Heart-pod hoary cress

Lepidium draba (heart-pod hoary cress, whitetop) is a perennial herb of the mustard family (Brassicaceae) growing 10–80 cm tall from a vigorously creeping, deep horizontal rhizome that gives rise to extensive vegetative colonies. The stout, erect to spreading, often branched stems are sparsely covered with short greyish hairs giving the foliage a softly hoary appearance. The basal leaves are 4–10 cm long, shortly stalked, lance-shaped to ovate, and shallowly toothed, while the alternate stem leaves are smaller, oblong, sessile, and clasp the stem with arrow-shaped (sagittate) auriculate bases. From late spring to early summer the stems terminate in dense, slightly domed, corymb-like clusters of numerous small four-petalled cruciferous white flowers, each petal 3–5 mm long and about twice the length of the sepals. The fruit is a small, hairless, distinctly heart-shaped (cordate) silicle 3–5 mm across, indehiscent or only tardily splitting, containing two small reddish-brown seeds. Native to dry, disturbed, nutrient-rich open ground across Europe and western to central Asia, hoary cress has been widely introduced and is regarded as a noxious invasive weed in much of North America, Australia, and elsewhere.