daily until germination ceased.
The experiment was
conducted twice.
A second germination study was conducted using seeds
from the seven collection sites to determine the effect of
light on spotted knapweed seed germination.
One hundred
spotted knapweed seeds from each seed source were placed on
blotter paper in 10 cm diameter petri dishes.
Ten ml of
distilled water was added to each petri dish; half of the
petri dishes received water in complete darkness and were
then wrapped in aluminum foil to exclude light.
Petri
45
dishes were placed in an incubator at 21° C.
Seeds
receiving light were exposed to 15 hours of light
(combination of incandescent and fluorescent lights
producing 660 ^mol s 1 m 2) per day.
determined after 5 days.
times.
Percent germination was
Each treatment was replicated five
This experiment was also conducted twice.
Results and Discussion
Seed Burial Study
The PVC seed containers used for seed burial worked well
since seed recovery was nearly 100% throughout the term of
the study (Table I).
Even when seeds had germinated during
the first year, the remains of the original seeds were
easily distinguishable after 5 years of burial.
This
indicates that the outer protective seedcoat is very durable
and resistant to decomposition.
Table I. Recovery of Buried Spotted Knapweed Seed.
Months of
Burial
______Seed Recovered_____
Bozeman
Three Forks
%
12
24
36
48
60
OVab1
99a
99a
99a
94c
95b
100a
97ab
9Vab
97ab
1Means within a column followed by the same letter are not
significantly, different at the 5% level using the LSD test.
46
In situ germination of artificially buried spotted
knapweed seeds
averaged approximately 38 and 65% after 5
years of burial at Bozeman and Three Forks, respectively
(Figure I).
This relatively low amount of germination after
5 years indicates that conditions during burial induce and
enforce dormancy in spotted knapweed seed since seeds
germinated rapidly when removed from soil (Table 2).
Bozeman
D
Three Forks
M O N T H S OF BURIAL
F i g u r e I. Percent Germination of in situ Spotted Knapweed
Seeds Recovered Annually From Buried Containers at Bozeman
and Three Forks.
Variability in germination occurred among replications
from year to year, especially at the Three Forks site, due
47
to uneven settling of the rings after burial.
Originally
the top surface of each burial ring was flush with the soil
surface.
After settling, some rings were slightly exposed
and some were covered with soil causing a large differential
in germination response.
Some of the rings that had settled
below the soil surface wene saturated with standing water on
occasion which stimulated higher rates of in situ
germination.
We have found that spotted knapweed seeds
germinate in only 18 hours when floated on water, compared
to 3 to 5 days when placed on wet filter paper (unpublished
results).
This may partially explain why spotted knapweed
is adapted to extremely well drained, gravelly sites where
standing water does not occur.
The germination variability
associated with small microsite changes in this experiment
shows spotted knapweed seed germination response and
longevity are strongly influenced by environmental
conditions.
Viability of ungerminated spotted knapweed seeds
recovered from burial rings averaged approximately 90%,
which indicates that dormant seeds are well protected from
decomposition (Table 2).
Over 50% of the original buried
seeds were dormant 60 months after burial at both locations
(Figure 2).
Recovered seeds germinated within 24 hours of
the start of imbibition under laboratory conditions
suggesting that light or some other rapidly reversible
48
factor related to burial is responsible for maintaining
dormancy.
Table 2. Viability of Intact, Ungerminated Spotted Knapweed
Seed Recovered Over a 60 Month Period at Bozeman and Three
Forks M T .
Months of
Burial
12
24
36
48
60
Bozeman
91a1
83b
78b
84b
83b
Germination
Three Forks
--- % --------------96a
92a
50b
91a
97a
1Means within a column followed by the same letter are not
significantly different at the 5% level using the LSD test.
Lewis (1973) investigated the survival characteristics
of crop and weed seeds in undisturbed soil in a 20 year seed
burial study.
He concluded that the survival potential of
seeds is revealed during the first 4 years of burial. After
four years seed viability decreased approximately 5% per
year.
At this rate, in the spotted knapweed study, it would
take 77 years to reduce the current number of approximately
50 viable spotted knapweed seeds per ring to one seed per
ring.
Lewis (1973) reported that viability of creeping
buttercup (Ranunculus repens L.) seed was 51 percent 20
years after burial in undisturbed soil, a figure almost
identical to the viability he measured after 13 years.
The
long term survivability of this species was attributed to
49
TH R EE FORKS, M T
BOZEM AN, M T
M O N T H S A F T E R B U RIAL
Figure 2.
The Number of Viable Spotted Knapweed Seeds
Recovered During a 60 Month Burial Period at Three Forks and
Bozeman.
50
the durable achene which protects the seed from
degenerativesoiI conditions.
Additionally, important
enzymatic biological repair processes would remain active
during burial since the seeds were imbibed intermittently
during dormancy.
Spotted knapweed achenes, like creeping
buttercup, have a thick, durable pericarp that protects the
seed but permits water imbibition (Chapter 4).
A second burial study was established by the author in
October, 1989 that was designed to last as long as 100 years
if necessary to determine the extent of longevity of buried
seed.
Viability of Natural Seed Populations
When the soil seed reserve in spotted knapweed infested
soil was measured in June, 1982, there were 501 and 647
viable spotted knapweed seeds recovered per m? to a depth of
7.6 cm at Ovando and Harlowton, respectively.
The number of
viable seed decreased 87 and 72%, respectively, at these
locations in the 10 month period from June, 1982 through
April, 1983 (Chicoine 1984, Lacey 1985).
