Garlic Mustard

Garlic mustard (Alliaria petiolata) is an invasive herbaceous biennial weed which has become an increasing problem in forests, in botanic gardens and arboreta, in landscape plantings and in parklands. Its appearance changes from its first year to its second year in its life cycle, making identification difficult. Best control involves use of glyphosate herbicide on first year growth or very early hand-pulling to prevent seed production.

Origins and history: Garlic mustard is native to Europe and was brought to the U.S. as an herb with a first report in the U.S. on Long Island in New York in 1868 and the first report in Ohio in 1899. For unknown reasons, in recent years, garlic mustard infestations in natural areas and parks have exploded.

Biology: Garlic mustard is a non-woody biennial herb, producing only short stems and leaves in its first year after seed germination, with longer stems, leaves, flowers, fruits and seeds in its second year of growth before it dies back. The rosette of first year leaves persists as green tissue well into the fall and winter. Garlic mustard flowers in early spring, about the time that spring beauty (Claytonia virginiana) blooms (early April). Seed produced in that second year may survive in soil for up to five years and possibly longer.

Description: The first year plants are distinctly different from the second year plants. Leaves occur in a basal rosette and the plant is only several inches in height. The leaves are small, scalloped and kidney-shaped with a mild garlic aroma. In the second year of growth, the stem elongates to 3-4 feet in length, leaves are triangular and sharply toothed, and tiny 4-petaled white flowers and long slender seedpods ("siliques") develop.

Comments: Garlic mustard is a successful invasive for several reasons. It produces large amounts of seed. In its first year of growth, the rosette of kidney-shaped leaves are often unidentified as garlic mustard. It is not beloved by deer. It is believed to produce allelopathic chemicals which deter growth of other plants competing for the same niche. Garlic mustard also thrives in rich woodland or garden sites with high earthworm activity and rates of organic decomposition.

Control: Because the rosette of first year leaves is green and growing in the fall and into the cold of winter it can be controlled with glyphosate herbicide (eg. Roundup) which is absorbed through the leaves and then translocated to the roots, killing the garlic mustard plant. This control approach is particularly effective because these are times when glyphosate can be applied without non-target damage to other plants which are dormant during that period. Hand-pulling second year plants is risky as a control measure if garlic mustard has reached even the early flowering stages. Research shows that viable seed may be produced even on flowers of plants that are pulled up and left in the garden or parkland site. Only if pulled flowering plants are bagged in plastic or completely removed from the site will control with hand-pulling be assured.