Garlic mustard
Alliaria petiolata

History and impacts

Garlic mustard is an invasive non-native biennial herb that spreads by seed. It is difficult to control once it has reached a site; it can cross-pollinate or self-pollinate, it has a high seed production rate, it out competes native vegetation and it can establish in a relatively stable forest understory. It can grow in dense shade or sunny sites. The fact that it is self fertile means that one plant can occupy a site and produce a seed bank. Pant stands can produce more than 62,000 seeds per square meter to quickly out compete local flora, changing the structure of plant communities on the forest floor. Garlic mustard is also allelopathic, producing chemicals that inhibit the growth of other plants and mychorrizal fungi needed for healthy tree growth and tree seedling survival.

Legal status in King County, Washington

First identified in Seattle in 1999 and listed as a Class A noxious weed in 2000, the King County Noxious Weed Program is working closely with landowners to prevent new infestations and eradicate existing infestations. Without cooperation we will lose the battle to keep garlic mustard out of woodland areas throughout western Washington.

Biology and morphology

Garlic mustard is a biennial herb that grows to about 3 feet tall. The small, white 4-petaled flowers appear in early spring and seed production soon follows. Seedlings develop into basal rosettes by mid summer. The plants overwinter as a basal rosette with kidney-shaped leaves. When the plants bolt in early spring, the mature leaves are triangular, becoming smaller toward the top of the plant. In early spring the roots and new leaves smell like garlic. Each plant usually produces one flowering stem. If a plant is cut or stepped on, many stems will form. Roots typically have a characteristic s-shaped bend that helps the plant hold on to the soil even on steep slopes with loose soil. Garlic mustard is competitive in a wide range of soils, sun, shade and moisture. It grows in wet soil near creeks and on dry, steep slopes. It can grow under the shade of other plants like nettles or in bright sunny spots. Flowering plants range in size from several feet tall to tiny plants with just a few seed pods. Seeds can last in the soil for at least 10 years.

Prevention is the recommended control option. Familiarize yourself with the flower, the plant and the habitat where it grows. Monitor sites regularly to remove plants prior to seed set.

Additional information on garlic mustard

• Washington State Noxious Weed Control Board (external link)
• Garlic mustard weed alert (1.02 MB) Acrobat file).
• For in-depth information on impacts, biology, identification and control of garlic mustard in King County, please read the garlic mustard best management practices (294 KB Acrobat file)

What to do if you find this plant in King County, Washington

Please notify us if you see garlic mustard growing in King County. Our program staff can provide the property owner or appropriate public agency with site-specific advice on how best to remove it. We map all known locations of regulated noxious weeds such as gaelic mustard in order to help us and others locate new infestations in time to control them.

Garlic mustard photos - click thumbnail for larger image