Alliaria petiolata is an obligate biennial herb of the mustard family (Brassicaceae). Seedlings emerge in spring and form basal rosettes by midsummer. Immature plants overwinter as basal rosettes. In the spring of the second year the rosettes (now adult plants) produce flower stalks, set seed, and subsequently die.
Basal leaves are dark-green and kidney-shaped with scalloped edges, 6-10 cm diameter. Stem leaves are alternate, sharply-toothed, triangular or deltoid, and average 3-8 cm long and wide, gradually reducing in size towards the top of the stem. All leaves have pubescent petioles 1-5+ cm long. New leaves produce a distinct garlic odor when crushed. The fragrance fades as leaves age, and is virtually non-existent by fall.
Plants usually produce a single unbranched or few-branched flower stalk, although robust plants have been recorded with up to 12 separate flowering stalks. Flowers are produced in spring (usually May) in terminal racemes, and occasionally in short axillary racemes. Some plants produce additional axillary racemes in mid-summer. Flowers are typical of the mustard family, consisting of four white petals that narrow abruptly at the base, and 6 stamens, two short and four long. Flowers average 6-7mm in diameter, with petals 3-6mm long. Fruits are linear siliques, 2.5-6cm long and 2mm wide, held erect on short (5mm), stout, widely divergent pedicels. Individual plants produce an average of 22 siliques (range 2 to 422; Nuzzo unpublished). Siliques contain an average of 16 seeds (range 3- 28; Nuzzo unpublished), arranged alternately on both sides of a papery sinus. Seeds are black, cylindrical (3mm x 1mm) and transversely ridged, and range in weight from 1.62-2.84mg.
Adult plants range in height from 0.05m to 1.5m, and average 1.0m, at the time of flowering. As plants of all sizes are found in the same cluster, plant height is likely a response to competition rather than genetically determined.
Immature plants can be confused with other rosette forming species, especially violets (Viola sp.), white avens (Geum canadense), and Cardamine sp. Alliaria petiolata can be distinguished from these plants by the strong garlic odor in spring and summer. In fall and winter Alliaria can be distinguished by examining the root system. Alliaria has a slender, white, taproot, with a distinctive "s" curve at the top of the root, just below the root crown (Nuzzo, personal observation). Axillary buds are produced at the root crown and along the upper part of the "s".
Chromosome number of 2n=36 has been recorded for European material, and 2n=24 for North American and European material (Cavers et al. 1979).
Excellent illustrations are contained in Cavers et al. (1979). Descriptive characteristics derived from Cavers et al. (1979) and Gleason and Cronquist (1991) except where otherwise noted. There is one other species in this genus (Gleason and Cronquist 1991).