Carduus pycnocephalus 1
Carduus pycnocephalus
Italian thistle, slender thistle, shore thistle, sheep thistle (Australia) winged thistle (New Zealand)
Asteraceae
The following description of Carduus pycnocephalus is adapted from Munz and Keck (1973), Parsons
(1973), and Goeden (1974).
The name Carduus comes from the old Latin name for thistles. Carduus pycnocephalus, a member of the
Thistle tribe of the Composite family, is an erect winter annual herb 3-18 dm tall. The stems are slender and
narrowly spiny-winged, especially below. The lanceolate leaves are pinnatifid, to about 12 cm long, with
spiny-tipped lobes and teeth, white-woolly below, greenish but more or less arachnoid above. Branches end
with a cluster of one to five rose-purple subcylindric flowerheads; the disk 1.5 cm wide. The phyllaries are
not membranous- margined but are more or less persistently floccose-tomentose. The corolla lobes are
about three times as long as the throat.
The akenes are light tan or buff, about 20- nerved, and 5-6 mm long. The pappus is dull and 15-20 mm long.
It reproduces only by its windborne or otherwise easily disseminated pappus achenes. Heterocarpy is
well developed in Carduus pycnocephalus (Bendall 1974). Hybridization between Carduus pycnocephalus
and Carduus tenuiflorus has been reported in Europe (Batra et al. 1981).
Carduus pycnocephalus can be distinguished from other thistle species by its relatively small and few
terminal flowerheads and narrow phyllaries with copious tiny, firm, forward-pointing hairs, especially on the
midrib (Hitchcock and Cronquist 1973).
Carduus pycnocephalus originated in western and southern Europe but today is widespread throughout
temperate parts of the world. It is a serious pest in Australia, New Zealand, South Africa, Pakistan, Iran, and
Europe. In the U.S. it is found in only a few parts of Texas and Arkansas but is rapidly spreading and "out
of control" in most of California (Dunn 1976).
Italian thistle apparently arrived in California during the 1930s (Goeden 1974). It displaces desirable forage
or cover plants, but more commonly colonizes disturbed habitats where interspecific competition is less
intense. It is most abundant in coastal areas and occurs as a weed of pastures, ranges, roadsides, rural areas,
fallow cropland, railroad rights-of-way, field margins, and ditchbanks (Goeden and Ricker 1978).
Carduus pycnocephalus reproduces only by seed. It prefers soils of high fertility, and its seedlings establish
best on bare or disturbed sites (Wheatley 1971, Parsons 1973). If there is reasonable ground cover during
the late summer and autumn the thistle will not invade a site, but it will come in following overgrazing or
creation of fire breaks (Parsons 1973). Drought favors a rapid increase in the thistle population. On soils of
naturally high fertility, thistle invasion can be expected at an earlier stage than on poorer soils. Thistles will
invade basalt soils earlier than granite soils, and granite soils before sedimentary soils (Wheatley and Collett
1981). Carduus pycnocephalus commonly colonizes disturbed habitats where interspecific competition is
less intense (Goeden 1974). However, this weed does displace more desirable forage or cover plants.
The blanketing effect of over wintering rosettes can severely reduce the establishment of other plants, as the
leaves of the rosette can become erect in dense stands (Parsons 1973).
Italian thistle does not reproduce vegetatively, but its seeds are well equipped for dispersal by wind because
of the large pappus and relatively small size. The distance that seeds can be spread by wind is not known,
but it is at least several hundred meters. Seeds are also spread when infested pastures are cut and the hay fed
to animals on clean areas. Seed dispersal by water and on animals and machinery is less important (Parsons
1973). Ants may also play a role in dispersing the seeds (Uphof 1942). Italian thistle seeds are
mucilaginous, unlike most other thistles. The mucilage is abundant and adhesive enough to aid in seed
Preparer: Don Pitcher April 12, 1986 (revised by Mary J. Russo) for The Nature Conservancy, California Field Office,
785 Market Street, 3rd Floor San Francisco, Ca 94103 (415) 777-0487
Carduus pycnocephalus 2
dispersal (Evans et al. 1979). Italian thistle seeds exhibit polymorphism, with brown seeds that have less
mucilage and germinate at lower temperatures than silver seeds. The brown seeds do not usually dehisce
from the seedheads, and this may be important in the establishment of these seeds in the seedbed litter
(Evans et al. 1979).
