Interference to Hardwood Regeneration in
Northeastern North America: Assessing and
Countering Ferns in Northern Hardwood
Forests
Heather M. Engelman and Ralph D. Nyland, Faculty of Forest and Natural Resources
Management, SUNY College of Environmental Science and Forestry, Syracuse, NY
13210.
ABSTRACT: The extremely dense shade cast by spreading ferns, particularly hayscented, New York, and
bracken ferns, interferes with the survival and development of tree seedlings in northern hardwood forests.
Excessive bracken frond litter and hayscented fern root mats can also prevent adequate germination and
seedling development. In addition, the herbaceous cover may harbor detrimental small herbivores, while
large ones often preferentially browse seedlings that grow through this layer. Increased understory light
levels after an overstory disturbance, abundant soil moisture, fire, and herbivory promote ferns, whereas
excessive and repeated cold or drought deter fern development and propagation. The most promising
control methods repress ferns until seedlings cast adequate shade to inhibit further development of the fern
layer. When ferns cover more than 30% of the understory, well-timed applications of either glyphosate or
sulfometuron methyl have successfully controlled hayscented, New York, and bracken ferns. Two carefully
timed mowings annually for at least 2 years have also provided long-lasting control on level, accessible
sites. Deer populations must be reduced where browsing prevents development of desirable plants. North.
J. Appl. For. 23(3):166 –175.
Key Words: Pteridium aquilinum, Dennstaedtia punctilobula, Thelypteris noveboracensis, shade, herbicides, mowing.
T
he survival and development of tree seedlings and
sprouts of northern hardwoods, and other desirable woody
species around the world (Table 1), can be significantly
inhibited by dense bracken (Pteridium aquilinum (L.)
Kuhn), hayscented (Dennstaedtia punctilobula (Michx.)
Moore) and New York (Thelypteris noveboracensis (L.)
NOTE:
166
Heather M. Engelman can be reached at (315) 470-4877; Fax
(315) 470-6956; engelman@syr.edu. Ralph D. Nyland can be
reached at rnyland@mailbox.syr.edu. This paper is the third in
a five-part series on Interference to Hardwood Regeneration in
Northeastern North America. The first paper appeared in the
March 2006 issue and the second paper appeared in the June
2006 issue. Drs. Ruth Yanai, Susan Stout, Robin Hoffman,
Christopher Nowak, Tsutomu Nakatsugawa, and two anonymous reviewers provided comments on early drafts. This research was supported by the Cooperative Forest Research Unit
(CRFU), University of Maine, and the NY Center for Forestry
Research and Development of SUNY-ESF. CFRU published a
companion annotated bibliography that presents detailed
abstracts of publications reviewed here. For a copy contact
Dr. Robert Wagner, Director, CFRU, 5755 Nutting Hall, University of Maine, Orono, ME 04469-5755; E-mail:
bob㛭wagner@umnefa.maine.edu. Copyright © 2006 by the Society of American Foresters.
NJAF 23(3) 2006
Nieuwland) ferns. In general, these plants become a problem because of their low palatability to insects and other
animals (Dent 1971, Cooper-Driver et al. 1977, Jarrett 1982,
Schreiner et al. 1984), their suspected allelopathic effects on
competing species (Torkildson 1950, Gliessman and Muller
1972, Stewart 1975, Maquire and Forman 1983, Horsley
1977a, 1977b, 1989, 1993b, Drew 1990, Dolling 1996b),
their shading of shorter tree seedlings (Horsley 1993b), their
high propagule mobility (Page 1985), and their wide climatic and edaphic tolerance (Cox 1915, Page 1979). Further, spore banks and buried rhizomes survive fires and
readily restore the population after surface burning (Dyer
and Lindsay 1992).
Widespread spore banks (Raynor et al. 1976, Schneller
1988, Milberg 1991, Dyer and Lindsay 1992, Penrod 1994,
Penrod and McCormick 1996) spread ferns to new areas,
but vegetative spread of their rhizomes increases the extent
and density of these ferns where they already exist (Furuya
1978, Leck and Simpson 1987, Schneller 1988, Milberg
1991, Dyer and Lindsay 1992, Penrod and McCormick
Table 1. Partial list of commercial tree species inhibited when overtopped by Pteridium aquilinum (P),
Dennstaedtia punctilobula (D), or Thelypteris noveboracensis (T).
