Field Pennycress (Thlaspi arvense)
An unimposing little weed, but it delights children by producing round, flat seedpods that look like coins. This one grew in a meadow near Cranberry, where it was blooming and already seeding in the middle of June.
Gray describes the genus and the species:
THLASPI [Tourn.] L. PENNY CRESS
Pod orbicular, obovate, or obcordate, flattened contrary to the narrow partition, the midrib or keel of the boat-shaped valves extended into a wing. Seeds 2-8 in each cell. Cotyledons accumbent. Petals equal.—Low plants, with root-leaves undivided, stem-leaves arrow-shaped and clasping, and small white or purplish flowers. (Name from thlaein, to crush, from the flattened pod.)
T. arvense L. (FIELD P. or MITHRIDATE MUSTARD.) Smooth annual; lower leaves wing-petioled, the upper sagittate-clasping; broadly winged pod 1.2 cm. in diameter, deeply notched at top; style minute. Waste places; not common, except along our northern borders, where too abundant and called “FRENCHWEED.” (Nat. from Eu.)