The seed reserve
had declined 96 and 92% at Ovando and Harlowton,
respectively, by April, 1985, 34 months after termination of
seed production.
One month later Lacey (1985) applied
treatments, designed to stimulate germination of the
remaining seeds.
None of these treatments were effective in
accelerating the reduction of seed populations (Table 3).
51
The seed population remaining in soil did not decline
measurably from October, 1985 to October, 1989 (Figure 3).
In October 1989, approximately 22 and 17 viable seeds
remained.per 0.5 m2 (440,000 and 340,000 seeds per ha) at
Ovando and Harlowton respectively, 88 months after seed
production was halted.
Table 3. Changes in the Seed Reserve in Soil 12 Months After
Treatments were Applied in May, 1985 in an Attempt to
Stimulate Germination of Spotted Knapweed Seed at Harlowton
and Ovando.
Viable iseeds ner 0.5 m2
Treatment
Harlowton
May, '85 May, '86
Fertilization
Glyphosate
Rototilling
2,4-D amine
No treatment
25
22
22
10
54
±
±
±
±
±
18
36
36
18
68
32
21
32
10
21
±
±
±
±
±
33
18
33
18
18
Ovando
May, •85 May, '86
32
21
42
10
10
±
±
±
±
±
32
18
19
18
18
31
10
31
21
21
±
±
±
±
±
10
18
10
18
37
The rapid decline in the spotted knapweed seed reserve
during the first 34 months of the experiment is probably due
to germination of seeds located in microsites suitable for
germination, on or near the soil surface.
I propose that
the small but vital population of seed, remaining 3 years
after seed production was halted, represents dormant buried
seed that has acquired a light requirement for germination.
This will be discussed below.
52
CM
22
28
34
40
46
SPOTTED
KNAPW EED
SEEDS
PER
0 .5
OVANDO
52
58
64
NUMBER
OF
V IA B L E
H A R LO W TO N
22
28
34
40
46
52
58
64
76
88
M O N T H S W IT H O U T SEED PR O D U C TIO N
3. The Decline in the Spotted Knapweed Seed Population
Following Termination of Seed Production in 1982 at Ovando and
Harlowton MT.
Figure
53
Germination Behavior
The germination of freshly harvested spotted knapweed
seed from.seven locations was 21% higher when placed on the
soil surface than when buried 0.6 cm deep (Figure 4).
It is
unlikely that the difference in germination of buried and
surface placed seeds was due to inadequate moisture,
temperature or soil atmosphere surrounding the buried seed
since I) frequent, uniform watering was provided by the
automated misting system, 2) temperature conditions were
quite constant, and 3) the shallow seeding depth and well
drained soil allowed for adequate gas exchange.
Spotted knapweed seed from 7 locations exposed to light
during imbibition germinated 24% higher than seed kept in
the dark (Figure 5).
Approximately 13% of the seed from
each location did not germinate initially in either the
light or dark treatment.
Several months after collection,
germination was nearly 100% indicating a portion of the
seeds were innately dormant at the time of collection and
required a period of after-ripening.
If individual seeds
with a light requirement become buried before conditions
favorable for germination occur, they will remain in a
dormant state until the light requirement is either
satisfied or terminated.
Polymorphic germination behavior, where seeds produced
on the same plant or within a population of plants possess
54
different germination requirements, is a common phenomenon
among plant species (Bewley and Black, 1982).
It permits
4
Effect of Spotted Knapweed Seed Placement on the
Soil Surface
or Buried 0.6 cm Deep |g| on Germination
of Seed Collected at 7 Locations.
Figure
seed germination over time insuring long-term survival of at
least a portion of each seed crop (Popay and Roberts, 1970).
In addition plants such as spotted knapweed that exhibit
asynchronous flowering behavior are more likely to display
germination polymorphism due to the influence of variable
environmental conditions during seed ontogeny (Fenner,
1985).
55
Since the soil surface is continually changing due to
climatic factors and biotic activity, microsites (and their
conditions) are likewise changing (Harper, Williams and
100
Z 90
LSD ■
0.05 I
O
h- 80
<
Z 70
2
60
DC
LU 50
0
LU 3°
CC 20
S
10
Drummond Bozeman
Anaconda
Bonner
Butte
Hyalite
Florence
COLLECTION LOCATION
Figure 5.
Light
Germination of Spotted Knapweed Seed in
or Complete Darkness ■ § .
Sagar, 1964).
Seeds that are dispersed into microsites
where conditions are unfavorable for germination may remain
in a state of enforced dormancy for many years (Harper,
1959).
Wesson and Wareing (1969) demonstrated that burial
induces light sensitivity in seeds of some weed species that
were previously light insensitive.
56
Spotted knapweed seeds may fail to germinate immediately
after being shed because of environmental factors such as
supra or suboptimaI temperature and insufficient moisture,
or innate dormancy factors such as after-ripening.
Subsequent germination of these after-ripened seeds is
stimulated by exposure to light caused by a soil disturbance
(Wesson and Wareing, 1969).
Furthermore, Nolan and
Upadhyaya (1988) showed that 50-60% of freshly harvested
spotted knapweed seed displayed a light requirement.for
germination.
If these seeds became buried a portion might
remain dormant for long periods of time prior to exposure to
light.
This supports the hypothesis that the small
population of ungerminated spotted knapweed seed remaining
in the natural decline study, and the seeds remaining in ttie
burial study, represent dormant seed which require light to
germinate.
