Thlaspi arvense aka Common Penny Cress

Thlaspi arvense, or field pennycress, belongs to the mustard family. This annual grows wild across temperate Eurasia, its native range. It is also native to Britain and Ireland. It now occurs throughout North America. It grows in fields, beside roads, and on disturbed ground. Other names include stinkweed, wild cress, and boor's mustard.

Plants grow upright and reach 1–2 feet [0.3–0.6 metres]. They grow fast and reach maturity within one year. A shallow root system makes them easy to pull. Young plants first form a low rosette of green leaves. These leaves have rounded tips and smooth or wavy edges. They fade as the flowering stem grows. The stem leaves alternate and clasp the stem without stalks. Their edges bear rounded or pointed teeth. The hairless stems are green, ribbed, and sometimes winged. They may branch near the top. Crushed leaves give off a faint garlic scent.

Small white flowers form long racemes at the stem tips. The RHS gives May to July as the flowering season. North Carolina State lists spring bloom. Each flower has four petals and six pale anthers. The flowers have no scent. New flowers open above while pods develop below. The flat, round to oval pods have a notched tip. They reach about 0.04 feet [0.013 metres] long. Each pod has two sections, each holding up to eight seeds. The shell turns tan and papery as it dries. It then splits and releases nearly black seeds.

Grow field pennycress in full sun or partial shade. It needs moist soil that drains well. The RHS favors rich loam; North Carolina State notes tolerance of poor soil. It also grows in clay, sand, and chalk. It accepts acid, neutral, or alkaline soil. The RHS rates its hardiness H5 for UK conditions. Seed provides the means of propagation, and plants self-sow freely. They need no pruning. The RHS reports few pest or disease problems.

Field pennycress can suit informal gardens and wildflower meadows. Small bees and flies may visit its nectar. Some white butterflies and moths lay eggs on the leaves. Yet its fast growth and free seeding can make it weedy. Grazing animals tend to avoid its mustard-garlic taste. Cows that eat it can produce tainted milk. The evidence does not establish safe human food use. The ASPCA lists it as poisonous to horses. Its isothiocyanates can cause colic, diarrhea, and skin inflammation.