Family Cruciferae or Brassicaceae.
Not common, but locally abundant. This is a summer-blooming mustard with small yellow flowers; tall when it’s allowed to grow, but it will bloom short in a mowed lawn. When the (annual) plant is finished, it dries up and detaches from its roots, tumbling along the ground. Unlike a tumbleweed, Tumble Mustard is well and truly dead when it tumbles, but while the corpse travels it flings its seeds everywhere. These plants were blooming in Mount Oliver in late August.
Gray describes the genus and the species:
SISYMBRIUM [Tourn.] L. HEDGE MUSTARD. Pod terete, flattish or 4-6-sided, the valves 1-3-nerved. Seeds oblong, margin less, in 1 or 2 rows in each cell. Cotyledons incumbent. Calyx open.—Flowers small, white or yellow. Pubescence spreading. (Latinized from an ancient Greek name for some plant of this family.) Ours are mostly annuals or biennials.
Leafy-stemmed; leaves pinnate or pinnatifid.
Pods linear-cylindric, longer.
Pods firm, 6-10 cm. long.
S. altíssimum L. (TUMBLE MUSTARD.) Tall; leaves deeply pinnatifid with narrow segments, flowers pale yellow; pods rigid, very long, divergent, hardly thicker than the short thickish pedicels.—Waste places, roadsides, etc., a recent immigrant, locally abundant as a pernicious weed. (Nat. from Eu.)