Introduced. Annual and winter annual. Propagates by seeds. Time of bloom: As soon as snow melts in spring, beginning on autumn plants already budded; spring seedlings bloom later and continue until fall. Seed-time: Autumn plants ripen seed in early July. Spring seedlings mature fruit in August. Both continue seeding until winter. Range: Northern and Middle Western United States; in all Canadian provinces, but most abundant in Manitoba and the Northwest Territory. Habitat: Grainfields, meadows, roadsides and waste places.
This weed is perhaps the most hated enemy of the western farmer, and is considered to have caused greater loss than any other intruder in the grain fields of Minnesota, the Dakotas, and western
Canada. It is immensely prolific and its seeds have long vitality. Cold does it no harm and chemical sprays that kill other Mustards do not in the least affect it. Other crops cannot crowd it out, for it is the better crowder, seeding in dense timothy sod almost as readily as in a mellow fallow. Blooming "from snow to snow" and constantly developing fruit, it requires and absorbs much of the food and moisture in the soil, starving the accompanying crops almost to worthless-ness. T. N. Willing, Chief Weed Inspector of the Northwest Territory, says, "It will pay well to drop all other work and fight this weed when it is first noticed." (Fig. 120.)
Stem six inches to two feet tall, smooth, bright green, often simple but usually branching at the top. Root-leaves long oval, broadest at tip, with long petioles; stem-leaves lance-shaped and clasping with a pair of pointed ears at the base; all leaves coarsely toothed. When bruised, the plant exhales a most disgusting garlicky odor; if it is eaten by milch cows, the dairy products are spoiled. Flowers clear white, very small, in thick, flat terminal clusters; beginning to mature at the bottom of the cluster, they leave behind a long raceme of the fruits, standing out on slender, wiry, upcurved pedicels about as long as themselves. Silicles flat, about three-fourths of an inch across, pale green at first, broadly winged at the sides, notched at the top, two-celled, the division being across the narrowest part, as in Shepherd's Purse; each side contains two to eight seeds. As the pods ripen they turn to a rusty orange color, making the weed very conspicuous when growing with grain or clover. Seeds deep reddish brown, flattened ovoid, roughened with fine curved ridges about a central groove.
Fig. 120. - Penny Cress (Thlaspi arvense). X 1/5.
A very few of these seeds ground by accident with a grist of wheat, ruins the flour, and grain that contains them is very sharply cut in price.
Sow clean seed. If the infestation is new, hand-pull and destroy all plants before any fruits mature, even though the task be very strenuous. In grain fields, if seeds have been allowed to ripen, burn over the stubbles for the purpose of destroying those that have fallen on the ground. Give surface cultivation in order to encourage germination of such seeds as are in the soil, and plow the young plants under while still in the rosette stage of growth. But never turn under any plants bearing developed pods, even though they may be green, for they go on ripening on the stalks, when under the warm soil, quite as well as or better than above it. Autumn-grown plants are the most obnoxious, since they come earliest into bloom and fruit the next season, and every effort should be made to kill as many of these as possible. Spring seedlings may be dragged out of grain fields with small-toothed weeding harrows, beginning when the grain is only about three inches high and repeating the operation once or twice afterwards - a treatment which greatly benefits the crop at the same time that it kills the weeds. If practicable, put the ground to a cultivated crop, which should be given very thorough tillage, before the land is used again for grain.