This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Native. Annual. Propagates by seeds.
Time of bloom: July to October.
Seed-time: August to November.
Range: Rhode Island to Nebraska and Arizona, southward to Florida and Mexico. Also in Europe and Asia. Habitat: Gardens, fields, roadsides, and waste places.
Satisfied if the soil is only moderately moist, this weed often makes itself troublesome in fields of Indian corn and other cultivated crops, maturing seeds after the horse-hoe culture has ceased. Stems slender with many spreading branches, one to five feet tall, erect, smooth, and four-sided. Leaves pinnately twice or thrice divided, the segments broadly lance-shaped, deeply cut and toothed, opposite or the uppermost sometimes alternate; petioles slender and grooved. Heads usually numerous, on long, ridged, and angular peduncles; outer bracts of the involucre linear, shorter than the inner ones which are broader and acutely pointed; rays small and few, pale yellow with dark veins; disk-florets yellow and five-lobed. Achenes brown, nearly three-fourths of an inch long, slim, spindle-shaped, four-angled, usually tipped with four rather short, awllike, diverging awns, barbed downward. (Fig. 331.)
Prevent seed production, continuing the tillage of cultivated crops late or hand-pulling the late-flowering remnant of the weed growth. All waste-land plants should be cut several times during the growing season.
Fig. 331. - Spanish Needles (Bidens bipinnata). X 1/4.