Nolan and Upadhyaya (1988) reported three forms of
polymorphic germination behavior in populations of spotted
and diffuse knapweed seed: I) non-dormant seed which
germinated readily under suitable conditions,
2) primary
dormant seed which required light to germinate, and
3)
primary dormant seed which remained dormant in spite of
light exposure.
The evidence presented here suggests that
the light-sensitive and light-insensitive primary dormant
spotted knapweed seed that becomes buried in soil shortly
f
57
after dehiscence may be the small but extremely important
portion of the seed produced each year that remains dormant
to permit long-term survival of spotted knapweed.
58
CHAPTER 3
LONG TERM CONTROL OF SPOTTED KNAPWEED
WITH REDUCED RATES OF PICLORAM
Introduction
Spotted knapweed is the most troublesome weed in
Montana.
Its invasive nature and the absence of most of its
natural enemies permits establishment of dense, near­
monoculture stands.
Infestations of this perennial weed can
reduce forage production 90% (Watson and Renney, 1974).
Herbicides are an effective means of controlling spotted,
knapweed.
Piclofam provided I to 5 years of complete
control of spotted knapweed when applied at 0.28 kg a.i./ha
(Fay et. al., 1989).
The exact longevity of complete
control depends on site conditions, especially the presence
of significant amounts of soil organic matter (Cranston,
1984).
Grover (1971) showed a positive correlation of
picloram adsorption with soil organic matter content.
Leaching of picloram is minimized in high organic matter
soils (Keys and Friesen, 1968; Scifres et al., 1969),
providing longer periods of control than in light textured
soils low in organic matter.
The length of time between
application of picloram and the occurrence of precipitation
greatly influences the loss of picloram through
59
photodecomposition (Hall et al., 1968? Johnsen and Martin,
1983; Rice, 1989).
The period of control of spotted knapweed provided by
0.28 kg a.i./ha of picloram often extends well beyond the
predicted period of residual activity for the herbicide.
Haymaker et ad. (1967) reported that 90% of picloram applied
at 0.28 kg a.i./ha will dissipate in 12 months; therefore,
no more than 0.03 kg a.i./ha should remain in the soil
during the second year following application.
In contrast,
Lacey (1985) determined through a bioassay that spotted
knapweed seedlings are sensitive to 0.02 kg a.i./ha
picloram. Thirty months was required for picloram residues
to degrade to this level following application of 0.28 kg
a.i./ha at Harolwton and Ovando.
The longevity of spotted
knapweed control measured at Harlowton and Ovando was
partially dependent upon the presence of phytotoxic levels
of picloram in soil, therefore the residual period of
picloram in Montana was 40 to 50% longer than the period
predicted by Haymaker.
Since the viability of spotted
knapweed seed exceeds 7 years (Chapter 2), factors other
than picloram residues, such as plant competition, influence
the rate of reinfestation of spotted knapweed.
The objective of this study was to determine the
longevity of control of spotted knapweed using quantities of
picloram at and below the present recommendation of 0.28 kg
a.i. picloram per hectare.
60
Materials and Methods
Field trials were established in June, 1982 by T . K.
Chicoine, former graduate research assistant. Plant and Soil
Science Department, Montana State University (Chicoine,
1984) .
Site one is located near Ovando on the BlackfootClearwater Game Refuge.
The study was established on gently
rolling foothills characterized by a grassland community
type dominated by timothy (Phleum pratense), smooth
bromegrass (Bromus inermis), and Kentucky bluegrass (Poa
pratensis).
The dominant forb was spotted knapweed.
The
site had been cultivated in 1954 and seeded to timothy.
There was no grazing use by domestic livestock or wildlife
for the duration of the trial, however the area was
inadvertently cut for hay by Fish, Wildlife, and Parks
Department personnel during the summer of 1986.
The elevation of the study site is 1837 m and average
annual precipitation during the 7 year study period was 32.6
cm. measured at Ovando approximately 10 miles east of study
site.
The soils are classified as loamy-skeletal, mixed,
Frigid Typic Haplustols.
Site two is located on the Bill Jones ranch
approximately 15 miles south of Harlowton on a floodplain.
The site consisted of a grassland vegetation community
dominated by western wheatgrass (Agropyron smithii),
Kentucky bluegrass needle-and-thread (Stipa comata), and
blue grama (Bouteloua gracilis).
dominant forb.
Spotted knapweed was the
The study area is located within a 245
hectare pasture that was heavily stocked (700 animal units)
with cattle each year from November through May.
Although
supplemental feed was provided, the area received heavy
grazing pressure during the early spring months of 1982
through 1986.
The experimental area was fenced in the fall
of 1986 to exclude livestock.
The elevation at the site is 2041 m and average annual
precipitation measured at Harlowton was 34.4 cm during the 7
year study period.
The soil is classified as a loamy-
skeletal, mixed, Frigid Ustic Torrifluvent.
Further details
of the plant community and soil characteristics of both
sites were described by Lacey (1985).
Herbicide treatments were applied by Chicoine in June,
1982 with a C02-pressurized backpack sprayer delivering 137
1/ha spray solution.
Plots (3.4 X 12.2 m) were arranged in
a split block design with 3 replications.
Picloram was
applied at 0.07, 0.11, 0.14, 0.22, 0.25, and 0.28 kg a.i./ha
as main plot treatments, and 2,4-D amine was applied at 2.24
kg a.i./ha to half of each plot as the subplot treatment.
