This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Introduced. Annual. Propagates by seeds.
Time of bloom: May to September.
Seed-time: June to October.
Range: Throughout North America except the extreme North.
Habitat: Grain and clover fields, meadows, waste places.
A very noxious weed because of its immense productiveness -more than fifteen thousand seeds having been taken from a single thrifty plant - and also because of the exceedingly long vitality of the seed when in the soil.
Stem one to three feet tall, erect, branching toward the top, roughened with short, stiff hairs. Lower leaves pinnatifid, with the terminal lobe large, and the few lateral lobes small, the petioles rather stout and short; upper leaves narrowly rhombic, sessile or nearly so; all irregularly toothed and somewhat hairy; small blotches of brownish red show on the stem at the junction with the leaves. Flowers in racemose clusters at the ends of stem and branches, bright yellow, fragrant, each about a half-inch broad, the calyx-lobes spreading; they begin to open at the bottom of the cluster, which lengthens as the season advances and the pods form and ripen, so that there may be emptied pods below and forming buds above. Silique, or pod, one to two inches long, round and somewhat constricted between the seeds, veined and ribbed, and tipped with a long, two-edged beak; it may contain three or four to a dozen or more seeds, one of which is usually sticking in the beak when the pod splits. Seeds globular, dark reddish brown, under a lens seen to be delicately pitted.
They are a common impurity of grass and clover seeds. (Fig. 128.)
In grain fields seedlings should be harrowed out, with one of the light harrows known as weeders, when the grain is but a few inches tall; or later, but before the grain begins to head, the Mustard may be almost entirely destroyed by the use of Iron sulfate or Copper sulfate spray. Stubbles should be surface-cultivated immediately after harvest in order to stir into growth such seeds as may be lying on the ground; the young plants to be plowed under, or they may be profitably grazed off by sheep. Plants of waste places, fence rows, and roadside should be hand-pulled or closely cut when the flowers are first noticed.