Lepidium didymum L. (also known as Coronopus didymus (L.) Sm.) is well known from southern Ontario and northern Ohio, and will doubtless turn up in Michigan at some point. It is a mostly prostrate to spreading weed of lawns and sidewalk cracks, with deeply pinnately divided leaves and distinctive rugose fruits. A similar but less common species, L. coronopus (L.) Al-Shehbaz (Coronopus squamatus (Forssk.) Asch.), has also been collected along the Detroit River in Windsor, Ontario. It also has rugose fruits, but larger and ridged.
1. At least upper cauline leaves sessile and auriculate, sagittate, or clasping at base.
2. Flowers yellow; seeds winged; uppermost leaves cordate-clasping; plants glabrous above.
2. Flowers white; seeds wingless; uppermost leaves auriculate to sagittate; plants puberulent or pubescent above.
3. Plants annual, not rhizomatous; fruit broadly winged at apex; racemes elongated in fruit.
3. Plants perennial, rhizomatous; fruit wingless at apex; racemes not elongated in fruit.
4. Fruit pubescent, inflated, spherical, valves not veined.
4. Fruit glabrous, flattened, cordate, valves veined.
1. Cauline leaves petiolate or subsessile, never auriculate, sagittate, or clasping at base.
5. Fruit (4–) 5–7 mm long, 3–5.5 mm wide; upper cauline leaves deeply lobed or pinnatifid.
5. Fruit 1.5–3.2 (–4) mm long, 1.5–3 mm wide; upper cauline leaves entire or dentate.
6. Plants perennial; stamens 6.
6. Plants annual; stamens 2.
7. Fruit elliptic; fruiting pedicels puberulent all around; basal leaves (often withered in fruit) 1-or 2-pinnatisect.
7. Fruit orbicular or obovate; fruiting pedicels glabrous at least below; basal leaves dentate or pinnatifid.
8. Fruit obovate, widest above middle; petals absent or often rudimentary; rachis of raceme puberulent with cylindrical or clavate hairs.
8. Fruit orbicular, widest at middle; petals present or rarely rudimentary; rachis of raceme puberulent with curved hairs, rarely glabrous.