SPECIES CHARACTER
Description
Garlic mustard produces a characteristic fragrance of garlic from all parts of
the plant. Adult plants grow 2-48 inches (5-125 cm) high. Basal rosettes have
dark green, kidney-shaped leaves that differ somewhat in shape from the sharply-
toothed, triangular, alternate, petioled leaves on the stems. In Illinois,
garlic mustard usually blooms in May. Numerous small white flowers, 0.25
inches (6-7 mm) across, are borne in a terminal raceme at the apex of the stem,
and also at some leaf axils. Plants usually produce 1 flowering stem, but may
have as many as 10 stems from a single root. Each flower is composed of 4 white
petals that narrow abruptly at the base. Black seeds are produced in 1-4.7 inch
(3-12 cm) long, narrow, linear capsules called siliques.
Similar Species
Garlic mustard is easily distinguished from all other woodland mustard plants by
its characteristic odor of garlic and the 2-4 foot (0.6-1.2 m) tall flower stalks
covered with numerous small white flowers in May. In northern Illinois, garlic
mustard is the only tall, white-flowered, four-petalled plant that blooms in
May. The alternate, coarsely toothed, broadly triangular stem leaves with a
distinct petiole are also characteristic. The garlic odor gradually dissipates
by autumn, and garlic mustard rosettes may then be mistaken for violets (Viola
spp.) or immature white avens (Geum canadense). Garlic mustard can be
distinguished from these species by examining the roots. Garlic mustard has a
white slender taproot, with a characteristic crook or "s" shape at the top of the
root, just below the base of the stem. Garlic mustard should be accurately
identified before attempting any control measures. If identification of the
species is in doubt, the plant's identity should be confirmed by a knowledgeable
individual and/or by consulting appropriate books.
Distribution
Garlic mustard is native to Europe, and also occurs in northern Africa, Sri
Lanka, and India. In North America, garlic mustard is now distributed from
Quebec and Ontario, south to North Carolina and Kentucky, and west to Kansas and
North Dakota. In Illinois it occurs in at least 41 counties. It is abundant
and widespread in the northern half of Illinois, and occurs south to a line from Calhoun to Clark County, with
a local occurrence in Jackson County in southwestern Illinois.
Habitat
This species occurs most frequently in upland and floodplain forests, savannas,
and along roadsides. It invades shaded areas, especially disturbed sites, and
open woodland. It is capable of growing in dense shade and occasionally occurs
in areas receiving full sun.
Life History
Garlic mustard is a biennial herb. Seeds germinate in early spring, young
plants overwinter as basal rosettes, and adults bloom from May-June the
following year. Each plant dies after producing seed. Seeds disperse when the
siliques burst at maturity in August. Seeds have a 20 month dormancy period and
do not germinate until the second spring after ripening. The species reproduces
readily from the numerous seeds produced.
Effects Upon Natural Areas
Garlic mustard aggressively has invaded numerous forested natural areas and is
capable of dominating the ground layer in many areas. It is a severe threat to
many natural areas where it occurs because of its ability to grow to the
exclusion of other herbaceous species.
CONTROL RECOMMENDATIONS
RECOMMENDED PRACTICES IN NATURAL COMMUNITIES OF HIGH QUALITY
Initial effort in areas of heavy infestation
Fall or early spring burning is an effective control treatment in oak woods.
Repeated burns over several years may be necessary to achieve adequate control
and to eliminate plants produced from the seed bank. Prescribed fires should be
of sufficient intensity to burn the affected site thoroughly. Low intensity
fires that leave unburned areas will not control garlic mustard effectively.
Any isolated plants that are not burned should be removed by hand prior to flower
production.
Research by Victoria Nuzzo indicates that cutting flowering stems at ground
level results in 99% mortality, while cutting at 4 inches (10 cm) above ground
level produces 71% mortality and reduces total seed production by 98%. Plants
cut near ground level when in full flower usually do not resprout. Viable seed
may be produced after stems are cut: pending further research, cut stems should
be removed from the site when possible.
The Nature Conservancy has successfully controlled or eliminated this plant from
several sites by a combination of spring burning, hand-pulling, and cutting
flowering stems with a scythe. When garlic mustard occurs in nearly pure
populations with few other plants, scything is advantageous in that large areas
can be covered quickly and the soil is not disturbed.
