Treacle mustard

Erysimum cheiranthoides (treacle mustard, wormseed wallflower) is a slender, erect, herbaceous annual of the mustard family (Brassicaceae), growing 15–100 cm tall (rarely to 150 cm) on a simple or sparingly branched, ridged green stem bearing scattered short two- to three-rayed appressed (medifixed) hairs. The alternate leaves are lanceolate to elliptic, 2–11 cm long, narrowed to short petioles below and becoming sessile above, with margins that vary from entire to coarsely toothed. From June to August the stem terminates in a long, narrow, gradually elongating raceme of small bright yellow four-petalled cruciferous flowers 5–12 mm in diameter on slender ascending pedicels. The fruit is a slender erect cylindrical siliqua 1–3 cm long (rarely to 5 cm) held away from the stem on short stalks and splitting at maturity to release numerous small pale to dark brown seeds. Native to temperate Eurasia and now widely naturalised across North America, New Zealand, and Argentina, it grows from sea level to about 3,000 m elevation in disturbed open habitats including arable fields, gardens, roadsides, riverbanks, and dry stream beds.