Tree
Fern
Source
Abies balsamea
P
Abies grandis
Acer rubrum
P
D
Betula allegheniensis
Fraxinus america
Liriodendron tulipifera
D
D
D
Picea engelmannii
Picea glauca
P
P
Picea rubens
P
Pinus contorta
Pinus radiata
Pinus strobus
Pinus sylvestris
Populus spp.
Prunus serotina
P
P
D
P
P
D
Prunus serotina
Prunus serotina
Pseudotsuga menquisii
P
T
P
Quercus rubra
D
Place 1952; Cody and Crompton
1975
Ferguson and Adams 1994
Maquire and Forman 1983;
George and Bazzaz 1999a
George and Bazzaz 1999a
Bowersox and McCormick 1987
Bowersox and McCormick 1987,
McCormick and Bowersox
1997
Comeau et al. 1993
Place 1952; Cody and Crompton
1975
Place 1952; Cody and Crompton
1975
Ferguson and Adams 1994
Karjalainen and Boomsma 1989
George and Bazzaz 1999a
Jones 1947, Dolling 1996b
Dolling 1996a
Horsley and Marquis 1983,
Horsley 1977b, 1989, 1993a,
Drew 1990
Horsley 1977a
Horsley 1977b
McCulloch 1942, Stewart 1975,
Stewart et al. 1979
Hanson and Dixon 1985, 1987,
Bowersox and McCormick
1987, McCormick and
Bowersox 1997, Lyon and
Sharpe 1996, George and
Bazzaz 1999a, 1999b
1996). On forested sites, the most aggressive fern species
may form into a dense continuous layer that shades seedlings (Horsley 1993a, 1993b). Bracken in particular may
also intercept falling seed, preventing germination. Its senescing fronds, either with or without the additional weight
of snow, may also smother or crush other plants (Place
1952).
These species of fern quickly proliferate and develop
after understory light levels and other resources increase
after an overstory disturbance (Horsley 1984b, Daniels
1986, Hollinger 1987, Chazon and Pearcy 1991, Hughes
and Fahey 1991, Brach et al. 1993, Hurd 1995, Demchik
and Sharpe 2001, Hill and Silander 2001). The resultant
dense fern cover has little impact on nutrient availability to
desired plants (Horsley 1988b, 1989, 1993a, Hurd 1995,
Demchik and Sharpe 1999). Rather, the extremely dense
shade from overlapping fronds prevents tree seedling establishment after germination (Bowersox and McCormick
1987, Horsley 1993a, 1993b, Hippensteel and Bowersox
1995, George and Bazzaz 1999a) and encourages fern persistence (Hall 1955). An intact hayscented fern root mat
beneath these dense beds may also inhibit regeneration of
desirable species (Hippensteel and Bowersox 1995, de la
Cretaz and Kelty 1999). Allelopathic interactions have been
reported (Gliessman and Muller 1972, Horsley 1977a, 1979,
Maquire and Forman 1983, Drew 1990, Dolling 1996a), but
the impact on germination and seedling establishment is
minor compared to that of shading by a dense bed of these
ferns (Horsley 1993b). In addition, browsing by deer may
prevent height growth of established seedlings that grow
through the fern layer (MacCammon 1938, Marquis and
Grisez 1978, Kelty 1979, Tilghman 1989, Stroymayer and
Warren 1997, Horsley et al. 2003), further encouraging fern
persistence.
Regeneration and Growth of Ferns
Ferns are often distinguished by their growth habits,
described as either clumping or spreading. Clumping ferns
such as cinnamon fern (Osmunda cinnamomea L.) and
spinulose woodfern (Drypopteris spinulosa (O.F. Muell.)
Watt, Dryopteris carthusiana (Vill.) H.P. Fuchs, and
Dryopteris austriaca (Jacq.) Woynar ex Schinz & Thellung
var. spinulosa (O.F. Muell.) Fisch.) generally grow in selfcontained clusters. Spreading species such as bracken, hayscented, and New York ferns proliferate as dense beds of
individual stalks that arise from rapidly spreading rhizomes
and pose a much greater concern for forest managers. They
occur throughout the eastern deciduous forest (Montgomery
and Fairbrothers 1992), and have a wide tolerance of site
conditions (Page 1979). In fact, their presence indicates
very little about site quality.
Bracken Fern
Bracken fern is the only species in its genus (Cody and
Crompton 1975). Two subspecies and many varieties have
been reported, including Austral bracken (P. aquilinum var.
esculentum (formerly P. esculentum (Forst) Nakai)) (Tryon
1941). Bracken fern is a large aggressive fern of open
habitats with an almost worldwide distribution (Cody and
Crompton 1975, Montgomery and Fairbrothers 1992). It
often dominates the understory where it grows beneath
forest cover (Tryon 1941, McCulloch 1942, Watt 1976,
Biggin 1982, MacLeod 1982, Montgomery and Fairbrothers
1992, McDonald et al. 1999).
