Alliaria petiolata
(Garlic Mustard)

Where it originates:
Native to Europe, and brought to North America by settlers most likely in the early 1800's. Because of its availability in the early spring and its high A and C vitamin content, the plant served as a good food source for use in salads or stews.

Identification:
Alliaria petiolata is a biennial herb, emerging in the early spring and overwintering as a rosette. During the mid-spring of its second year, it will rapidly grow 1 to 3ft tall and develop small white clusters of 4-petaled flowers (~ 1/4 in diameter). The plant will die shortly after flowering but the tall flower stalks will linger most of the summer, as well as the seedpods which split as they dry to release tiny black seeds.

Leaves are kidney shaped and have scalloped edges during the rosette stage; leaves appear much more triangular and toothed once the plant develops into the flower stalk. In both years the plant is easily distinguished by its garlic-odor.

Habitat:
Invades typical habitat disturbed by humans (roads, forest edges), and is well suited to moist soils along river and stream banks. Once established in these areas, A. petiolata is also very adept at invading non-disturbed habitat, and may occur in the interior of forests where few trails or roads exist.

A. petiolata is widespread from Quebec, south to Virginia, and as far west as Wisconsin; but the plant has been spotted in Georgia and as far west as Washington.

Ecological Impacts:
A. petiolata displaces native wildflowers, and can out-compete young shrubs and saplings by siphoning off valuable light and space resources. Dense stands can also produce chemicals that reduce the growth of mycorrhizal fungi, which native plants require to draw nutrients from the soil. A single plant can produce thousands of seeds, and will rapidly colonize new areas if left unchecked.

A. petiolata also is posing a threat to many butterfly species by thinning populations of native plants such as toothwarts, spring beauties, and trilliums. Butterfly eggs laid on A. petiolata leaves instead of natives fail to develop, likely again due to chemicals within the plant. A rare butterfly, the West Virginia White, is especially threatened due to increased scarcity of its reproductive plant.

The spread of A. petiolata also deprives the habitat from any nectar, seeds, nuts, or other essential resources that multitudes of organisms seek each spring. For people, perhaps the most apparent impact is the loss of the visual brilliance of spring wildflowers so many hikers and botanists highly value.

Control:
Plants are easily pulled, both as a rosette and as a flower stalk. Cutting may lead to re-sprouting, so getting the entire root is essential. Although easily pulled - the challenge in controling A. petiolata is in continuing to pull it for 3-5 successive years to exhaust its seed bank.

Herbicide is effective and may be necessary for large dense areas (Glyphosate - i.e. Round-Up), however threats to other native groundcover can be high and must be considered before spraying. 

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References:
Kaufman, Sylvan. Invasive Plants. 1st. Mechanicsburg, PA: Stackpole Books, 2007. 277-279. Print.

Rowe, Pamela. "Garlic Mustard." Plant Conservation Alliance's Working Alien Plant Group Least Wanted. 07 Jul 2009. Web. 3 Nov 2009.

Photos:
Elaine Haug @ USDA-NRCS PLANTS Database, Jim Stasz @ USDA-NRCS PLANTS Database