Biennial or perennial herb 20 - 80 cm tall Stem: upright, with stiff, appressed, pick-shaped hairs. Usually single but occasionally two or three arise from the base. Flowers: in loosely branched clusters, white or yellowish white, 3 - 5 mm long. Petals four. Stamens six. Fruit: a narrow pod, upright or pressed against the stem, 3 - 5 cm long, 0.5 - 1 mm wide, linear, flat. Seeds in one row. Basal leaves: short-stalked, oblong, hairy, in a rosette. Stem leaves: alternate, upright, clasping the stem, stalkless, 1 - 3 cm long, oblong to lance-shaped, toothed on lower leaves, becoming non-toothed on upper leaves, hairless or nearly so.
Similar species: Regular A. hirsuta does not have appressed hairs.
Flowering: mid-May to late June
Habitat and ecology: Extremely rare in the Chicago Region because of the scarcity of its special habitats. It grows on shaded, moss-covered limestone boulders along the borders of streams in small rock canyons. It also grows on the vertical walls of limestone canyons. Another habitat is shaded floodplains. Near Romeoville in Will County, Illinois it is found on a disturbed gravelly prairie. This variety has also been observed growing in sandy soil.
Occurence in the Chicago region: native
Etymology: Arabis comes from the Latin word for Arabia (where some species of these herbs originated). Hirsuta means hairy. Adpressipilis means "with adpressed (flattened) hairs."
Author: The Morton Arboretum