Seed germination rate in Carduus pycnocephalus is very high, ranging between 83-96%. The seeds
germinate at a wide variety of constant and alternating temperatures. The greatest diurnal fluctuation
that supported optimum germination was 10 C for 16 hours and 35 C for 8 hours in each 24-hour period.
Even with freezing temperatures during the daily cold period, germination was optimum if warm-period
temperatures were from 5 to 20 C (Evans et al. 1979). No after-ripening is required, and seeds can
germinate either rapidly or after a long dormancy period. Seeds of Italian thistle exhibit rapid germination
(within 2 weeks) at optimum temperatures (Evans et al. 1979). Bendall (1974) found that 85% of Carduus
pycnocephalus seeds produce germination inhibitors, but they are readily leached. The length of time the
seeds can survive in the soil is not known but appears to be at least 8 years (Parsons 1973). Italian thistle
can germinate at a variety of soil depths. Generally it does poorly on the surface of a bare seedbed, but on
the surface of clay soils it shows 70% germination. At a depth of 0.5-2.0 cm germination is highest, but
some seeds germinate to a depth of 8 cm (Evans et al. 1979). Seeds buried 1.3 cm deep show the
highest percentage emergence, whereas 20 to 25% of seeds buried 5 to 10 cm deep remain dormant. The
growth of Italian thistle is favored more by the addition of nitrogen than by phosphorus or potassium. High
pH (6.5) also favors growth (Bendall 1975).
Italian thistle has been rapidly spreading on rangelands previously dominated by alien annual grasses (Evans
et al. 1979). This is partly due to its germination requirements and timing. Italian thistle germinates at
temperature and moisture regimes and in seedbed environments which would inhibit the germination of the
alien annual grass species that presently dominate California grasslands. The seeds start to germinate in the
fall with the first effective rain. Seedlings grow through the winter as rosettes and produce flowering stalks
in the late spring before the summer drought
Though some Carduus species are known to accumulate nitrates in toxic quantities, Italian thistle has
apparently not been incriminated as a toxic weed (Goeden 1974). The primary threat of this weed is
its ability to dominate sites throughout California. Italian thistle is a problem on Nature Conservancy
property in this state and presents additional problems on grazed pastures. It reduces the establishment of
annual grasses and reduces the value of hay and other crops due to the blanketing effect of the overwintering
rosettes, high rate and timing of germination, and its broad range of germination conditions.
With the right combination of control measures, it should be possible to eliminate Italian thistle from
selected areas. Its inability to reproduce vegetatively makes control easier, but constant monitoring will be
necessary due to its potentially long seed dormancy (to 8 years).
Monitoring is needed to determine the effectiveness of any Italian thistle control measures. The necessity
for constant monitoring is all the more important given this weed's potentially long seed dormancy period (to
8 years).
A great deal of research has been conducted on Carduus species throughout the world, but much more needs
to be done on Carduus pycnocephalus. Additional research needs to be conducted on insects that can be
used to control Italian thistle and on the potential impact of these insects upon endangered native Cirsium
species (Kok et al. 1982). Several management techniques appear promising, and integrated control
operations involving a combination of practices need investigation. The effects of prescribed fire on Italian
thistle also need investigation.
Preparer: Don Pitcher April 12, 1986 (revised by Mary J. Russo) for The Nature Conservancy, California Field Office,
785 Market Street, 3rd Floor San Francisco, Ca 94103 (415) 777-0487
Carduus pycnocephalus 3
Control of Italian thistle requires active management once it becomes established in an area. Without
management it cannot be eliminated and may completely carpet the site.
Cultivation before seed production will eventually eliminate thistles, but only if repeated for several years.