Individual plots were spatially separated from each other by
a 2.1 m buffer zone treated with a tank mix of picloram and
2,4-D amine at 0.37 and 2.24 kg a.i./ha respectively to
prevent contamination from spotted knapweed seed production
in neighboring plots.
The perimeter of the experimental
62
area was further separated by a buffer zone 3.4 m wide which
was sprayed with the same mixture of picloram and 2,4-D
amine.
The buffer zones were retreated in June, 1986 with
the same treatment described above.
Spotted knapweed plant counts were taken in June, 1982
prior to herbicide application, and in July from 1983
through 1989.
Four I m2 frames were placed at random within
each plot and mature spotted knapweed plants were counted.
Biomass production was measured 2, 14, 26, and 86 months
following herbicide application.
Two 0.5 m2 frames were
placed at random locations within each plot.
The standing
crop was clipped at ground level and the current year's
growth separated into grasses or spotted knapweed, the only
forb present in significant frequency.
Vegetation was oven-
dried at 50° C for 48 hours to determine herbage dry weight
production.
Results and Discussion
All rates of picloram reduced spotted knapweed density
60 and 84 months after application at Harlowton and Ovando,
respectively.
Complete control of mature spotted knapweed
plants 36 months following treatment was obtained with rates
ranging from 0.14 to 0.28 kg a.i./ha at Ovando (Table 4).
Rates of 0.11 to 0.28 kg a.i./ha provided 100% control of
mature spotted knapweed 24 months after herbicide
application at Harlowton; however, only the 0.28 kg a.i./ha
63
rate provided complete control after 36 months (Table 5).
The addition of 2,4-D amine as a subplot treatment did not
significantly improve control over picloram alone (Lacey,
1985).
Table 4. Number of Mature Spotted Knapweed Plants per m2 in
July, 1983 Through 1989 Following Herbicide Application in
June, 1982 at Ovando.
Picloram
(kg/ha)
0.28
0.25
0.22
0.14
0.11
0.07
0.00
_____________________Year_______________________
1983
1984
1985
1986
1987
1988
1989
0. Oa1 0.0a
0.0a
0.0a
0.0a
0.0a
0.0a
0.0a
3.2a
4.6a
41.2b 21.1b
65.8c 58.4c
0.0a
0.0a
0.0a
0.0a
2.2a
4.1b
57.2C
0.0a
0.0a
0.1a
0.2a
0.2a
1.4b
54.2c
0.6a
0.4a
0.5a
1.7a
0.2a
4.7b
41.7c
0.3a
0.2a
0.5a
0.8a
1.3a
2.0b
52. Oc
0.2a
0.1a
0.6a
0.9a
0.
5a
1. la
46.5b
1Means in a column followed by the same letter are not
significantly different at the 5% level using the LSD test.
Table 5. Number of Mature Spotted Knapweed Plants per m2 in
July, 1983 Through 1989 Following Herbicide Application in
June, 1982 at Harlowton.
Picloram
(kg/ha)
0.28
0.25
0.22
0.14
0.11
0.07
0.00
_____________________Year
1986
1983
1984
1985
1987
1988
1.7a
1.5a
2.5a
3.2a
2.7a
2.7a
62.2b
8.8a
12.3a
6.5a
8.4a
10.8a
10.2a
61.9b
21.0 a
15.8a
17.5a
25.4a
22.8a
12.3a
30.8a
0.0a1 0.0a
0.0a
0.0a
0.0a
0.0a
0.0a
0.0a
0.0a
0.0a
2. Ia
2.1a
126.7a 84.1b
0.0a
0.2a
1.1a
1.2a
1.2a
1.8a
75.7b
1989
15.2a
15.3a
9.4a
16.4a
11.8a
15.2a
15.3a
1Means in a column followed by the same letter are not
significantly different at the 5% level using the LSD test.
Picloram soil residues dropped below the sensitivity
level of spotted knapweed seedlings by 36 months after
64
application of 0.28 kg a.i./ha picloram at Ovando and
Harlowton (Lacey, 1984).
Reinvasion by spotted knapweed
seedlings started 2 to 4 years after application depending
upon picloram rate in treated plots at Harlowton.
By July,
1988, the density of mature spotted knapweed plants within
picloram treated plots was not significantly different than
in untreated control plots (Table 5).
Reinvasion also occurred at the Ovando site.
Low
densities of mature spotted knapweed plants were present in
plots treated with picloram doses lower than 0.25 kg a.i./ha
48 months after treatment.
Picloram ranging from 0.11 to
0.28 kg a.i./ha resulted in less than one mature spotted
knapweed plant per m2 in 1989, 84 months following
application, compared to 47 plants per m2 for the untreated
control (Table 4).
Picloram at 0.14 to 0.28 kg a.i./ha
provided 100% control of spotted knapweed 26 months after
application, decreasing spotted knapweed herbage from 1990
and 2316 kg/ha to zero kg/ha at Harlowton and Ovando
respectively (Tables 6 and 7).
Grass production increased
200% at Harlowton and 700% at Ovando 26 months after
treatment in response to the elimination of spotted knapweed
(Tables 8 and 9).
?
