Yellow rocket, Barbarea vulgaris,. R. Br., [BARVU, barbarée vulgaire, Herb barbara, Herb of St. Barbara, Winter cress, herbe de Sainte-Barbe, cresson d'hiver] Usually biennial or perennial, but some plants flowering, setting seed and dying after their first growing season; reproducing only by seed. Root system on young plants a taproot, becoming much-branched and fibrous with age; young plants producing a rosette of smooth, shiny, dark green leaves during the first year, these staying green throughout the winter or turning slightly purplish by spring; stems forming in the spring, 1 to several stems per plant, erect, 20 - 80 cm (8 - 32 in.) high, branched; rosette and lower stem leaves long-stalked, hairless, divided into 1 large rounded terminal lobe and smaller lobes along each side; upper leaves alternate (1 per node), short-stalked or stalkless, coarsely toothed, or without teeth, or sometimes deeply lobed, but always with a pair of basal lobes which clasp the stem; flowers similar to Wild mustard but golden-yellow and somewhat smaller, 10 - 16 mm (2/5 - 2/3 in.) across; the seedpods and their stalks either nearly erect and overlapping one another forming a dense a raceme as in the typical botanical variety (B. vulgaris var. vulgaris) or as in the second botanical variety (B. vulgaris var. aracuata (Opiz. Fries), the stalks spreading with the seedpods standing outwards or curving upwards and usually not over-lapping one another, thus forming an open raceme; stalks 3 - 6 mm (1/8 - ¼ in.) long; pods 1.5 - 3 cm (3/5 - 1¼ in.) long with a slender, seedless beak 2 - 3 mm (1/12 -1/8 in.) long; seeds egg-shaped 1 - 1.5 mm (1/25 - 1/16 in.) long, metallic greyish-brown. Flowers from mid-May to early July and sometimes again briefly in late autumn.

Yellow rocket is common throughout most of Ontario in meadows, pastures, waste areas, roadsides, railways and along watercourses, being especially common in moist rich soil and is apparently still spreading rapidly in such areas. Its occurrence in grainfields is increasing.

It is similar to Wild mustard and often mistaken for it, although the two are easily distinguished. Yellow rocket is a perennial or biennial, so flowers much earlier in the season than Wild mustard and has smaller and deeper golden-yellow flowers. Its leaves are dark glossy green or somewhat purplish, hairless, and distinctly clasp the stem, and the seedpod is tipped by a very slender beak which does not have a seed in its base.

(Source: Ontario Ministry of Agriculture and Food, Publication 505, Ontario Weeds)