Coronopus is a small, fast-growing, and fast-maturing, non-native plant. It takes advantage of disturbed areas and often grows quickly to maturity hidden by a larger plant. As such it requires great diligence to treat effectively. Infested areas usually have to be mostly devoid of other vegetation for Coronopus to be detected prior to seeding. The heavily lobed leaf shape of Coronopus is unique on Kure, making it relatively easy to identify. Coronopus also has a very strong and distinctive odor. If you learn its smell you can sometimes discover infestations by the nose! Coronopus is somewhat seasonal, most abundant in the winter but occurring year-round in some areas.
Coronopus cotyledons (left) are very long and slender, widening gradually from the base to a rounded tip. They can be distinguished from Chenopodium (right), Amaranthus, and Ciclospermum, which have a wide base and narrow towards the tip.
The first true leaves of Coronopus are rounder and may begin to show dentate (toothed) margins. In the photo below left both of the smaller leaves have a single subtle tooth. After the 4-leaf stage, the characteristic toothed leaf structure is apparent.
Coronopus seeds come packaged in woody corrugated shells that hinge open much like a clam. At more than a millimeter in diameter they are actually relatively large for the size of the plant. If you dig in Coronopus infested areas these seeds can be found in horrifying numbers.
Description: Foetid annual or biennial with short-loved rosette and procumbent to ascending glabrous or sparsely hairy stems to 40 cm long. Leaves 1-2-pinnatifid, narrow-oblong to ovate, glabrous except for sparse spreading hairs on petioles of some basal leaves; rosette leaves petiolate, 6-10 x 1.5-2 cm; stem leaves smaller, usually pinnatifid, rarely 2-pinnatifid, 1-4 x 0.5-1.5 cm. Racemes (1)-3-5 cm; rachis with sparse spreading hairs; pedicels spreading, 2.5-4 mm long. Sepals c. 0.5 mm long. Petals white, less than sepals or 0. Stamens usually 2, rarely 4. Silicle broader than long, emarginate, c. 1.5 x 2-2.5 mm; valves finely warty or ridged, separating at maturity; stigma minute, almost sessile in apical notch of silicle. Seed comma-shaped, light brown, c. 1 mm long. (Webb et al., 1988; p. 419)