65
Table 6. Spotted Knapweed Herbage Production 2, 14, 26, and
84 Months After Herbicide Application at Harlowton.
Picloram
Rate
(kcf/ha)
0.28
0.25
0.22
0.14
0.11
0.07
0.00
Months After Herbicide Application
2
14
26
84
Spotted Knapweed Herbage Production fka/ha)
659.Vab1
0.0a
O.Oa
466.5a
589.9ab
0.0a
763.9ab
0.0a
963.6b
3.7a
1022.7b
52.2a
1607.5c
2106.6b
O.Oa
O.Oa
O.Oa
0.0a
O.Oa
78.3a
1990.0b
1797.0a
1740.0a
1390.0a
1533.0a
2097.0a
1687.0a
1510.0a
1Means in a column followed by the same letter are not
significantly different at the 5% level using the LSD test.
Table 7. Spotted Knapweed Herbage Production 2, 14, 26, and
84 Months After Herbicide Application at Ovando.
Picloram
Rate
fkg/ha)
0.28
0.25
0.22
0.14
0.11
0.07
0.00
Months jAfter Herbicide Application
2
14
26
84
Spotted Knapweed Herbage Production fkq/ha)
1267.5a1
1242.8a
1573.1a
972.7a
1413.9a
1563.4a
3991.9b
0.0a
O.Oa
0.0a
1.7a
17.9a
210.4b
1446.6c
0.0
0. Oa
O.Oa
0.0a
23.0a
597.3b
2316.0c
163.3a
226.7a
110.0a
393.3a
490.Oab
566.7ab
1113.0b
1Means in a column followed by the same letter are not
significantly different at the 5% level using the LSD test.
Spotted knapweed and grass herbage production within all
picloram treated plots was not significantly different than
the untreated plot 84 months following application at
Harlowton (Tables 6 and 8).
Heavy grazing pressure by
cattle during the spring of 1983, 1984, and 1985 provided
66
enough soil disturbance to permit significant germination of
dormant spotted knapweed seed to occur.
With the picloram
soil residues depleted by 1983, and grass competitiveness
reduced, spotted knapweed quickly reinfested the treated
plots.
Table 8. Perennial Grass Production 2, 14, 26 and 84 Months
After Herbicide Application at Harlowton.
Picloram
Rate
fka/ha)
0.28
0.25
0.22
0.14
0.11
0.07
0.00
Months .
After Herbicide Aoolication
26
2
14
84
Perennial Grass Herbaae Production fka/ha)
898.Sab1
727.9ab
516.4ab
693.lab
698.9ab
704.4ab
429.5b
1700.8a
1747.4a
1287.6a
1466.2a
1472.2a
1437.4a
321.2b
807.2a
795.2a
841.6a
702.Oa
640.4a
659.6a
256.7b
196.7a
230.0a
121.0a
130.0a
280.0a
286.7a
63.3a
1Means in a column followed by the same letter are not
significantly different at the 5% level using the LSD test.
Table 9. Perennial Grass Production 2, 14, 26 and 84 Months
After Herbicide Application at Ovando.
Picloram
Rate
fka/ha)
0.28
0.25
0.22
0.14
0.11
0.07
0.00
Months After Herbicide Aoolication
14
26
84
2
Perennial Grass Herbaae Production fka/ha)
560.5a1
307.1a
682.3a
440.5a
316.5a
350.2a
360.7a
1630.8ab
1474.Obc
1655.4ab
1433.8b
1275.0b
1021.2bc
332.Od
2612.8b
2686.4b
2739.2b
2896.8a
2517.6b
1636.4c
418.4d
803.3ab
1200.Oab
1330.0a
1050.Oab
910.Oab
766.7b
93.3c
1Means in a column followed by the same letter are not
significantly different at the 5% level using the LSD test.
67
While the recolonizing spotted knapweed plants were in
low density, they were much larger than individual plants
within the control plots due to reduced interspecific
competition.
The large plants produced a heavy seed crop,
leading to increased spotted knapweed density. . Powell,
Risley and Myers (1989) measured the biomass, population
density and seed production of recolonizing diffuse
knapweed plants 3 years following treatment with picloram.
They found that while the density of knapweed was less than
one tenth of the control population, individual plants were
much larger.
Plant biomass and seed production per mz was
greater than from plants in unsprayed areas.
These results
suggest that recolonization occurs rapidly once individual
knapweed plants become established following dissipation of
picloram residues on sites devoid of plant competition.
The suspected allelopathic influence of spotted and diffuse
knapweed may also contribute to their ability to invade and
displace native vegetation (Harris and Cranston, 1979;
Kelsey and Locken, 1987; Muir and Majak, 1983).
Perennial grass herbage production increased 700% at
Ovando by 26 months after herbicide application of picloram
at 0.14 to 0.28 kg a.i./ha (Table 9).
Spotted knapweed
herbage production averaged 178 kg/ha in plots treated with
picloram at rates of 0.14 to 0.28 kg a.i./ha compared to
1113 kg/ha for the untreated control (Table 7).
68
Grass production at Ovando 84 months after treatment
was 2.5 times lower than it was 26 months following
treatment.
This reduction was not due to reinvasion from
spotted knapweed as at Harlowton.
While the grass herbage
production in picloram treated plots was 11 times greater
than in the untreated plot 84 months after treatment, the
reduction in grass production in treated plots from 1984 to
1989 is substantial (Table 9).
The overall decrease in
grass production observed at Ovando may be due to a
sodbound condition of the smooth brome dominated site
following years of heavy grass production without
utilization as evidenced by the heavy thatch of dead grass
in treated plots.
In addition to reducing grass
productivity, the heavy thatch may inhibit spotted knapweedseed germination by filtering out red light (Nolan and
Upadhyaya, 1988).
