Adapted from: Element Stewardship Abstract
From: Exotic Pests of Eastern Forests, Conference Proceedings - April 8-10, 1997, Nashville, TN, Edited by: Kerry O. Britton, USDA Forest Service & TN Exotic Pest Plant Council
Garlic mustard (Alliaria petiolata) is a biennial herb that invades
forested communities and edge habitats, where it spreads rapidly and apparently
displaces native herbaceous species, often within ten years of establishment.
The plant has no natural enemies in North America, and is difficult to eradicate
once established. Thus, the best and most effective control method for Alliaria
is to prevent its initial establishment.
In shaded and partially shaded communities lacking Alliaria the
preferred method is to monitor annually, and remove all Alliaria
plants prior to seed production. Once Alliaria is established, the
management goal is to prevent seed production until the seed bank is depleted,
potentially 2-5 years. Cutting of flowering stems provides the most effective
control with minimal or no side effects, but has a high labor cost. Burning
and herbicide application both provide effective control at a lower labor
cost, but each has potential drawbacks: Fire may increase total presence
of Alliaria unless a second and third consecutive fire are conducted; fire
may alter groundlayer composition; and herbicides may negatively impact
some native groundlayer species.
Alliaria petiolata [(M. Bieb.) Cavara and Grande] is an obligate
biennial herb of the mustard family (Brassicaceae). The genus name Alliaria
refers to the garlic or Allium-like fragrance of the crushed leaves,
an unusual odor for the mustard family. The species name petiolata
refers to the petiolate leaves.
Alliaria seeds germinate in early spring, beginning in late February
or early March, and concluding by mid May in northern states and Canada
(Cavers et al. 1979, Kelley et al. 1991, Roberts and Boddrell 1983). In
northern Illinois, germination coincides with emergence of spring beauty
(Claytonia virginica) and false mermaid weed (Floerkea proserpinacoides).
Seedling density in heavily infested forests was recorded at 5,080/m2
at the cotyledon stage, and 2,235/m2 at the 2-3 leaf stage, in Illinois
(Nuzzo unpublished), and approximated at 20,000/m2 in Ohio (Trimbur 1973).
By June seedlings develop the characteristic rosette of first year plants.
Basal leaves are dark-green and kidney-shaped with scalloped edges, 6-10
cm diameter, and have pubescent petioles 1-5+ cm long (flowering stem leaves
are alternate, sharply-toothed, triangular, 3-8 cm long and wide, and gradually
reduced in size towards the top of the stem).
Immature plants can be confused with other rosette forming species, especially
violets (Viola sp.), white avens (Geum canadense), and Cardamine
sp. Alliaria petiolata can be distinguished from these plants by
the strong garlic odor in spring and summer. In fall and winter Alliaria
can be distinguished by examining the root system. Alliaria has a
slender, white, taproot, with a distinctive "s" curve at the top
of the root, just below the root crown. Axillary buds are produced at the
root crown and along the upper part of the "s."
First year rosettes are sensitive to summer drought (Byers 1988) and
approximately 95% die by fall (Nuzzo 1993b). By mid-fall rosettes average
4-10 cm diameter and are dark green to purplish in color. The rosettes continue
to grow in winter during snow-free periods when temperatures are above freezing
(Cavers et al. 1979). Natural mortality continues through winter: Total
survival rate from seedling to adult stage varies from 1% (Nuzzo 1993b)
to 2-4% (Cavers et al. 1979).
Alliaria is an obligate biennial: all plants that survive the
winter produce flowers, regardless of size, and subsequently die (Cavers
et al. 1979, Byers and Quinn 1988, Bloom et al. 1990). Plants only 5 cm
tall, with 3-4 leaves, have been observed with flowers and seeds. The majority
of plants are taller, averaging 0.7 to 1.0 m when in flower. Flower stalks
begin to elongate in March or April, and flowers open early April through
May. This is some 6-10 weeks after new seedlings germinate; in established
populations generations overlap, and two cohorts can be seen from March
through June. Alliaria flowers can be self-or cross-pollinated (Cavers
et al. 1979, Babonjo et al. 1990).
Plants usually produce 1-2 flowering stems, although robust plants have
been recorded with up to 12 separate flowering stalks. Flowers are produced
in spring in terminal racemes, and occasionally in short axillary racemes.
Some plants produce additional axillary racemes in mid-summer. Flowers are
typical of the mustard family, consisting of four white petals that narrow
abruptly at the base, and 6 stamens, two short and four long. Flowers average
6-7 mm in diameter, with petals 3-6 mm long. Seeds develop in a linear silique,
with siliques forming on the lower part of the inflorescence while flowers
are still opening on the upper part. Alliaria produces an average of 16.4
(+ 3.0) seeds/silique (range 3 to 28), and 21.8 (+ 22.5) siliques/plant
(range 2 to 422; Nuzzo unpublished, Cavers et al. 1979). Seeds ripen and
disperse between mid-June and late September (Cavers et al. 1979, Kelley
et al. 1990).