Common bracken fern has been found in uncultivated
places and rocky hillsides (Underwood 1881), in woodlands, and on dry slopes (Poel 1961). Northeastern bracken
(P. aquilinum var. latiusculum (Desv.) Underw.) grows in
dry open woods, open fields, and borders of wetlands from
New Foundland to Minnesota; south to the uplands of North
Carolina, Tennessee, Missouri, and Oklahoma, as well as in
Mexico, Europe, and eastern Asia (Montgomery and Fairbrothers 1992). In persistent openings within the Allegheny
hardwood forest, it grows in association with wild oat grass
(Danthonia compressa Aust.), rough goldenrod (Solidago
rugosa Mill.), and flattop aster (Aster umbellatus Mill.)
(Horsley 1981). Despite its exceedingly wide range and
competitive nature, young plants are unusual in the field.
Spores are commonly consumed by insects (Conway 1953)
and sporelings succumb to low temperature or the fungal
pathogen Botrytis (Conway and Stephens 1957).
Where dominant, bracken fern spreads in a regular pattern that appears to maximize coverage and minimize competition between ramets. Its long-creeping rhizomes spread
NJAF 23(3) 2006
167
forward, and die away behind as it advances; when competition is encountered, the direction of growth changes accordingly (Watt 1976). Although resources are allocated to
existing buds, and then to new rhizomes that facilitate
spreading (Dolling 1999), bracken may both readily spread
in area and increase in density to thousands of plants per
acre. Dormant buds on the rhizomes provide a reservoir of
potential fronds that replace those lost to frost, fire, or other
agents (Biggin 1982, Daniels 1985, Ferguson and Boyd
1988). Although bracken fern spreads in clearcut areas
where it has been present before cutting, it generally does
not increase in frond density or colonize new ground in
closed forests (Watt 1976, Dolling 1999).
In general, bracken requires both abundant moisture
(Biggin 1982, Thomson et al. 1985) and adequate drainage
(Poel 1961). In dry weather, emerging fronds die and
shrivel. In wet weather, they become chlorotic and die. If
persistent, these conditions kill short shoots and cause death
of fronds (Poel 1961, Watt 1976, Thomson et al. 1985).
Excessive cold is also a naturally occurring deterrent to
bracken fern. Unless adequately insulated by snow cover,
litter, or other plants, shallow rhizome apices may be killed
at low temperatures (Watt 1950, 1976). In fact, bracken is
rarely found in consistently frosty areas where weather
extremes may cause the loss of dormant buds and the
depletion of stored carbohydrates (MacCammon 1938, Watt
1950, Williams and Foley 1976, Smith 1990).
Allelopathy has been reported for bracken fern—the
inhibition of germination, growth, or metabolism of one
plant because of the release of organic chemicals from
another (Stewart 1975). Bracken’s phytotoxicity increases
during and after frond senescence (Gliessman and Muller
1972). Allelopathic chemicals from bracken fern have limited the growth and survival of aspen (Dolling 1996a) and
orchard-grown black cherry seedlings (Horsley 1977a,
1979). The impact is lessened as litter becomes incorporated
into the soil (Stewart 1975).
The litter layer beneath bracken fern can act as a mechanical barrier against seeds reaching the soil and for
seedlings emerging from below the litter mat (George and
Bazzazz 1999a). It takes several years for holocellulose and
lignin in bracken fern fronds to decompose, and much of the
dry matter may remain for 11 years or more, particularly
when moisture or nitrogen is limiting (Frankland 1976).
Once seedlings have emerged from the frond mat, light
levels rather than litter depth, or other factors associated
with excessive litter, limit growth and development (George
and Bazzazz 1999b).
Hayscented Fern
Hayscented fern is a large terrestrial fern of forest or
forest border, and has wide ecological amplitude. It is
common throughout the Appalachian Mountains where it
may remain the dominant herb indefinitely (Horsley and
Marquis 1983, Montgomery and Fairbrothers 1992, Stromayer and Warren 1997). It develops best on well-drained
stony or sandy soils, and often forms thick beds from
perennial rhizomes (Conard 1908, Cody et al. 1977, Horsley
168
NJAF 23(3) 2006
1984b). Hayscented fern is a common understory component of oak-transition hardwoods-hemlock (Hill and Silander 2001), sugar maple-beech (Berglund 1980), black cherry-maple (Marquis 1980), hemlock-hardwood (Maquire and
Forman 1983), and transition hardwood-white-pine-hemlock types (George and Bazzaz 1999a). It generally occurs
in open woods, clearings, and on roadside banks (Conard
1908, Flaccus 1959, Siccama et al. 1970, Hughes and Fahey
1991, Montgomery and Fairbrothers 1992, Hill and Silander
2001).
