Stems pilose, trichomes appressed, malpighiaceous, or minutely stalked, forked, plants rarely glabrescent. Basal leaves: blade surfaces pubescent, trichomes sessile, forked, and/or stellate. Fruits (4-)4.5-6(-6.5) cm; style 0.5-1.3 mm.

Flowering Mar-Jun. Ravines, pastures, cliffs, calcareous talus, dolomite glades, rich woods, bluffs, rocky ledges; 0-300 m; Ont.; Ill., Ind., Iowa, Kans., Mo., N.Y., Ohio, Pa., Tenn., Va.

R. C. Rollins (1941, 1993) recognized both pycnocarpa and adpressipilis as distinct varieties of Arabis hirsuta; G. A. Mulligan (1996) treated adpressipilis as a synonym of A. hirsuta var. pycnocarpa. In my opinion, the differences in trichome morphology of the stems and leaves are significant enough to justify recognition of infraspecific taxa of one species.

Infrequent to rare in the state and probably absent from some of the central counties. It grows in sandy soil in alluvial bottoms, in crevices of rocks, and on rocky slopes and high banks of streams. Since all of my specimens are from the borders of streams and lakes, its affinity for them is apparent. This species is variable in its pubescence which is sometimes restricted to the lower part of the plant. Its pubescence, strict habit, and its habit of sending up several erect branches from the base usually identify it.

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