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Alliaria petiolata (M. Bieb.) Cavara and Grande
(garlic mustard, hedge garlic, sauce-alone, jack-by-the-hedge, poor man's mustard, jack-in-the-bush, garlic root, garlicwort, mustard root)


Summary: known invader sighted in new areas
Small populations of Alliaria petiolata have been reported in Washington, Utah, Colorado, Idaho, and British Columbia. This is a range expansion for this well-known weed.

Alliaria petiolata was first collected in the USA in 1868 on Long Island, New York. It has since spread to 30 eastern/midwestern states and 3 Canadian provinces. Garlic mustard invades forested communities and edge habitats where it rapidly spreads and displaces native herbaceous species. Displacement occurs rapidly, often within 10 years of establishment. Once established, garlic mustard is very difficult to control. Annual monitoring and rapid removal of plants are the most effective measures in preventing the establishment of garlic mustard.

Description: known invader sighted in new areas
(Click on thumbnail images for a closer view)

An infestation

Fruit

Flowering plant

In a forest

Young plant

In West Virginia

Alliaria petiolata (Brassicaceae -- the mustard family) is a biennial herb. Seedlings emerge February to early March, and form basal rosettes by midsummer. In the spring of the plant's second year, the basal rosettes generate flower stalks. The plant dies after setting seed.

Basal leaves are dark green and kidney shaped. The leaves have scalloped edges and are 6-10 cm in diameter. Stem-leaves are alternate, sharply toothed, and triangular in shape. The stem leaves are 3-8 cm in both length and width, and are smaller towards the top of the stem. The leaf petioles are pubescent and 1-5 (or more) cm long. Leaves produce a distinct garlic odor when crushed, although mature plants have less odor.

Each plant usually produces a single, unbranched (or weakly branched) flower stalk in early April through May. Robust plants have been recorded with up to 12 separate flowering stalks. Flowering plants average 0.7-1 m in height, but tiny 5 cm tall flowering specimens have been observed. The mustard-like flowers consist of four white petals that narrow abruptly at the base with 6 (2 short and 4 long) stamens. The flowers are 6-7 mm in diameter with 3-6 mm long petals. Alliaria petiolata will set seed even if pollinators are not present.

Fruits are linear, 2.5-6 cm long and 2 mm wide. The fruit is held erect on short (5 mm), stout, and widely divergent pedicels. Fruit ripen between mid-June and late September. A single plant produces 2-422 fruit (the average is 22). Since each fruit contains an average of 16 seeds, a single plant produces 194--8,000 seeds. The seeds are black, cylindrical (3 mm x 1 mm) and grooved. The seeds lay dormant for at least one year before germinating in the spring.

In the basal rosette stage, Alliaria petiolata can easily be confused with many other plants, but its strong garlic odor (in the spring and summer) is distinctive. Each plant has a slender white taproot with an "S" curve at the top, just below the crown.

Scientific and Common Names:
The genus name, Alliaria, is in reference to the Allium-like or garlic fragrance of the crushed leaves. The specific epithet refers to the petiolate leaves. Garlic mustard is the common name used in North America. Other common names (listed above) are used predominantly in England. Common names refer to the European use of Alliaria petiolata as a potherb and as an edge plant. Synonyms for A. petiolata include "Alliaria officinalis Andrz.", "A. alliaria L. (Britton)", "Sisymbrium alliaria Scop.", "S. officinalis DC", and "Erysimum alliaria L."

Considerations and Impacts:
1. It dominates areas and displaces natives, altering species composition.
2. It decreases fuel load for burns.
3. Rich soils, disturbed areas such as forests with fallen trees, habitat edges such as roads or streams, and disturbance from trails or agriculture can encourage invasion.
4. In North America, it invades wet to dry-mesic deciduous forest and in the partial shade of oak savanna, forest edges, hedgerows, shaded roadsides, and urban areas. It occasionally occurs in full sun.
5. It grows on sand, loam, and clay soils, and has also been found on limestone and sandstone substrates.
6. Its seeds can remain viable for 5 years.
7. Garlic mustard spreads exclusively by seed. Seeds are spread mostly by floodwaters or on humans, animals, and vehicles. Wind dispersal is minor.

Native Range:
Alliaria petiolata is native mostly to the northern areas of Europe (from England east to Czechoslovakia, and from Sweden and Germany south to Italy).

Range As An Invader:
Alliaria petiolata has spread to North Africa, India, Sri Lanka, New Zealand, and North America. In the USA, it is most abundant in New England and other midwestern states. Populations have established as far west as North Dakota and Kansas, and south to Tennessee and North Carolina. Populations have been known in Oregon since at least 1974. It has newly been recorded in other western states (Washington, Utah, Colorado, and Idaho).

Control:
Preventing infestations by monitoring and removing pioneering plants is the best control. Where populations are established, seed production must be prevented for 2-5 years to deplete the seed bank.
1)Minimize disturbance by reducing foot traffic, overgrazing and erosion.
2)Monitor once or twice annually for seedlings and new invasions. Check for garlic mustard in very early spring or late fall for immature rosettes. Monitor in early to mid spring for flowering adults. Immediately remove small populations before seeds are produced.
3)Hand pulling works well but must be continued until the seed bank is exhausted. The plant pulls easiest early in the season. Remove the plant and at least the upper half of the root. Adventitious buds on the upper half of the root can send up new flower stalks if not removed.
4)Natural mortality of seedlings is high (approximately 95%). Chemical control is most economical in late fall or early spring.
5)Fair control was observed with a 3% concentration of glyphosate (Roundup) applied with a sprayer in Indiana. A decline in population was observed with a 5% solution of glyphosate (Roundup) sprayed on in Kentucky. Glyphosate is a non-selective herbicide.
6)2,4-D is not effective for the control of garlic mustard.
7)A spring application of triclopyr (Garlon 3A) killed 92% of garlic mustard rosettes in a small test. Triclopyr was applied at 7 oz/5 gal water (a little over 1%).
8)Cutting before seed set is effective if the infestation is minor. Cutting of large infestations are not as effective. Repeated cuttings at ground level produce the best control results. If feasible, cut stems should be removed.
9)Burning can provide control if it burns completely through the affected area. Scattered plants may survive in some areas. In Wisconsin, plants have been observed resprouting after a burn. New seedlings may germinate after a fire.

References:
1)Alliaria petiolata 1998-99 Weed Survey Report compiled by the Wildlands Invasive Species Team, The Nature Conservancy.
2)Nuzzo, V. 1994. Alliaria petiolata Element Stewardship Abstract (ESA), The Nature Conservancy.
3)Simon, B. 2000. Personal Communication.
4)Reichard, S. 2000. Personal Communication.

--Tunyalee Martin/Wildland Invasive Species Team; April 2000



Updated January 2007
©The Nature Conservancy, 2000