Beneath closed forests, hayscented fern fronds and
clones generally are sparsely distributed and do not noticeably interfere with the growth of a diversity of understory
plants. The formation of dense colonies of hayscented fern
is facilitated where deer browsing prevents development of
other understory plants (MacCammon 1938, Marquis and
Grisez 1978, Kelty 1979, Tilghman 1989, Stroymayer and
Warren 1997, Horsley et al. 2003), especially after fires
(McCulloch 1942, Cody et al. 1977, McGee et al. 1995) and
where other overstory disturbances (e.g., thinning) increase
light to the understory (Marquis et al. 1975, Cody et al.
1977, Horsley and Marquis 1983, Horsley et al. 2003). As
with bracken fern, the direction of growth of the main axis
facilitates forward spread until competition is encountered.
Yet because rhizome buds are located on the rear-facing
side of the rachis, rhizomes may reverse direction of growth
if an obstacle prevents additional forward growth (S.B.
Horsley, personal communication). Stems and roots from all
portions of the rhizomes buried 5 to 15 cm beneath the soil
surface may spread rapidly when conditions are suitable
(Conard 1908, Horsley 1984b). Even small viable segments
separated from the main rhizome mass may regrow to form
dense beds that once more dominate the understory in as
little as 5 years after control measures reduced the density
and coverage of this fern (Horsley 1988a). In addition,
viable sporebanks readily germinate on moist mineral seedbeds exposed by disturbance (Penrod and McCormick
1996).
Seedlings having a wide range of shade tolerances (Acer
rubrum, Betula, Prunus, and Tsuga canadensis) often occur
at only low densities beneath hayscented fern. This suggests
allelopathic interactions (Maquire and Forman 1983). Allelopathy was also suspected when growth of previously
germinated black cherry did not immediately increase after
hayscented fern was removed, just as the seedlings did when
released from overtopping asters (Drew 1990). Heavy shading by the ferns was eventually hypothesized as the cause
after greenhouse results could not be duplicated in a forested environment (Horsley 1977b). The effect of shading
was finally demonstrated in field trials in which black
cherry seedlings failed on plots having a closed fern cover,
but grew well on plots where low fences held back the fern
fronds to expose the seedlings to better light (Horsley
1993b).
New York Fern
New York fern occurs in mesic to damp woods and in
full to partial shade, from Newfoundland to Michigan, south
to Georgia, Mississippi, and Arkansas (Montgomery and
Fairbrothers 1992). It is very hardy and grows relatively
rapidly at optimal temperature, light, and nutrients
(Hoshizaki and Moran 2001). New York fern is mediumsized, with creeping rhizomes. It forms colonies (Montgomery and Fairbrothers 1992) and is a common understory
component of black cherry-maple (Marquis 1980), beechmaple (Smith 1980), and transition hardwood-white pinehemlock types (George and Bazzaz 1999a). Because they
are often found together (Horsley 1981), New York and
hayscented ferns had been thought to have similar growth
habits. However, New York fern spreads more slowly than
hayscented fern, posing less of a problem in forest regeneration efforts (Hill and Silander 2001). Further, its abundance tends to be highest, although not significantly so,
under canopies of red (Acer rubrum L.) and sugar (A.
saccharum Marsh.) maples at sites with abundant soil moisture, and lower under stands of other species or in the open.
Spinulose Woodfern
Spinulose woodfern is a medium-sized fern common to
moist woods and swamps. Its erect rhizome system facilitates clumping of fronds, and results in much slower spreading (Marquis et al. 1992). Like many of the less invasive
fern species, it only becomes restrictive when the density of
its fronds prevents light from reaching new seedlings (S.B.
Horsley, personal communication). Yet hayscented and
New York ferns and spinulose woodfern are all considered
potential threats to forest regeneration. They are counted
together in tallies of interfering plants, with the importance
of spinulose woodfern counts weighted by one-half that of
hayscented or New York fern (Marquis et al. 1992).