Seeds are dormant at maturity and require 50 to 100 days of cold stratification
to come out of dormancy (Byers 1988, Lhotska 1975, Baskin and Baskin 1992).
The dormancy period lasts eight months in southern locales (Baskin and Baskin
1992, Byers 1988) and 22 months in northern areas (Cavers et al. 1979).
Unlike some forest crucifers that fail to germinate under leaf cover,
Alliaria seeds germinate in both light and dark after dormancy is
broken (Bloom et al. 1990, Byers 1988). Light alone will not stimulate germination
during cold stratification (Byers 1988). The majority of seeds germinate
as soon as dormancy is broken (Roberts and Boddrell 1983, Baskin and Baskin
1992). A small percentage of seed remains viable in the seed bank for up
to four years (Roberts and Boddrell 1983, Baskin and Baskin 1992).
Alliaria spreads exclusively by seed (Cavers et al. 1979). Seeds
typically fall within a few meters radius of the plant. Wind dispersal is
limited, and seeds purportedly do not float well, although seeds readily
attach to moist surfaces (Cavers et al. 1979). Anthropogenic distribution
is the primary dispersal mechanism (Lhotska 1975, Nuzzo 1992b, 1993a). Seeds
are transported by natural area visitors on boots and in pant cuffs, pockets,
and hair, and by roadside mowing, automobiles and trains (Nuzzo 1992b).
Seeds are widely dispersed in floodwaters. Seeds may be dispersed by rodents
or birds; isolated plants are frequently found at the bases of large trees
in forest interiors. Seeds may possibly be distributed directly or indirectly
by white-tailed deer (Odocoileus virginianus).
In southern locales Alliaria populations are even-aged, alternating
annually between immature plants and adult plants (Baskin and Baskin 1992),
probably due to the 8 month seed dormancy. In northern climates Alliaria
populations can be even-aged in early stages of invasion, and then become
multi- aged as the seed bank builds up. Alliaria is frequently overlooked
at low density levels. In many sites Alliaria can be present for
a number of years before appearing to "explode" in favorable years.
Once Alliaria reaches this level of infestation control is difficult
Alliaria is one of the few alien herbaceous species that invades
and dominates the understory of forested areas in North America. The phenology
is typical of cool-season European plants, and Alliaria grows rapidly
during early spring and late fall when most native species are dormant.
Alliaria invades forested communities and edge habitats, where it
spreads rapidly and apparently displaces native herbaceous species, often
within ten years of establishment. After just four years of co-occurrence
with Alliaria, cover of the ephemeral herb toothwort (Dentaria
laciniata) was reduced >50% (Nuzzo 1992a). Toothwort plants associated
with Alliaria were stunted, yellowed, and failed to flower.
Alliaria is most widespread in the midwestern and northeastern
United States and in southern Ontario, where it invades wet to dry-mesic
deciduous forest (Cavers et al. 1979, Nuzzo 1992a, 1993a), and also occurs
in the partial shade characteristic of oak savanna, forest edges, hedgerows,
shaded roadsides, and urban areas, and occasionally in full sun (Nuzzo 1991).
Alliaria is common in river-associated habitat, particularly in the
Northeast (Nuzzo 1993a), and in both upland and floodplain forest communities
in the Midwest.
Alliaria grows on sand, loam, and clay soils, and on both limestone
and sandstone substrates, occurs rarely on drained peat soils, and does
not occur on muck soils. Alliaria frequently grows in well- fertilized
sites (Cavers et al. 1979), and is described as a nitrophile by Passarge
(1976) and Wilmanns and Bogenrieder (1988).
The goal of Alliaria management is to prevent seed production.
Alliaria spreads only by seed, and has a short-lived (2-5 years)
seed bank; in theory, preventing seed production for a maximum period of
five years should result in elimination of Alliaria from a site,
if no additional seeds are introduced.
The primary management objective in areas lacking this plant is to prevent
establishment, by annually monitoring for and removing all Alliaria plants.
The primary management objective in infested sites is to prevent seed production.
Cutting flowerstalks is effective in small populations. Fire and herbicides
are useful for larger populations but both have potential side effects.
No method provides 100% control.
Growing season mortality reduces Alliaria seedling populations
by >95% between spring and late fall (Nuzzo 1993b); hence, control is
most economical when undertaken in late fall or early spring, prior to flower
production. Late fall is usually the preferred season for control, as native
plants are dormant and management can be conducted until snow covers the
ground. If weather is unfavorable in fall, control can still be conducted
in early spring. Delaying control until spring can be risky, as native herbs
may begin growth earlier than anticipated, and weather may limit or prevent
Biological control may be the only regionally effective method of controlling
this species, but as of 1997 no formal program had been established.
Prescribed burning can provide effective control of Alliaria when
fires burn completely through the affected area, and are conducted for at
least two consecutive years (Nuzzo 1991). Kill appears related to a critical
increase in rootcrown temperature, effected by a slow fire, or by a fast
fire that also removes all litter.
