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. Plant 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.
 Garlic mustard is a Class A noxious weed with a very limited distribution in Washington, and eradication is required state-wide. Most of the known infestations in King County are on City of Seattle Parks properties or nearby private properties, but garlic mustard is moving into new areas and has also been found in a few other counties in Washington State. Early detection, containment and eradication of new sites is of the highest priority. If you think you see this plant, please contact our program as soon as possible.
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. 
For more information on garlic mustard identification and control, please download our 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, 43 seconds on 56K modem).
If you find garlic mustard in King County, please notify us through our online infestation form.
To find out where we have records of this weed in King County, use our interactive noxious weed map and search  for garlic mustard.
Garlic Mustard Photos - click thumbnail for larger image
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