Scientific name: Alliaria petiolata
Common name: Garlic Mustard
(Information in this Species Page was compiled in part by Andrea Boysel as part of the course requirements for Biology 220M in Spring 2001 at Penn State new Kensington)
Garlic mustard is a conspicuous understory plant often found in incredible abundance in the shaded floor of moist deciduous forests. It is also frequently found near disturbed areas like trails, hedgerows, shaded roadsides, and forest edges. Garlic mustard is not a native plant of North America. Its natural range is throughout Europe ("from Italy to Sweden and England to Russia") where it is an integrated component of natural forest flora. It was carried, probably inadvertently, to North America as seeds on boots or in the soil of intentionally transplanted plants. The first recorded observation of garlic mustard in the United States was on Long Island, New York in 1868. Since 1868 garlic mustard has spread to thirty U.S. states and three Canadian provinces. In North American ecosystems garlic mustard grows and spreads rapidly and invasively, an uncontrolled "weed", choking out native plant species and negatively impacting the herbivores that depend upon them for food.
Mature garlic mustard plants stand one to three feet tall with alternate, heart-shaped, toothed leaves that are three to eight centimeters in diameter (leaves get smaller as you go up the plant stem). In spring and early summer
flower stalks form on which there are small (1/4 to 1/3 inches wide), white flowers with four petals arranged in a cross shape pattern, each bearing six stamens (four long and 2 short). The leaves smell strongly of garlic when crushed. These mature, flowering plants represent the second year's growth of this biennial species.
The flowers of garlic mustard are either pollinated quite non-specifically by a variety of insects
(solitary bees, a variety of flies, and on rare occasions, honeybees or bumblebees) or they can self-pollinate if these insect interactions do not occur. The seeds of the pollinated plants have been shown to be more vigorous than those produced by the self-pollinated plants. The ability to self-pollinate, though, does confer a great advantage on the survivability of an invading population founded by a single, established individual. Garlic mustard also has a slender, white taproot from which adventitious buds that can form flower stalks can arise. This root budding ability further adds to the difficulty of population control of these invasive plants.
A pollinated flower produces a fruit that is linear, 2.5 to 6.0 cm long and about 2 mm wide. A single plant can produce two to over four hundred fruits (with an average of twenty-two per plant). The fruits ripen between mid-June and September. Each fruit contains approximately sixteen seeds. A single plant, then, is capable of producing up to 8000 seeds in a single season! Seeds are contained in tan seedpods that are capable of ejecting the ripened seeds several feet away from the stem of the plant. Seeds must lay dormant for at least one year before germinating in the spring. Cold stratification is required to trigger seed germination. The seeds can remain viable in the soil for up to five years (a factor that further complicates the removal of an established garlic mustard population). Seeds can be transported great distances on boots of hikers or even on the tires of vehicles.
Seedlings emerge from February to early March and form basal rosettes by the middle of summer. These first-year rosettes are dark green with kidney shaped basal leaves and are very sensitive to drought. Sixty to ninety percent of these first year individuals fail to survive this first summer season. The surviving plants remain green over the winter and then grow very rapidly into the mature, second year form early in the next spring (typically early March) prior to the leafing of the over-story deciduous trees. The ability of garlic mustard to over-winter in a green (chlorophyll rich) form and to survive and grow in the near freezing temperatures of late fall and early spring enables it to take over a system's habitat space and growth resources before the less cold-tolerant plant species can become established.
Garlic mustard is a threat to the forest ecosystems of the midwestern and eastern United States. It is able to form monospecific stands that dominate the understory of even relatively undisturbed forests and actively displace native understory plant species. Once garlic mustard is established in an area it is almost impossible (because of characteristics discussed above) to be eliminated. As a permanent component of a site's flora, it increases its presence every year and grows out of control until it completely dominates the site. Tree seedlings (especially oaks) and many wildflowers (including spring beauty, trout lily, wild ginger, bloodroot, Dutchman's breeches, hepatica, toothwort, and trillium) cannot survive the explosive growth and spread of garlic mustard. In addition, invertebrates and other consumers that rely on these natural plant species for food are harmed by the spread of this invasive "weed". Garlic mustard also produces root exudates that inhibit the growth of important soil fungi and leaf chemicals that kill native butterfly larvae that feed on the plant.
Garlic mustard is edible and is used in its native range for a great variety of dietary and medicinal purposes. It is rich in vitamins A and C and makes a spicy addition to salads, sandwiches, or cooked dishes. Its crushingly negative impact, though, on native plants and native forest ecosystems renders these very minimal uses quite trivial and unimportant.