Hairy rockcress

Arabis hirsuta (hairy rockcress) is a slender biennial or short-lived perennial herb of the mustard family (Brassicaceae) growing up to about 75 cm tall, usually with a single unbranched, sparsely leafy stem clothed in stiff, branching, bristly hairs. The plant produces a flat overwintering basal rosette of obovate to oblanceolate leaves with shallowly toothed margins, while the alternate stem leaves are smaller, narrowly oblong, sessile, and clasp the stem with arrow-shaped (sagittate) auricles. From June to August the stem terminates in a long, tightly elongating raceme of small white four-petalled cruciferous flowers about 5–7 mm across, the petals roughly twice as long as the sepals. The fruit is a long, slender, cylindrical, erect siliqua 1.5–5 cm long held pressed close against the stem and containing two rows of small reddish-brown, narrowly winged seeds. Native to temperate Eurasia and parts of North Africa and widely naturalised in North America, hairy rockcress grows on dry, well-drained calcareous soils on chalk and limestone slopes, sand dunes, hedgebanks, old walls, and rocky outcrops.