Long-term control of spotted knapweed with reduced
rates of picloram appears to be more dependent on site
conditions and management practices than on the rate of
picloram applied.
Picloram applied at 0.14 to 0.28 kg
a.i./ha provided the same level of spotted knapweed control
for 84 months at Ovando, a highly productive, low
disturbance site.
The same concentrations of picloram
provided similar control for 36 months at Harlowton,
followed by a uniform rate of reinfestation throughout all
69
treated plots.
In addition, the Harlowton site represents
a case of extreme disturbance from high density animal
grazing.
70
CHAPTER 4
SEED MORPHOLOGY AND GERMINATION
CHARACTERISTICS OF SPOTTED KNAPWEED
Introduction
Spotted knapweed belongs to the Compositae
(Asteraceae) family which contains about 1310 genera and
13,000 species.
The knapweeds are classified in the genus
Centaurea, tribe Cynareae and subtribe Centaureinae
Durmort.
There are 500 to 600 members of the genus
Centaurea, mostly originating in the Mediterranean area
and Southwest Asia (Dittrich, 1977).
Grouping plants in
tribes and subtribes is commonly based on differences in
flower and fruit morphology.
Fruit characteristics serve
major functional roles in seed dissemination, water
uptake, seed positioning on a soil surface, speed of seed
germination and longevity of seed survival in soil
(Fenner, 1985; Lewis, 1973; Sheldon, 1974).
Scanning electron micrographs, speed of imbibition,
and point of water uptake were investigated to determine
their relationship to long-term survival of spotted
knapweed seed.
71
Materials and Methods
Scanning Electron Micrographs
Spotted knapweed achenes collected at Bozeman in 1987
were sputter-coated with a 60 nm thick layer of gold and
viewed through a JEOL IOOCX electron microscope utilizing
a ASID-4D scanning attachment located at the Veterinary
Research Laboratory, Montana State University.
The
achenes were scanned at magnifications ranging from 20 to
80OX.
Polaroid micrographs were taken of the achene
pericarp, pappus, hilum, epidermal hairs, nectary and
micropyle.
Longitudal cross section micrographs of the
embryo, testa, radicle, cotyledons, and internal air space
were taken.
The protective pericarp and testa of unburied
seeds and seeds recovered after 5 years of burial
(obtained from the burial study in chapter 2) were
fractured and compared.
Imbibition Curves
Spotted knapweed seed was collected at Bozeman, MT in
August, 1988 and stored at 20° C.
The seeds contained 5%
moisture by weight at the initiation of the study.
Ninety- six lots of 50 seeds were weighed and placed
between two sheets of moist blotter paper in 100 mm
diameter petri dishes.
Seeds were incubated in a growth
chamber at 25° C with 14 hours of light.
At I hour
intervals seeds in two petri dishes were blotted dry on
72
absorbent tissue papper for 3 minutes and weighed.
This
procedure was repeated until germination (protrusion of
the radicle by 0.5 mm) occurred.
Percent water imbibition
was determined by dividing the initial seed weight by the
imbibed seed weight.
and multiplied by 100.
This value was subtracted from one
An imbibition curve for spotted
knapweed seed was constructed by plotting the percent
increase in seed weight against the duration of
imbibition.
Point of Water Uptake
The location of water imbibition of the spotted
knapweed achene was determined by coating portions of
achenes with paraffin and recording germination over time.
Four paraffin treatments were compared: I) pappus end
coated, 2) hilum end coated, 3) entire achene coated and
4) no paraffin treatment.
Four replications of 100 seeds
for each treatment were placed in 100 mm diameter petri
dishes containing moist blotter paper.
Percent
germination was recorded every 6 hours until germination
of the uncoated achenes reached 100% (48 hours).
Viability of seeds that had not germinated after 48 hours
was determined by a tetrazolium test.
This experiment was
conducted twice and data were analyzed by analysis of
variance.
In a second study 100 spotted knapweed seeds were
imbibed in a solution of 0.1% methylene blue which stains
73
tannin and pectic substances blue (Sheldon, 1974).
Every
two hours, 10 seeds were randomly selected and the outer
pericarp and testa were removed by hand to observe the
degree and location of staining by viewing through a
dissecting microscope at 20X magnification.
Schematic
drawings of the embryo were used to demonstrate the
pattern of staining during imbibition.
Results and Discussion
Scanning Electron Micrographs
The achene of spotted knapweed is 2 to 3 mm long, 1.5
mm wide and olive green in color with 4 prominent light
colored stripes evident to the base and with finer lines
between. The seed is dull or slightly lustrous, obovately
shaped, somewhat bossed on the abaxial side in the upper
third and always flattened at the sides.
The pappus is
white with the longest bristles 1.5 to 2 mm long (U.S.
Dept. Agr. Handbook 219, 1958).
Scanning electron micrographs provided clear, highmagnification images of spotted knapweed achenes.
From
these SEM's a schematic diagram of a spotted knapweed
achene (cypsela) was drawn and morphological structures
identified (Figure 6).
The hilum is a scar from the original point of
attachment of a seed to the maternal plant.
The hilum is a
74
In n e r
S t y le
Pappus
.C o ro lla
O u te r
Pappus
M ic r o p y le
C o ty le d o n s
E m b ry o
P e r ic a r p
T e s ta
R a d ic le
H ilu m
F u n ic u lu s
Figure 6.