Guidelines for Assessing Fern Interference
In northern hardwood stands with ⱕ60% canopy cover,
sufficient light reaches the understory to promote the establishment and development of raspberries and blackberries,
American beech, pin cherry, striped maple, hobblebush,
sedges, ferns, and forbs (Hannah 1991, Nyland et al. 2005a,
2005b). When tall and dense, at least some of these plants
may interfere with the development of more desirable tree
species (Leak 1988, Nyland et al. 2006a, 2006b). In fact,
some guidelines suggest that where undesirable species
represent 50% or more of the advance regeneration, they
should be controlled before shelterwood cutting (Hannah
1991, Sage et al. 2003). Those for Allegheny hardwoods
prescribe site preparation when interfering plants occur on
ⱖ30% of the stand area (Marquis et al. 1975). The control
must remain effective for 3 to 4 years after an overstory
cutting to ensure successful development of mixed-species
regeneration (Horsley 1984b). Browsing pressure must also
remain low (MacCammon 1938, Hough 1953, Marquis
1978, Horsley and Marquis 1983, Horsley et al. 1994,
2003).
Management Strategies to Inhibit Fern
Development
Complete, long-term eradication of ferns is generally not
necessary in forested communities. Rather, control is re-
quired only until trees of desirable species grow taller than
fern height. Also, for some species, little action may be
required. For example, pin cherry (Prunus pennsylvanica L.
f.) and blackberries (Rubus allegheniensis Porter) grow
through dense fern cover (Horsley and Marquis 1983, Horsley 1984a), as will yellow (Betula alleghaniensis Britt.) and
black (B. lenta L.) birch (Horsley and Marquis 1983, Horsley 1984a, de la Cretaz and Kelty 1999), eastern hemlock
(Horsley et al. 2003), and white pine (de la Cretaz and Kelty
1999, Horsley et al. 2003). Substantial numbers of northern
red oak (Quercus rubra L.) can grow through a fern canopy
if not first consumed by insects and rodents (George and
Bazzaz 1999b), but in reduced numbers (George and Bazzaz
1999b) and with slower height development (McCormick
and Bowersox 1997). Other tree species of the northern
hardwood forest will not survive beneath ferns nor develop
through them.
Managers may find fern cover excessive in a stand and
elect to control it as part of their regeneration program
strategy. Yet they must follow federal and state regulations
regarding threatened and endangered species. In New York,
for example, picking, plucking, severing, or otherwise killing “exploitably vulnerable” New York fern may be done
only with the consent of a landowner (NY State Dept. of
Environmental Conservation. 1989. New York State protected native plants. Title 6, New York Codes, Rules and
Regulations, Section 193.3. State of NY, Albany). In other
states, the intentional or negligent acts that result in the
death of threatened or endangered plants may be generally
prohibited. Ultimately, regulations such as these will determine the options available to a landowner.
Manual and Mechanical Controls for Ferns
In limited areas, pulling up shallow rhizomes or mowing
the fronds close to the ground will control the vegetative
spread of ferns (Brooks 1979, Biggin 1982, Hoshizaki and
Moran 2001). For larger areas, other measures are required.
Hayscented Fern
Early studies showed that repeated mowing prevented
mature fronds from developing sufficiently to maintain
starch reserves necessary for production of new hayscented
fronds (Cox 1915). More recently, de la Cretaz and Kelty
(1999) compared scalping (root-raking), scarification, and
mowing as control measures for hayscented fern in red and
white pine (Pinus resinosa Soland and P. strobes L.) plantations. Clipping the fronds and leaving the litter in place
provided nutrients to seedlings and also suppressed germination of seed-bank herbaceous species. Only a moderate
number of seedlings become established, but there was less
competition around them. Other evidence suggested that
mowing the ferns for two growing seasons might substantially reduce fern abundance during the third growing season. Nevertheless, repeatedly clipping the fronds while also
leaving existing tree seedlings unharmed could not easily be
applied commercially.
NJAF 23(3) 2006
169
Bracken Fern
Bracken fern has been considered resistant to cutting,
slashing, and bruising unless the treatment is continuous
(Cody and Crompton 1975). Even so, a cutting during
mid-June to early August, followed by a second mowing 5
to 6 weeks later, may provide adequate control (Preest 1975,
Marrs et al. 1998). In fact, the levels of bracken in plots cut
twice yearly for 6 years stayed low long after (i.e., 12 years)
the treatment was stopped (Marrs et al. 1998).