Low-intensity fires are ineffective (Nuzzo 1991). A slow mid-intensity
fire can reduce adult density by 50% (Nuzzo 1991). A fast high-intensity
fire that removes most litter can effectively reduce adult cover (Nuzzo
et al. 1996). However, fast fires may leave a thin layer of litter. This
1-2 cm layer is sufficient to protect root crowns, which subsequently produce
multiple flower stalks from axillary buds, increasing total seed production
(Nuzzo et al. 1996). Removal of the litter layer increases seedling survival
after fire, and can result in a larger population after a single burn (Nuzzo
et al. 1996). Thus, after a single fire, total Alliaria cover can
increase due to survival of adult plants, and/or to enhanced seedling survival.
After two consecutive fires total cover is greatly reduced (Nuzzo et al.
Spring and fall fires are equally successful in reducing cover of Alliaria
rosettes (Nuzzo 1991). Spring fires also reduce seedling presence if conducted
during the germination period (Nuzzo 1991). However, burning enhances survival
of seedlings that germinate after fire, by removing the smothering leaf
litter (Nuzzo et al. 1996).
Use of fire as a management tool should be tailored to the specific community.
Removal of the litter layer may facilitate invasion by disturbance adapted
species, including Alliaria, particularly if there is little native
groundlayer present at the site. Fires should only be conducted when at
least two consecutive fires can be scheduled; burning only once may increase
Alliaria abundance. Impact of consecutive fires on the community
should be considered, including changes in groundlayer composition.
Fire is not a realistic management tool in upland communities that have
become fire-resistent, due to decreased fuel loads and flammability resulting
from replacement of overstory oaks (Quercus sp.) by cherry (Prunus
serotina, P. virginiana), ash (Fraxinus sp.), black walnut
(Juglans nigra), and hackberry (Celtis occidentalis). Invasion
of the understory by buckthorn (Rhamnus cathartica) and honeysuckle
(Lonicera tatarica, L. xylosteum) also reduces fuel loads.
Dormant season herbicide application can provide effective control of
Alliaria, but poses a potential threat to native herbaceous and graminoid
Round-up (glyphosate) applied at 1%, 2%, and 3% concentrations to dormant
rosettes in late fall or early spring reduced adult cover by >95% (Nuzzo
1991, 1996). Control was slightly greater with higher Round-up concentrations.
Seedlings that germinate after application are not affected by the herbicide,
as Round-up has no soil residual. Roundup applied after germination will
significantly reduce seedling populations (Nuzzo 1991).
Round-up results in some native species loss, particularly when applied
in spring, as it is a non-selective herbicide. Semi-evergreen species including
phlox (Phlox divaricata), wild ginger (Asarum canadense) and
sedges (Carex sp.) are reduced by Roundup (Nuzzo 1996). At the community
level, Round-up did not affect mean species richness or total mean herbaceous
cover, but did significantly reduce cover of both sedges and grasses, at
both 0.5% and 1% concentrations (Nuzzo 1996).
Growing season application of Basagran (Bentazon) at 0.56-1.12 kg AI/ha
(0.50-1.0# AI/acre) reduced rosette cover by 90-95% (Nuzzo 1994). Dormant
season application nonsignificantly reduced rosette cover >90% (compared
to 70% reduction in the control plots)(Nuzzo 1996).
Basagran did not affect species richness or herb cover, and had minimal
effect on graminoid cover. Alliaria seedlings were not affected by
treatment. Basagran is a post-emergent contact herbicide that targets dicots
and is used to control mustards in agricultural fields.
Cutting flowering plants at ground level results in 99% mortality, and
eliminates seed production. Cutting at 10cm above ground level results in
71% mortality and reduces seed production by 98% (Nuzzo 1991). Ability of
cut flowerstems to form viable seed is unknown. Cavers et al. (1979) suggest
that vivipary (germination of seeds while still in the silique) does not
occur, although all seeds remained viable during the observation period.
Until more information is available, cut stems should be removed from the
site, or piled and composted.
Cutting with a weed whip provides quick removal of flowering stems, but
also may remove other desirable species. Some native species, such as Trillium,
are severely impacted if cut. Most other species are not substantially damaged,
and the benefits of removing Alliaria outweigh the temporary reduction
in growth and reproduction of native groundcover species.
Pulling is very labor intensive but effective if the upper half of the
root is removed. Alliaria frequently snaps off at or just below the
root crown when the flower stalk is pulled, leaving adventitious buds which
send up new flower stalks. Pulling can result in substantial soil disturbance,
damaging desirable species and bringing up Alliaria seeds from the
seedbank. Soil should be thoroughly tamped after pulling to minimize chances
for re-establishment of garlic mustard or other weedy species. Alternatively,
soil may be kept disturbed to stimulate germination of Alliaria seeds
and subsequent depletion of the seedbank, if seedlings are removed before
maturing. In general, cutting is a less destructive method of control than
pulling but is effective only during flower stalk elongation, whereas pulling
can be conducted throughout the growing season.
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