Noxious perennials to 0.4(0.5) m tall, with creeping horizontal roots that vigorously produce new plants. Cardaria species are problematic in natural areas and many crops, especially irrigated crops such as alfalfa and sugar beets. Not until 1933 were 3 distinct Cardaria species known to exist in North America. Consequently, taxonomic references to Cardaria species in articles published prior to 1933 are unclear. Although similar, each species differs in chromosome number and herbicide resistance. All are self-incompatible. Lens-podded (2n = 80) and hoary cress (2n = 62, 64) can hybridize, but only first generation hybrids survive under natural conditions. Lens-podded and hairy whitetop were introduced from Central Asia. Hoary cress (2n = 16) was introduced from Eurasia

SEEDLINGS :Develop tap roots to a depth of 25 cm and lateral roots with shoot buds within 1 month.Hoary cress: Cotyledons oval to elliptic, 7-9 mm long, ~ 2.5 mm wide, unequal, pale gray-green, with peppery taste. First leaves ovate to oblong, dull, scaly, somewhat larger than the cotyledons, often with slightly wavy margins. Subsequent leaves resemble first leaves, but sometimes have short fine hairs along the margins. First and subsequent leaves have bases tapered to petioles equal to or longer than the length of the blades. No descriptions are available for lens-podded and hairy whitetop. However, seedlings of these species likely resemble those of hoary cress.

MATURE PLANT: Stems +/- erect, sparse to densely covered with simple short hairs. Leaves alternate, gray-green, variable, obovate, (ob)lanceolate, oblong to elliptic. Surfaces, especially lower, sparsely to densely covered with simple, short white hairs. Margins irregularly toothed to entire. Basal leaves short-stalked. Upper leaves sessile, with rounded-acute- to acute-lobed bases that clasp the stem. Leaves to 9 cm long and 4 cm wide, sometimes smaller. Leaf base lobes often rounded-acute. Hairs sparse to dense

ROOTS and UNDERGROUND STRUCTURES: Plants develop extensive systems of persistent, deep, vertical and horizontal roots that vigorously produce new shoots at irregular intervals. Root fragments can generate new plants. Vertical roots can penetrate the soil to depths of 2 m or more. Roots can account for 75% of the total plant biomass and, as a result, store considerable amounts of carbohydrates. Carbohydrate reserves typically accumulate to maximum levels by mid-summer and are minimal in early to mid-spring. Roots survive cold winter climates and periods of drought. Mycorrhizal associations do not develop.

FLOWERS: Infloresecences often +/- flat-topped (compound corymbs). Flowers fragrant, numerous, 4-petaled, white. Insect-pollinated