Schematic Representation of a Spotted Knapweed
75
distinctive means of identifying the point of insertion of
achenes to maternal tissue and helps to distinguish among
subtribes of Cynareae.
The fruit insertion of the Centaurea
spp. is described as lateral-adaxial, concave (Dittrich,
1977).
A portion of the maternal receptacle tissue remains
attached to the achene at the hilum (Figure 7) and consists
of maternal tissue cell walls arranged in a "honeycomb" like
matrix (Figure 8).
The developing ovule receives
nourishment from the maternal plant through the funicular
attachment between the ovule (seed) and the receptacle.
Figure 7.
Proximal End .of Spotted Knapweed
Achene Showing Hilum (a) and Epidermal Hairs (b)
(XlOO) .
The chalaza is that portion of an ovule where the
integuments (testa) originate.
The orthotropous ovules of
Centaurea species are the simplest type of ovule arrangement
76
in which the ovule is erect, with the micropyle at one end
and the funiculus at the other.
The chalaza of orthotropous
F i g u r e 8 . Cross Section of Hilum Showing
"Honeycomb" Cellular Structure (X500).
ovules is located directly below the funicular attachment.
In this area the testa apppears to be thinner than in other
parts of the seed (Figure 9).
Upon germination this is the
area of the seed coat that splits, allowing emergence of the
radicle, perhaps indicating a weak spot in the testa.
The
surface of the dome-shaped receptacle tissue remaining
attached to the achene appears waxy with numerous pores
which may serve as ports of entry for water (Figure 10).
These observations suggest that water uptake by spotted
knapweed achenes would likely occur at the hilum end of the
achene.
77
Figure 9.
Cross Section of Proximal End of
Achene With Funicular Attachment (arrow) (XlOO) .
Figure 10. Hilum Surface Showing Waxy Texture
with Numerous Pores (arrows) (X800).
78
The pappus and micropyle are located at the apical end
of the achene (Figure 13).
In the Centaureinae the pappus
is characterized by multiserial bristles defined as a double
pappus containing two distinct whorls of bristles which
display different morphological features.
The bristles in
the outer whorl are longer on the inside than the outside
(Figure 11) and have deeply serated margins with barbs
arranged in a herringbone pattern (Figure 12).
The inner
whorl of bristles are shorter and flatter than the bristles
of the outer whorl and have thickened, spoon-like bases
(Figure 13).
Located within the inner pappus is the disc (nectary) to
which the style is attached (Figure 14).
The micropyle, the
79
integumentary opening of the ovule through which the pollen
tube enters prior to fertilization, is located within the
nectary.
This small rudimentary opening may also serve as a
port of entry for water during imbibition.
Figure 12.
(X300) .
Bristles on Outer Whorl of Pappus
The protective tissues surounding the embryo are
comprised of two layers of integuments making up the testa
(seed coat) and the outer pericarp (fruit wall)
(Figure 15).
The inner integument is relatively thin and made up of
parenchyma tissue.
The thick outer integument consists of
sclerenchyma tissue, having cells arranged in a palisade
layer.
These palisade cells have radial walls that are
thickened and lignified.
The pericarp surrounds the testa
and is thinner than the outer integument.
The pericarp is
80
Figure 14.
(X300).
Disc (Nectary) (a) and micropyle (b)
81
olive green and mostly glabrous with sparse monocellular
epidermal hairs of different length on both ends of the
achene (Figure 7).
The presence of chlorophyll in the pericarp may
influence the light sensitivity of the seed contained within
the achene.
Cresswell and Grime (1981) showed that if the
structures enveloping the seeds (ovary walls, calyx, bracts)
remain green throughout the maturation period of the seeds,
a light requirement for germination will be induced in the
seeds before they are shed.
Spotted knapweed achenes
acquire.a dark green color prior to dispersal and are
contained within the green bracts of the involucre during
seed development; therefore, they have a light requirement.
Spotted knapweed seeds germinate readily under favorable
conditions in a laboratory; however, 50% of seeds buried for
5 years remain viable (Chapter 2).
In the natural decline
of a spotted knapweed seed bank, 3-5% of the original seed
bank of viable seeds were recovered after 80 months without
seed production.
Seeds recovered after 5 years of burial
(Chapter 2) were compared to fresh seeds by examining the
outer protective structures of the seed.
The pericarp and
testa appeared to be weathered but fully intact in spotted
knapweed seeds recovered after 5 years of burial (Figure
16).
The ability of buried spotted knapweed seed to remain
viable for long periods of time can be largely attributed to
the durable seedcoat.
82
Figure 15. Fractured Surface of a Fresh Achene
Exposing the Testa (a) and Pericarp (b) (XlOO).
Figure 16. Fractured Surface of Achene After 5
Years of Burial Exposing the Testa (a) and
Pericarp (b) (X200).
83
Water Imbibition
Spotted knapweed seeds were fully imbibed after just 13
hours and germination started within 18 hours (Figure 17).
The ability of seeds to germinate quickly when conditions
are favorable for seedling establishment is an important
survival mechanism for plants of arid or mesic environments
(Fenner, 1985).
R -.98
Z
O
Z
Z
CC
LU
O
hZ
LU
O
CC
LU
0.
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
HOURS
Figure 17.
Spotted Knapweed Seed
Measured Over an 18 Hour Period
Germination.
Imbibition Curve
Culminating with
Spotted knapweed is well adapted to droughty, gravelly
sites.