Historically, harvested bracken fronds were sold in Great
Britain, providing a financial incentive for mechanical control. Fronds were mixed with grasses to feed livestock; used
to cushion packed fruit and other items; and burned as a
source of potash for glass (Tryon 1941) and soap making
(Fletcher and Kirkwood 1979). They were valuable for
stock-bedding, durable thatching, composting (Tryon 1941,
Frankland 1976, Rymer 1976, Fletcher and Kirkwood 1979,
MacLeod 1982), and as a fuel in brick making, brewing, and
heating (Rymer 1976). As the demand for these uses fell,
the impetus for harvesting waned, and bracken problems
increased in the region. Nevertheless, bracken fronds could
still be used as an opportunity energy crop if fronds were
converted to pellets or anaerobically digested to methane,
depending on the time of year of harvest (Callaghan et al.
1982, 1985, Lawson et al. 1984). Opening of such markets
could provide incentives to renew bracken fern harvesting,
particularly in open areas.
Bracken fern is inherently unpalatable to most animals
(Dent 1971, Cooper-Driver et al. 1977, Jarrett 1982, Schreiner et al. 1984), making grazing ineffective as a means of
control. Young bracken fronds are popular as food in Japan
and Korea, and many Japanese- and Korean-Americans
harvest them for social or recreational reasons (Hirono
1986, Chavez and Gill 1999, Anderson et al. 2000). Connoisseurs prefer to harvest the fronds in their fiddlehead or
crosier stage, and this occurs before the ferns shade shorter
plants and before carcinogenic spores are released and inhaled. Even so, cutting of fronds as a food source seems
unlikely to affect fern density over large areas. In addition,
consumption of bracken fiddleheads is linked to high rates
of digestive tract cancers (Hirono et al. 1972, 1987, Evans
1982a, 1982b, Soeder 1985, Hirono 1986), and that should
discourage widespread consumption. Hayscented and New
York ferns are not generally used for food.
Chemical Control of Ferns
Herbicide treatments limit regrowth of ferns by killing
their rhizomes, but no selective herbicides are labeled for
fern control in northern hardwood forests. Available broadspectrum herbicides can have the unwanted side effect of
removing desirable tree seedlings as well as ferns (Horsley
1981, 1982), so formulations, rates, and timing must be
carefully controlled. In addition, managers must ensure that
pesticides in their prescriptions are registered for silvicultural site preparation in their locales.
Table 2 summarizes current herbicide recommendations.
Users should consult the source publications for details
about formulations and application methods. The Herbicide
Handbook (Vencill 2002) and the Environmental Protection
Agency website (www.epa.gov/oppread201/international/
piclist.htm) provide extensive information about currently
available compounds, including those approved for use in
forestry operations and conditions of their registration. State
and local regulations and herbicide labels should serve as a
final reference with respect to any herbicide application
described here.
Hayscented and New York Ferns
Correctly timed and applied Oust (E.I. du Pont de
Nemours and Company, Wilmington, DE) herbicide (sulfometuron methyl) will control interfering hayscented and
New York ferns, grasses, and sedges for 3 (Horsley 1991) to
8 years (S.B. Horsley, personal communication), including
those that develop in the vehicle tracks after heavy machinery severs the rhizomes (Horsley 1988a, 1991). Late season
(early August to early October) application of 2 oz
product/ac (46.3 ml/ha) of Oust without a surfactant has
provided nearly complete control of hayscented and New
York ferns, while minimizing damage to valuable regeneration of Allegheny hardwoods and oaks in northwestern and
central Pennsylvania (Horsley et al. 1992). Applications
before frond maturance or after frond senescence resulted in
significantly less control than applications from July
through September (Horsley1988a).
Glyphosate will also control ferns. The time of its application affects the rate needed for effective control of hayscented and New York fern. July or August application at 1
lb ai/ac (1.1 kg ai/ha) can economically reduce their populations, but an earlier application may require up to twice
the dosage for a similar level of control (Horsley 1981).
When bracken fern is also present, the treatment should be
Table 2. Suggested herbicides for control of Dennstaedtia punctilobula (D), Thelypteris noveboracensis (T), and
Pteridium aquilinum (P) in northern hardwoods.
Chemical
Target
Sulfometuron methyl
D, T
Glyphosate
D, T
P
170
NJAF 23(3) 2006
Timing, rates, and comments
⫺1
Late season with 2 oz product ac (46.3 ml
ha⫺1) using a low-pressure sprayer
July 1 to September 1 with 1 lb ai ac⫺1 (1.1 ai
kg ha⫺1) using a small compressed-air
garden sprayer
August to September 1 with 1 lb ai ac⫺1 (1.1
ai kg ha⫺1) using a small compressed-air
garden sprayer
Source
Horsley et al. 1992
Horsley 1981
Horsley 1981, Karjalainen and Boomsma 1989
restricted to August (Horsley 1981). New York and hayscented ferns have not been adequately controlled with
bromacil or hexaxinone (Horsley 1981).
