This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Native. Perennial. Propagates by seeds. Time of bloom: June to September. Seed-time: August to November.
Range: Ontario to Minnesota and Wyoming, southward to Tennessee, Texas, and New Mexico. Habitat: Dry plains and prairies.
The range of this plant is increasing, mostly by the agencies of impure seed and baled hay. Stem ten to thirty inches tall, rather stout, obtusely four-angled, erect, simple or with a few branches above. Leaves ovate, pointed or sometimes obtuse, double-toothed, sessile or the lower ones with short petioles; the whole plant clothed with fine, white-woolly hair. Spikes very dense, rather stout, usually solitary but sometimes several in a panicle, becoming six inches to a foot in length when fruiting; corolla purple, large for a Vervain, being more than a quarter-inch long and the five spreading lobes about as broad. Its dense flowering habit makes the plant very productive.
Only by a short rotation of cultivated crops is it practicable to rid the ground of the perennial roots and the dormant seeds of this weed.