Garlic mustard (Alliaria petiolata) is a biennial flowering plant in the Mustard family, Brassicaceae. It is native to Europe, western and central Asia, and northwestern Africa, from Morocco, Iberia and the British Isles, north to northern Scandinavia, and east to northern India and western China (Xinjiang). First year of growth plants form attractive clumps of round shaped, slightly wrinkled leaves, that when crushed smell like garlic. The next year plants flower in spring, producing cross shaped white flowers in dense clusters, as the flowering stems bloom they elongate into a spike-like shape. When blooming is done, plants produce upright fruits that release seeds in mid summer. Plants are often found growing along the margins of hedgerows, giving rise to the old British folk name of Jack-by-the-hedge. Other common names include Garlic Root, Hedge Garlic, Sauce-alone, Jack-in-the-bush, Penny Hedge and Poor Man's Mustard. The genus name Alliaria, "resembling Allium", refers to the garlic-like odour of the crushed foliage.
It is a herbaceous biennial plant (sometimes an annual plant) growing from a deeply growing, thin, white taproot that is scented like a horse-radish. Plants grow from 30-100 cm (rarely to 130 cm) tall. The leaves are stalked, triangular to heart-shaped, 10-15 cm long (of which about half being the petiole) and 2-6 cm broad, with a coarsely toothed margin. In biennial specimens, first-year plants appear as a rosette of green leaves close to the ground; these rosettes remain green through the winter and develop into mature flowering plants the following spring. The flowers are produced in spring and summer in button-like clusters. Each small flower has four white petals 4-8 mm long and 2-3 mm broad, arranged in a cross shape. The fruit is an erect, slender, four-sided pod 2-7 cm long, called a silique, green maturing pale grey-brown, containing two rows of small shiny black seeds which are released when the pod splits open. Some plants can flower and complete their life-cycle in the first year. A single plant can produce hundreds of seeds, which scatter as much as several meters from the parent plant. Depending upon conditions, garlic mustard flowers either self-fertilize or are cross-pollinated by a variety of insects. Self-fertilized seeds are genetically identical to the parent plant, enhancing its ability to colonize an area where that genotype is suited to thrive.
The insects and fungi that feed on it in its native habitat are not present in North America, increasing its seed productivity and allowing it to out-compete native plants. A study published in 2006 concluded that Garlic Mustard harms mycorrhizal fungi that some North American plants, including native forest trees, require for proper growth. Additionally, because White-tailed Deer rarely feed on Garlic Mustard, large deer populations may help to increase its population densities by consuming competing native plants. Trampling by browsing deer encourages additional seed growth by disturbing the soil. A complication to the eradication of Garlic Mustard from an area is the longevity of viable seeds in the ground. Seeds contained in the soil can germinate up to five years after being produced.