Bracken Fern
Glyphosate at 1 lb ai/ac (1.1 kg ai/ha) is recommended
for controlling bracken fern in northern hardwoods, with
application during August. Higher rates may be necessary if
applied earlier or later in the season (Horsley 1981). The
effectiveness may be reduced when glyphosate becomes
bonded to metal ions found in hard water (Sandberg et al.
1978, Stahlman and Phillips 1979, Shea and Tupy 1984),
but can be restored with ethylenediaminetetraacetic acid
(Shea and Tupy 1984) or by decreasing diluent volume
(Sandberg et al. 1978, Stahlman and Phillips 1979).
Asulam has also successfully controlled bracken fern in
Europe and Australia (Martin 1976, Blatchford 1979, Heywood 1982). A foliar spray translocates to the rhizomes and
prevents buds from developing (Veerasekaran et al. 1978,
Heywood 1982, Kirkwood et al. 1982). The effectiveness
has been increased through the use of wetters and/or emulsifiable oils (Heywood 1982), and by windrowing the ferns
1 year before treatment (Karjalainen and Boomsma 1989).
Some additives increased translocation of asulam, at great
cost: using low concentrations (50 ppm) of 2,4-D or CEPA
(3-chloroethyl phosphoric acid) antagonized the activity of
asulam (Kirkwood et al. 1982), whereas adding diesel fuel
damaged desirable pine seedlings (Preest 1975). The recommended dosage is 1.18 gallons ac⫺1 (11 L ha⫺1) or 4.0
lb ac⫺1 (4.5 kg ha⫺1) in July or the beginning of August
(Williams 1977, Heywood 1982); or before planting at 2.5
lb ai ac⫺1 (2.8 kg ai ha⫺1) using an incremental drift
applicator ULV before frond tips unfurl or after planting 1.8
lb ai ac⫺1 (2 kg ai ha⫺1) using a mistblower (Blatchford
1979). Although it provides an effective level of control, its
carbamate-based formula has limited asulam labeling in
North America, and led to discontinued use in Canada (Pest
Management Regulatory Agency 2002, 2003). It is labeled
for the use in the United States to control western bracken
fern on noncropland areas and in Christmas tree plantations
(Asulox herbicide product label; Bayer CropScience LP,
Research Triangle Park, NC.).
Aminotriazole (3-amino-1,2,4-triazole) was also investigated for bracken control because of its ease of use in
comparison to asulam. It gave effective control when applied with ammonium thiocyanate at 1:1 or 1:0.05 formulations (Cook et al. 1982). However, aminotriazole’s registration was cancelled in 1988 at the manufacturer’s request
(S. Orme and S. Kegley. 2004. PAN pesticide database.
Pesticide Action Network, San Francisco, CA; available at
www.pesticideinfo.org). Other herbicides have proven ineffective for long-term control of bracken. These include
dicamba that provided only 31% control of bracken fern
(Radosevich 1979). Bromacil control was short-lived, and
high rates of hexaxinone and picloram proved less effective
than other compounds (Horsley 1981). Garlon 4 (Dow
AgroSciences LLC Indianapolis, IN), Escort (E.I. duPont de
Nemours and Company, Wilmington, DE), and Velpar L
(E.I. duPont de Nemours and Company, Wilmington, DE)
treatments actually promoted bracken fern to various degrees, rather than releasing desirable vegetation (McDonald
et al. 1999).
Reducing Browsing Also Essential
In the first year or two after overstory removal in Allegheny hardwoods, ferns share dominance with Rubus,
grasses, and sedges (Horsley et al. 2003). Once seedlings
emerge from beneath this layer, deer and insects begin to
feed on the trees (Horsley and Marquis 1983, George and
Bazzaz 1996a, 1996b) and Rubus (Hough 1953, Tilghman
1989). At some sites, intensive browsing by white-tailed
deer (Odocoileius virginianus) (MacCammon 1938, Marquis 1978, Tilghman 1989, Horsley et al. 2003), coupled
with overstory thinning that increased light near the ground
(Marquis et al. 1975, Cody et al. 1977, Horsley and Marquis
1983), led to the formation of dense hayscented fern and an
eventual failure of desirable tree regeneration. After shelterwood seed cutting, only the least palatable species developed. These included striped maple (Acer pennsylvanicum
L.), black cherry (Tilghman 1989), or American beech
(Curtis and Rushmore 1958). Yet at low levels of deer
browsing or no overstory cutting, only moderate fern cover
develops (de la Cretaz and Kelty 1999).
