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: Eastern Canada and United States to Minnesota, southward to the Gulf of Mexico. Habitat: Moist soil; damp grass lands, waste places, and along streams and ditches.
A pest of lowland clover fields, as it ripens its earlier seeds about the time of clover cutting. Stems two to five feet tall, somewhat hard and woody when old, and of rather branching and sprawling habit, the lower part smooth but the topmost leaves and the flower-stalks set with gland-tipped hairs. Leaves two to ten inches long, lance-shaped, with short petioles; sheathing stipules smooth and thin. Flowers in short, crowded, erect spikes, cylindric, often blunt at the end, deep pink; they are frequently affected with a smut or fungus which turns the heads into a mass of purple spores, destroying the fruits so that "purplehead" is a benefit from the farmer's point of view. Achenes black, lens-shaped, smooth, and shining. (Fig. 59.)
Fig. 59. - Pennsylvania Persicary (Polygonum pennsylvanicum). X 1/4.
Cut closely or pull before any seeds have matured. Rankly infested ground should be put under cultivation before being again used for clover or grass. Good drainage is an assistance in subduing this weed, for it likes the soil to be moist.