According to Wheatley and Collett (1981), hand-hoeing is effective for small patches, but make sure to sever
the root a good 10 cm below ground level. Mowing or slashing is not always reliable because the plant can
regrow from the base and produce seeds very quickly. Similarly, plants which are cut close to flowering
time can produce seed on the cut portion. A significant amount of seed can be produced even if thistles
are constantly mowed at 8 cm (Tasmanian Department of Agriculture 1977). Slashing is more effective than
mowing as it destroys the aerial part of the plant more thoroughly (Parsons 1973). For larger areas where
the thistles are dominant, cultivation and cropping is a successful method of control provided a
vigorous perennial pasture is established immediately after the cropping phase. In high fertility situations,
using a roller to compact the soil is recommended during seedbed preparation (but not during seed sowing).
This usually forces a massive germination of thistles that can be destroyed during cultivation (Wheatley and
Collett 1981).
Grazing by sheep, goats, and horses can be effective in controlling thistles, but cattle are of little value
(Parsons 1973). Bendall (1974) describes a grazing control method that has proven successful in Australia:
thistle-infested areas are closed to grazing in the fall when seedlings appear. They are left ungrazed until the
pasture has reached a height of 10-15 cm (about 6 to 10 weeks). The areas are then heavily grazed with
sheep at more than twice the normal stocking rate. Sheep selectively graze the tender thistles and will
kill 90-95% of the weeds. Only 2-3 weeks should be required for control. For this method to be successful,
the autumn grazing break is necessary so that vigorous growth of other plants is allowed to occur, forcing
the thistles to grow tall and tender. Continuous grazing significantly reduces thistle numbers but is not as
effective as the use of an autumn break (Bendall 1973).
Biological control techniques for Carduus species have been extensively studied. Carduus pycnocephalus
was one of the first weeds selected for biological control study by the USDA (Schroeder 1980). The search
for its natural enemies has included Italy, Greece, Iran, and Pakistan, as well as southern California (Baloch
et al. 1971, Baloch and Kahn 1973, Goeden 1974). According to Goeden (1974), Italian thistle serves as an
alternative food-plant or breeding host to a diversity of phytophagous insects, most of which are
euryphagous, ectophagous, sap- or foliage-feeding species. In southern Europe, more than 80 species are
associated with Italian thistle; about one third of them are stenophagous and restricted to host plants
belonging to the tribe Cynareae. In southern Europe, all major parts of Italian thistle plants are damaged by
one or more insect species, whereas in southern California the thistles are relatively free of insect damage.
In California, more than 40 species of indigenous or introduced phytophagous insects have adopted this
alien weed as an alternate food-plant, at least 15 of which also find it a suitable reproductive host.
Unfortunately, half of the identified species of insects found feeding on Italian thistle in southern California
are also pests of cultivated plants, thus not good choices for biocontrol.
Only three insect species appear to hold promise as biological control agents in California (Goeden 1974).
These species are Psylliodes chalcomera, Rhinocyllus conicus, and Ceutorhynchuys trimaculatus. All three
species are unknown as artichoke or safflower pests, apparently only reproduce on Cadruinae, cause injury
to vital plant parts at a critical growth stage of their host-plant (and thus appear capable of influencing the
reproductive potential of Italian thistle), and occur over a relatively wide geographic area.
However, according to Charles Turner (1985) of the USDA Biocontrol Lab in Albany, California, it
is possible that these insects may also prey on several of the endangered native thistles in the genus Cirsium.
Because of this concern, their use has been somewhat limited.
Preparer: Don Pitcher April 12, 1986 (revised by Mary J. Russo) for The Nature Conservancy, California Field Office,
785 Market Street, 3rd Floor San Francisco, Ca 94103 (415) 777-0487
Carduus pycnocephalus 4
Psylliodes chalcomera was a fairly consistent associate of Italian thistle in its vegetative and early
reproductive stages throughout central and southern Italy. The adults are foliage feeders, but
more importantly the larvae mine crowns of rosettes and tips of expanding and expanded shoots, blasting the
latter, and thus reduce the production of flowerheads. This species has been studied in depth by USDA
entomologists as a natural enemy of musk thistle, but certainly should also be considered for importation
into California for Italian thistle control" (Goeden 1974).