These findings suggest a management strategy that includes control of deer and ferns, and judicious cutting in the
overstory (Marquis 1981, Walters and Nyland 1989, Horsley 1984a, Fredericksen et al. 1998, Horsley et al. 2003),
particularly for thinning in even-aged stands (e.g., Marquis
et al. 1992). Fencing could limit deer access to the regeneration area (Marquis and Grisez 1978). Alternatively, hunting of antlerless deer in the area surrounding a regeneration
site can reduce deer pressure long enough for sufficient
numbers of trees to regenerate and grow to heights beyond
the reach of deer (Sage et al. 2003). In northwestern Pennsylvania, deer densities of ⱕ21 deer mi⫺2 (ⱕ8 deer km⫺2)
will allow adequate regeneration of many tree species in
large blocks of contiguous forest (Horsley et al. 2003).
Kelty and Nyland (1981) observed repeated success with
shelterwood seed cutting in New York’s Adirondacks after
deer density had been reduced from 27 to 14 deer mi⫺2 (10
to 5 deer km⫺2). The maximum deer density depends on
habitat quality.
Management Implications
Timber harvesting and other disturbances open the canopy, brightening the understory, and freeing moisture and
nutrients to residual plants. Heavy machinery used during
logging may disturb shallow rhizomes and promote vegetative proliferation of at least bracken, hayscented, and New
York ferns across the brightened understory. As a countermeasure, some benefit accrues from limiting harvesting to
dry periods (Groninger and McCormick 1992), or when the
soil is frozen and covered with snow. Keeping overstory
density sufficient to appropriately shade the ground also
helps to limit the spread of understory ferns after thinning,
NJAF 23(3) 2006
171
or after reproduction method cuttings that involve partial
overstory removal.
Because existing bracken, hayscented, and New York
ferns readily spread if an upper canopy disturbance increases light levels near the ground, landowners should
evaluate the status of ferns before any overstory treatment.
Seventy percent of the area beneath a stand must be free of
ferns for 3 to 4 years to insure adequate seedling development after a shelterwood seed cutting (Marquis et al. 1992),
or any other reproduction method. Site preparation is essential before any of these treatments if ferns cover more of the
area.
Repeated mowing will control ferns, but is impractical
for large areas, on uneven terrain, or in stands with uneven
tree spacing. Also, where landowners prefer this option, the
ferns must be cut twice per year for 2 years, with the
mowing done after full frond development and again 5 to 6
weeks later. Because of these constraints, herbicide treatments (Table 2) have proven more practical for site preparation in forests. Either 1 lb ai lb ac⫺1 of Oust in July or
August, or an early August to early October application of
2 oz/ac (46.3 ml/ha), is recommended for controlling hayscented and New York ferns. A single application of
glyphosate in August will control bracken, hayscented, and
New York ferns. Because the ferns will grow back after 3 to
4 years, landowners should couple the site preparation in
susceptible stands with shelterwood seed cutting or other
appropriate reproduction method to insure prompt establishment (Horsley 1982, Dolling 1996a).
Research has not identified the exact proportion of basal
area that can be safely removed during a seed cutting or
thinning without promoting the spread of competing ferns
and grasses in even-aged stands (Marquis et al. 1975, Horsley 1977b, Horsley et al. 2003). Management guides commonly recommend leaving a residual stand relative density
of 60% for thinning (e.g., Marquis et al. 1992), or 50 to 60%
stocking of upper canopy residual trees (21 to 25 m2/ha) for
shelterwood seed cutting in northern hardwood stands having a pre-existing fern layer (Kelty 1986). The selection
system may provide adequate shading to inhibit fern development, particularly if managers accept a shift to shade-tolerant tree species. The intermingled multilayered canopy
(Kenefic and Nyland 1999, 2002) and the crowns of residual
trees widen and shade the understory (Smith et al. 1997),
especially if cutting is infrequent.
Where browsing might prevent seedling development,
deer density control must precede the site preparation and
shelterwood cutting (Marquis 1981, Horsley 1984a, Walters
and Nyland 1989, Frederickson et al. 1998, Horsley et al.
2003, Sage et al. 2003), and remain in effect until the trees
have grown tall enough to exceed the reach of deer.
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