Spot application of 2% Roundup (a formulation of glyphosate) to the foliage of
individual plants is effective during spring and fall when most native
vegetation is dormant but garlic mustard remains green. Herbicide should be
applied when air temperatures are above 32o F (0o C). Managers should exercise
caution when applying herbicide to garlic mustard to avoid contacting nontarget
plants. Roundup is a nonselective herbicide (kills all vegetation) and should
not be used during the growing season in high-quality areas because of the
possibility of harming nontarget plants. Do not spray so heavily that
herbicide drips off the target species. The herbicide should be applied while
backing away from the treated areas to avoid contacting the wet herbicide. By
law, herbicides only may be applied as per label instructions and by licensed
herbicide applicators or operators when working on public properties.
Initial effort in areas of light infestation
Removal of plants by hand-pulling is effective if the root is removed. If the
stem snaps off from the root crown of a non-flowering plant, the plant may
resprout. When hand-pulling, disturb the soil as little as possible, and tamp
the soil firmly after removing the plant. Soil disturbance can bring garlic
mustard seed to the surface and create a favorable environment for garlic
mustard germination and growth.
Maintenance control
Vigilant monitoring and hand removal of first- and second-year plants prior to
flower production can be effective.
A regular burning regime in oak woods can control garlic mustard.
RECOMMENDED PRACTICES ON BUFFER AND SEVERELY DISTURBED SITES
Fall or early spring burning in oak woods can control this species. Repeated
burns may be necessary over several years. Spot application of 2% Roundup to
individual plants as described above can be used in severely disturbed woods.
Cutting or scything flowering stems, as described above, is effective.
Maintenance control is the same as given above.
In addition, hand spraying individual plants with an amine formulation of
2,4-D is an effective control when applied according to label instructions. To reduce vapor drift, use an amine formulation of 2,4-D rather than an ester formulation. A 1% solution of Mecamine (2,4-D plus Dicamba) applied to the foliage of young plants is also effective. Either herbicide should be applied only during spring or fall when most native vegetation is dormant but garlic mustard remains green. The herbicide 2,4-D amine is selective for broadleaf plants. As with Roundup, managers should exercise caution when applying these herbicides to garlic mustard to avoid contacting nontarget plants. Do not spray so heavily that herbicide drips off the target species.
FAILED OR INEFFECTIVE PRACTICES
Low intensity fires that do not burn through the leaf litter have no effect on garlic mustard.
REFERENCES
Cavers, P. B., M. I. Heagy, and R. F. Kokron. 1979. The biology of Canadian weeds. 35. Alliaria petiolata (M. Bieb.) Cavara & Grande. Canadian Journal of Plant Science 59: 217-229.
Fernald, M. L. 1950. Gray's manual of botany, eighth edition. American Book Co., New York. 1632 pp.
Heim, J. 1987. Natural heritage herbicide users guide. Illinois Department of Conservation, Division of Natural Heritage. 31 pp.
Mohlenbrock, R. H. and D. Ladd. 1978. Distribution of Illinois vascular plants. Southern Illinois University Press, Carbondale. 282 pp.
Nuzzo, V. In press. 1990. Experimental control of garlic mustard (Alliaria petiolata (Bieb.) Cavara & Grande) in northern Illinois using fire, herbicide, and cutting. The Natural Areas Journal.
Swink, F. and G. Wilhelm. 1979. Plants of the Chicago region. Morton Arboretum, Lisle, Illinois.
PERSONAL COMMUNICATIONS
Apfelbaum, Steve. 1988. Applied Ecological Services, Juda, Wisconsin.
Betz, Dr. Robert. 1988. Northwest ern University of Illinois.
McFall, Don. 1988. Division of Natural Heritage, Department of Conservation, Springfield, Illinois.
Nuzzo, Victoria. 1989. Native Landscapes, Rockford, Illinois.
Nyboer, Randy. 1988. Division of Natural Heritage, Department of Conservation, Springfield, Illinois.
Packard, Steve. 1989. The Nature Conservancy, Chicago, Illinois.
Schwegman, John E. 1988. Division of Natural Heritage, Department of Conservation, Springfield, Illinois.
Written for the Illinois Nature Preserves Commission by:
Victoria Nuzzo
Native Landscapes
124 Dawson Avenue
Rockford, IL 61107
Jill Kennay and George Fell
Natural Land Institute
320 S. Third Street
Rockford, IL 61108