Larvae of Rhinocyllus conicus feed within the flowerheads of Italian thistle, mining the receptacle and
destroying the developing achenes (Goeden 1974, 1978). This weevil mainly reproduces on certain thistles
belonging to the Carduus-Silybum-Cirsium complex. It survives best at low temperatures with short
photoperiods (Kok 1979). Rhinocyllus conicus was first introduced into Canada in 1968 for the biological
control of musk thistle (Carduus nutans L.) and plumeless thistle (Carduus acanthoides L.) (Harris and
Zwolfer 1971). It was imported into California in 1969 for the biological control of milk thistle (Silybum
marianum Gaertn.) (Hawkes et al. 1972) and in 1973 to control Italian thistle in southern California (Goeden
and Ricker 1978). After their introduction to See Canyon in southern California, the weevil destroyed 90%
of the achenes and infested 91% of the capitula; however, the population of Italian thistle did not decline
(Goeden and Ricker 1978). The weevil was released on infestations of Italian and slender-flower thistles at
16 sites in 11 counties in northern California during 1975-77 with establishment at most of the sites
(Hawkes et al. 1978).
Ceutorhynchus trimaculatus occurs in Europe and northern Africa (Bolt et al. 1980). "Larvae thought to be
Ceutorhynchus trimaculatus were recovered from mines in crowns of Italian thistle rosettes in central Italy,
though only adults were positively identified as fairly consistent associates of this plant. This weevil has
been studied in depth as a candidate biological control agent by USDA entomologists and may be usefully
employed against Italian thistle and other Carduus species in North America in the near future" (Goeden
1974).
The fungal rust Puccinia cardui-pycnocephali Sydow, is known to occur only on the genus Carduus (Batra
et al. 1981). Repeated greenhouse inoculations of the rust on growing rosettes significantly reduced
the growth of Italian thistle but not the number of florets (Oliveri 1984). In field situations, the added
hardship of intra- and interspecific competition may cause a greater effect on thistle populations. Optimum
conditions for rust infection (18 to 20 C, 90 to 100% relative humidity) are likely to occur in autumn, in
regions with a Mediterranean climate. This period also corresponds to the germination and vegetative
growth periods of Italian thistle (Oliveri 1984).
Two other rusts, Puccinia centaureae D.C. and Puccinia galatica Sydow are also reported to attack Carduus
pycnocephalus, but their impact has not been researched (Batra et al. 1981).
The use of herbicides to control Italian thistle may not be appropriate on natural areas such as Nature
Conservancy preserves. Near streams or lakes particular cautions should be taken when using herbicides.
Prior to using any herbicide, check with the County Agricultural Commissioner to determine which
chemicals are legal to use in a given situation. The labels also give more precise information on mixing and
safety precautions. A Certified Pest Control Applicator should be hired for large jobs or those
requiring nonselective herbicides.
A variety of herbicides have been used on Italian thistle, but they give only temporary control (Wheatley and
Collett 1981). Picloram (Tordon): Dr. Jim McHenry (1985) of the University of California, Davis,
recommends picloram to control Italian thistle on Nature Conservancy lands. Since it is a nonselective
herbicide, it must be carefully applied. It is most effective when applied in February or March at 1/8 to 1/16
lb acid equivalent per acre. Picloram kills half the test animals (LD50) at 8000 mg/kg body weight and is
Preparer: Don Pitcher April 12, 1986 (revised by Mary J. Russo) for The Nature Conservancy, California Field Office,
785 Market Street, 3rd Floor San Francisco, Ca 94103 (415) 777-0487
Carduus pycnocephalus 5
considered to be of "relatively no hazard." One drawback is its long persistence in the soil, up to 18
months.