Rapid germination of seeds lying on the soil surface
in response to precipitation enables seedlings to become
84
established quickly thus allowing spotted knapweed to
capture ephemeral niches.
Point of Water Uptake
Achenes uncoated with paraffin germinated 100% after 48
hours of imbibition (Figure 18).
Achenes that had the
pappus end sealed with paraffin but the hilum exposed
germinated at a slightly slower rate than the control group
but reached 100% germination by 48 hours.
—
The achenes which
UNCOATED
PAPPU S COATED
H IL U M C O A T E D
C O M P LE TE LY COATED
0.05
HOURS
Figure 18.
Germination of Spotted Knapweed Seeds with
Paraffin Covering the Hilum, the Pappus, the Entire Seed,
and Non-coated Seeds.
85
had the hilum end sealed with paraffin germinated only 32%
after 48 hours.
Only 4% of the achenes which were
completely coated with paraffin germinated which indicates
that the paraffin was an effective deterrant to imbibition.
Spotted knapweed seeds imbibed in a 0.1% solution of
methylene blue for varying lengths of time showed a distinct
progression of staining of the embryo (Figure 19).
There
was intense staining of the receptacle tissue surrounding
4V
the hilum and the nectary tissue surrounding the micropyle
after 2 hours of imbibition.
A^fter 4 hours the radicle
tissue of the embryo at the hilum end of the seed was
lightly stained, and by 6 hours almost one-half of the
embryo at the hilum end was stained.
By 8 hours some
staining of the cotyledons was apparent at the micropyle end
of the seed.
Two thirds of the embryo, proceeding from the
hilum end of the achene, was stained after 10 hours and by
12 hours the entire embryo showed some degree of staining.
Sheldon (1974) reported similar results with the achenes of
Taraxacum officinale when imbibed in a methylene blue
solution for 4, 16 and 24 hour periods.
These two experiments demonstrate the importance of the
hilum, and to a lesser degree the micropyle, in water
imbibition of spotted knapweed seed.
By restricting water
zuptake at only two specific locations in the testa, the
overall integrity of the hard seed coat can be maintained.
Additionally, the two water-permeable pores may serve as
86
semipermeable membranes which prevent or inhibit
introduction of detrimental microorganisms to the seed which
would serve to extend seed longevity in soil.
6 hr
8 hr
12 hr
19.
Schematic Representation of Cross-sections of
Spotted Knapweed Seeds Showing Staining Patterns Following
Imbibition in a 0.1% Methylene Blue Solution for 0, 2, 4,
6, 8, and 12 hours.
Figure
87
The orientation of achenes on the ground surface can
markedly affect the probability of germination.
Sheldon
(1974) reported the highest germination rate for T.
officinale was obtained when the seeds were placed with the
long axis at about a 45 degree angle to the horizontal.
This is the orientation that dandelion seeds would normally
assume on a flat surface if the pappus was still attached.
This orientation optimizes contact between the hilum,
through which water is initially absorbed, and the soil
surface.
The rigid bristles of the spotted knapweed pappus may
serve a similar function in positioning the seed for optimum
water uptake.
As the spotted knapweed achene matures and is .
exposed to large changes in humidity, the pappus extends to
an angle approximately 45 to 600 in relation to the long
axis of the achene (unpublished data).
Once the pappus is
fully extended it maintains this orientation permanently
regardless of humidity, similar to the pappus of Leontodon
autumnalIs (Sheldon, 1974).
The special morphological features of spotted knapweed
achenes promote rapid water imbibition optimizing timely
seed germination and seedling establishment.
The thick
lignified palisade layer of the outer integument protects
the inner seed, prolonging longevity in the soil.
The light
sensitivity observed in portions of spotted knapweed seed
88
populations (Chapter 2) may be associated with the green
color of the fruit wall surrounding the seed.
89
CHAPTER 5
SUMMARY
Spotted knapweed is an aggressive perennial weed
infesting millions of acres in Montana.
This extremely
competitive weed displaces native plant species resulting in
near monoculture stands which severely reduce the carrying
capacity of rangeland.
Spotted knapweed seeds buried in soil can remain dormant
for many years.
Over 50% of the seeds recovered after 5
years of burial were viable.
In a natural setting, over 90%
of the spotted knapweed seeds germinate or are lost from the
indigenous seedbank within the first 3 years following the
termination of seed,production.
After 7 years approximately
20 viable but dormant seeds remained per 0.5 m2 (390,000
seeds per ha).
A significantly greater number of freshly
harvested spotted knapweed seeds germinate in the light than
in the dark.
If individual seeds displaying this light
requirement become buried before conditions favorable for
germination occur, they will remain in a dormant state until
the light requirement is either satisfied or terminated.
Picloram is a very effective herbicide for long-term
control of spotted knapweed.
All rates tested provided 7
years of control at Ovando with a 700% increase in grass
90
production.
Spotted knapweed was controlled at Harlowton 4
years following treatment, resulting in a 200% increase in
grass production, but control rapidly diminished as. soil
residues of picloram dissipated and grass competition was
reduced by grazing cattle.
Site conditions and land
management practices greatly influence the longevity of
control achieved with picloram.
Spotted knapweed seeds rapidly imbibe water, enabling
germination to occur when environmental conditions favor
seedling establishment.
Imbibition occurs primarily through
the hiIum and to a lesser degree through the micropyle.
The
thick outer layer of the testa provides a durable protective
shield against degenerative soil microbes, thus allowing the
seed to remain in the soil for many years.
91
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