Garlic Mustard
Alliaria petiolata
Mustard family (Brassiaceae)

Description: This introduced biennial plant is 1-3' tall. During the 1st year it consists of a small rosette of leaves, while during the 2nd year it becomes a little-branched plant about 1-3' tall. The leaves of 1st year plants are up to 2" long and across. They are cordate-orbicular with margins that are dentate or wavy and their upper surface has a reticulated network of veins. The petioles of these basal leaves are rather long and slender. The alternate leaves of 2nd year plants have a similar appearance, except that they are usually longer than wide, spanning up to 3" long and 2" across. The lower and middle leaves along the stems are usually cordate with blunt tips, while the upper leaves are often ovate. Both stems and the petioles of 2nd year plants are occasionally hairy, otherwise they are glabrous like the blades of the leaves. The foliage is often light green or yellowish green in appearance. The upper stems terminate in narrow racemes of white flowers. While in bloom, these flowers are bunched together toward the top of the raceme. However, as the flowers mature and develop seedpods, the raceme becomes more elongated and they become more separated. Each flower is up to 1/3" across and consists of 4 white petals, 4 light green sepals, a short cylindrical stigma, and several stamens with pale yellow anthers. The pedicel of each flower is ¼" long or less. The blooming period occurs during late spring or early summer and lasts about 1-2 months. The flowers are replaced by elongated seedpods that are called "siliques." These seedpods are about 1½–2" long and narrowly cylindrical (although somewhat 4-angled in circumference). Relative to erect stalk of the raceme, they are spreading and ascending. Each seedpod contains a single row of black oblong seeds. The root system consists of a shallow taproot that is white and branches frequently. This plant often forms colonies by reseeding itself.

Cultivation: The preference is partial sun to medium shade, moist to mesic conditions, and a loamy fertile soil. Small rosettes of leaves are formed during the summer of the 1st year, which die down to the ground during the winter. However, during the spring of the following year, new leaves appear on stems that develop rapidly to produce flowers by early summer. This plant is well-adapted to deciduous woodlands and can reseed itself aggressively, forming dense stands that exclude other species. It has few problems with pests and disease.

Range & Habitat: Garlic Mustard has been reported primarily in NE and central Illinois, where it is locally common. In other areas of the state, this plant is apparently less common or absent, however it is rapidly spreading. There is little doubt that it is more common than official records indicate. This plant was introduced into the United States from Eurasia. Habitats include moist to slightly dry deciduous woodlands, woodland borders, semi-shaded areas in gardens and along fence rows, and partially shaded waste areas. This plant rarely strays far from the shade provided by woody vegetation and it is intolerant of regular mowing. At the present time, Garlic Mustard is the worst exotic invader of deciduous woodlands in Illinois as it has the capacity to crowd out and destroy all of the native wildflowers that bloom during the spring. Effective measures of control include pulling the plants by their roots and spraying the foliage with herbicides. Cutting the flowering stalks from their stems is not an adequate method of control because Garlic Mustard is capable of regenerating new flowering stalks from its side stems.

Faunal Associations: The flowers attract various kinds of bees and flower flies. In sunnier areas, the flowers may also attract the introduced butterfly Pieris rapae (Cabbage White). Apparently the seeds are little-used by birds and mammalian herbivores rarely bother the foliage, possibly because they're repelled by its garlic-like scent. There also appears to be few native insects that feed on the foliage and other parts of this plant. At the present time, ecologists in Switzerland are examining the insect pests of Garlic Mustard in the Old World to determine if any of them are suitable for introduction into the New World. Two species of leaf beetles appear to be the best candidates for biological control.

Photographic Location: The edge of a deciduous woodlands at Judge Webber Park in Urbana, Illinois.

Comments: Garlic Mustard was introduced into the United States as a potherb. The young leaves are edible to humans and quite nutritious – they can be added to salads or boiled in water and seasoned like spinach. The garlic-like aroma of the foliage is quite pronounced, which sets this species apart from many other members of the Mustard family (as well as plants from other families). The Cardamine spp. (Bitter Cresses) are somewhat similar in appearance to Garlic Mustard, but they have larger flowers on longer pedicels, and their leaves are never coarsely toothed. Some members of the Mint family have leaves that resemble those of Garlic Mustard, but the Mints always have opposite leaves and their stems are conspicuously 4-angled.