Garlic mustard identification and control

 Alliaria petiolata

Garlic mustard, a Class A noxious weed, is a biennial or winter annual herb that generally grows 2-3 (up to 6) feet tall. Lower leaves are kidney-shaped with scalloped edges. Leaves feel hairless, and the root has an "S" or "L" shape just below the stem base. In spring, roots and new leaves smell like garlic, and small, four-petal white flowers appear clustered at stem ends, followed by long, skinny seedpods. This weed spreads by seed and can self-pollinate, helping it rapidly displace native plants along trails, in forests, and on riverbanks, among other areas.

History and impacts

Introduced from Europe originally as a food plant, this species is now a serious concern in forests across North America.



Garlic mustard is an invasive non-native biennial herb that spreads by seed. Although edible for people, it is not eaten by local wildlife or insects. 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.

The majority of the known infestations in King County are on City of Seattle Parks properties and nearby private properties, but garlic mustard has also moved into Bellevue, properties along the Cedar River, North Bend, Tukwila, Shoreline and other parts of the county. 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.

Garlic mustard is also a growing problem in other parts of Washington as well as Oregon and Alaska. Regional collaboration to share information and build partnerships to combat garlic mustard will be the key to stopping this plant in the Pacific Northwest. See the PNW Garlic Mustard Working Group Poster sharing highlights from the October 2014 meeting. If you see garlic mustard in our region outside of King County, please notify the local or state weed board or conservation district office.

Legal status in King County, Washington

Garlic mustard is a Class A noxious weed with a limited distribution in Washington, and eradication is required state-wide. This species is also on the Washington quarantine list (known as the prohibited plants list) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts, seeds in packets, blends or "wildflower mixes" of this species, into or within the state of Washington. For more information, see Noxious weed lists and laws.

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 and vigilance we will lose the battle to keep garlic mustard out of woodland areas throughout western Washington.

Identification (see below for additional photos)

• 

  Biennial or winter annual herb that typically grows to about 3 feet tall, but can be anywhere from a few inches to over 6 feet tall depending on conditions
• Plants are usually single-stemmed, but may have more stems if they were cut
• Small, white 4-petaled flowers appear in early spring and are in clusters at the top of the stem
• First year plants are low-growing rosettes with rounded, kidney-shaped leaves, scalloped on the edges
• Leaves are not noticeably fuzzy or hairy (unlike most look-alike species)
• Upper leaves on mature plants are more triangular, becoming smaller toward the top of the plant, coarsely toothed
• Plants often smell like garlic, especially when leaves are crushed
• 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

Biology and morphology

Garlic mustard seeds typically germinate in fall or early spring and the plant first forms a low, mound of leaves called a rosette that grows from mid-summer through the following spring.  Plants typically bolt and form upright, flowering stems in March and April.  Flower buds can be seen on the tops of stems as the plants begin to bolt and then flowers open soon after stem elongates, usually late April through May. Seed production soon follows. 

Each plant usually produces one flowering stem. However, if a plant is cut or stepped on, many stems will form. Seeds can form on plants that are cut and left on the ground.  Roots crowns will grow new stems if they are not removed completely or if garlic mustard is cut.

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. Garlic mustard's curved root helps the plant hold on to the soil even on steep slopes with loose soil. It can grow under the shade of other plants like nettles or in bright sunny spots. Flowering plants can range in size from over six feet tall to tiny plants with just a few seed pods. Seeds can last in the soil for at least 10 years.

Seeds are small and easily spread on animals, people, vehicles and also by water, birds and other vectors.  Deer tracks and dog trails through infested forests are often lined with garlic mustard.

Control

Because this plant is so difficult to eradicate once it is established, familiarize yourself with the flower, the plant and the habitat where it grows to find infestations early. Monitor sites regularly to remove plants prior to seed set.  

Hand-pulling

Hand-pulling individual plants is effective if the entire root is removed.  Flowering or seeding plants must be put in a bag and discarded in the garbage. Carefully and thoroughly clean off boots, clothes and tools before leaving the area to avoid carrying the tiny seeds to new sites.

Chemical Control

Herbicide may be needed for large, dense infestations and should be applied in the spring or fall on seedlings and rosettes, with care taken to avoid native and other desirable plants.  Follow the product label and all laws and regulations regarding herbicide use on the site. Contact your local noxious weed program or county extension office for recommendations on herbicides.

Follow Up

After pulling or spraying dense infestations of garlic mustard, it can help to cover the bare areas with wood chip mulch to reduce seed germination. Infested sites should be carefully monitored every year for new plants, and checked for at least three or four years after no more plants have been found to ensure the population has been eradicated.

Additional information on garlic mustard

• Washington State Noxious Weed Control Board (external link)
• Garlic mustard weed alert (1.69 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)
• Wisconsin garlic mustard video (external link)

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 garlic mustard in order to help us and others locate new infestations in time to control them.

Garlic mustard photos