2,4-D is a phenoxy-type herbicide used for broadleaf weed control that works as a selective hormone or
growth regulator. 2,4-D does not affect grasses. It is foliar absorbed and translocated, making it effective in
destroying the roots. 2,4-D is available in ester, amine, emulsifiable acid, and low volatile ester
formulations. The chemical is noncorrosive and is generally considered nonharmful to wildlife. 2,4-D is
lethal to 50% of tested animals (LD50) at 300-1000 mg/kg body weight and is classified as being of "little"
to "some" hazard. It persists in the soil for 1-4 weeks. 2,4-D has been applied to Italian thistle with limited
success (Taylor 1977). 2,4-D ester should be applied when the thistles have a central stock height of no
greater than 0.25 m (Wheatley and Collett 1981). Application should be at the rate of 1-1.5 lbs/100 gallons
of water with 1 quart of surfactant/100 gallons. Surfactants affect the surface property of the spray by
lowering surface tension to increase the herbicide's effectiveness. 2,4-D can be used in combination with
biological control measures to control Carduus. Several recent studies (Kok 1980, Trumble and Kok 1980a,
1980b) have shown that the weevil Rhinocyllus conicus is not adversely affected by field applications of 2,4D.
There are several petroleum oils used for weed control. The herbicidal use of oils depends on their chemical
and physical properties. Most contact oils evaporate slowly and owe their plant toxicity to their high content
of aromatic compounds. Spraying oil on thistle will be effective only if entire plants are coated.
Herbicides can be applied uniformly over an area (for large infestations) or by spot spraying only the
individual plants. Dr. McHenry recommends using a flat-fan nozzle (Spraying Systems Co. #8003 or 8004
nozzle tip) rather than the cone nozzles available on most garden sprayers. Cone sprayers produce greater
atomization of the chemicals and increase the chance of drift into unwanted areas. Spraying should be done
on calm days with dry plants (dew or rain will dilute the herbicide, reducing its effectiveness). When
spraying large areas, a horizontal boom (6-8 feet long) made from aluminum tubing will improve herbicide
coverage.
Carduus pycnocephalus is present on both the Ring Mountain and Jepson Prairie preserves in California,
with Ring Mountain having the most significant infestation. Although other thistle species present greater
problems on these preserves, Italian thistle control has been included as an adjunct to other control efforts.
Former preserve manager Greg Wolley (1986) has used both hand pulling and cutting to destroy the plants at
Ring Mountain Preserve, CA. This has proven most effective during the spring and early summer. Contact:
Larry Serpa, Area Manager, 3152 Paradise Drive, Room 101, Tiburon, CA 94920 (415) 435-6465
Carduus pycnocephalus, a vigorous annual thistle, arrived in California during the 1930s and has since
become a serious weed problem. It occurs in a variety of disturbed habitats and germinates rapidly and in
large numbers. Hand pulling, cultivation, and grazing are all effective control measures. Biological control
agents, particularly the weevil Rhinocyllus conicus and the rust Puccinia cardui-pycnocephali, show
considerable promise in controlling Italian thistle. Chemicals such as Picloram and 2,4-D may be of some
use in controlling the weed, but an integrated management program involving a combination of techniques
will prove most effective.
Preparer: Don Pitcher April 12, 1986 (revised by Mary J. Russo) for The Nature Conservancy, California Field Office,
785 Market Street, 3rd Floor San Francisco, Ca 94103 (415) 777-0487
Carduus pycnocephalus 6
CITED REFERENCES
Baloch, G.M., A.G. Khan, and M. Mushtaque. 1971. Biological control of Carduus spp. (Compositae: I.
Insects associated with these in West Pakistan). Commonwealth Institute of Biological Control,
Technical Bulletin 14:51-58.
Baloch, G.M. and A.G. Khan. 1973. Biological control of Carduus species. II. Phenology, biology and
host-specificity of Terellia serratulae L. (Diptera: Trypetidae). Commonwealth Institute
of Biological Control, Technical Bulletin 16:11-22.
Batra, S.W.T., J.R. Coulson, P.H. Dunn, and P.E. Boldt. 1981. Insects and fungi associated with Carduus
thistles (Compositae). USDA Technical Bulletin No. 1616. 100 pp.
Bendall, G.M. 1973. The control of slender thistle, Carduus pycnocephalus L. and Carduus tenuiflorus
Curt. (Compositae), in pasture by grazing management. Australian J. Agricultural Research 24:831837.
Bendall, G.M. 1974. Slender thistles in pasture: control by grazing management. Tasmanian J. of
Agricultural Research 45:62-63.
