This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Introduced. Annual or biennial. Propagates by seeds.
Time of bloom: April to October.
Seed-time: May to November.
Range: New Brunswick to Ontario and Minnesota, southward to Florida and Arkansas. Habitat: Cultivated ground, waste places.
This weed flourishes best in cool weather, dying down in the heat of midsummer but recovering in autumn and maturing a late crop of seeds; autumn seedlings develop fruit very early in the spring, so that the soil is fouled with two abundant sowings each year. Stems six to eighteen inches long, slender, square, branching from the base and also from the lower axils, weak and spreading on the ground. Leaves opposite, rounded, deeply scallop-toothed, sparsely hairy, the lower ones with short petioles, the upper ones sessile and clasping. Flowers in small axillary and terminal clusters; calyx hairy, with five erect, awllike teeth: corolla-tube slender, with the upper lip erect, entire, and bearded, dark red, the lower one three-lobed, white, spotted with purple; stamens ascending against the upper lip, the anterior pair the longer. The flowers contain much nectar and honeybees are frequent visitors. Seeds four long, ovoid nutlets, dark brown, specked with white dots. These seeds are long-lived and tillage should begin early and be continued late, in order to prevent their development and distribution. (Fig. 246.)
Fig. 246. - Henbit (Lamium amplexicaule). X 1/4
For small areas destruction of seedlings by hoe-cutting in autumn or early spring; in fields thorough cultivation followed by heavy seeding with clover, which will crowd out the weed.