Bendall, G.M. 1975. Some aspects of the biology, ecology and control of slender thistle, Carduus
pycnocephalus L. and C. tenuiflorus Curt. (Compositae), in Tasmania. J. Austr. Institute of
Agricultural Science 41:52-53.
Bolt, P.E., G. Campobasso, and E. Colonnelli. 1980. Palearctic distribution and host plants of
Ceutorhynchus trimaculatus and Trichosirocalus horridus (Coleoptera: Curculionidae). Annals of
the Entomological Society of America 73:694-698.
Dunn, P.H. 1976. Distribution of Carduus nutans, C. acanthoides, C. pycnocephalus and C. crispus, in the
United States. Weed Science 24:518-524.
Evans, R.A., J.A. Young, and R. Hawkes. 1979. Germination characteristics of Italian thistle (Carduus
pycnocephalus) and slenderflower thistle (Carduus tenuiflorus). Weed Science 27:327-332.
Goeden, R.D. 1974. Comparative survey of the phytophagous insect faunas of Italian thistle, Carduus
pycnocephalus in southern California and southern Europe relative to biological weed control.
Environmental Entomology 3:464-474.
Goeden, R.D. 1978. Initial analyses of Rhinocyllus conicus (Froelich) (Col. Curculionidae) as an
introduced natural enemy of milk thistle (Silybum marianum (L.) Gaertner) and Italian thistle
(Carduus pycnocephalus L.) in southern California. Pp. 39-50 in K.E. Frick, Biological control of
thistles in the genus Carduus in the United States.
Goeden, R.D. and D.W. Ricker. 1978. Establishment of Rhinocyllus conicus (Col. Corculionidae) on
Italian thistle in southern California. Environmental Entomology 7:787-789.
Harris, P. and H. Zwolfer. 1971. Carduus acanthoides L., welted thistle, and C. nutans L., nodding thistle
(Compositae). Pp. 76-79 in Biological control programmes against insects and weeds in
Canada 1959-68. Tech. Commun. Commonw. Inst. Biological Control 4.
Hawkes, R.B., L.A. Andres, and P.H. Dunn. 1972. Seed weevil released to control milk thistle. California
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Hawkes, R.B., L.A. Andres, P.H. Dunn, and D.M. Maddox. 1978. Biological control of problem weeds in
non-cropland areas. Pp. 71-75 in Proc. 30th Anniversary California Weed Conference.
Hitchcock, C.L. and A. Cronquist. 1973. Flora of the Pacific Northwest. Univ. of Washington Press,
Seattle. 730 pp.
Kok, L.T. 1979. Influence of temperature and photoperiod on the mortality of diapausing Rhinocyllus
conicus (Coleoptera: Curculionidae). Annals of the Entomological Society of America 72:206208.
Kok, L.T. 1980. Compatibility of Rhinocyllus conicus, Trichosirocalus horridus and 2,4-D for Carduus
thistle control. Proc. V Int. Symp. Biol. Contr. Weeds, Brisbane, Australia:441-445.
Kok, L.T., T.J. McAvoy, G.R. Johnson, and P.H. Dunn. 1982. Further tests on Ceutorhynchus trimaculatus
R. as a candidate for the biological control of Carduus thistles. Crop Protection 1:67-74.
McHenry, J. 1985. University of California, Davis. Personal communication.
Preparer: Don Pitcher April 12, 1986 (revised by Mary J. Russo) for The Nature Conservancy, California Field Office,
785 Market Street, 3rd Floor San Francisco, Ca 94103 (415) 777-0487
Carduus pycnocephalus 7
Munz, P.A. and D.D. Keck. 1973. A California Flora and Supplement. University of California Press,
Berkeley. 1905 pp.
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pycnocephalus and C. TENUIFLORUS). Weed Science 32:508-510.
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Control Conference: 38-41.
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(Coleoptera: Curculionidae) and their compatibility for integrated control of Carduus thistles.
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Wolley, G. 1986. Former Preserve Manager, The Nature Conservancy, Ring Mountain Preserve, CA.
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Preparer: Don Pitcher April 12, 1986 (revised by Mary J. Russo) for The Nature Conservancy, California Field Office,
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