This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Introduced. Annual. Propagates by seeds. Time of bloom: May to September. Seed-time: June to October. Range: Newfoundland to Vancouver Island and southward to Florida and Texas. Most abundant on the Pacific Coast and also in the Atlantic States. Habitat: Gardens, lawns, fields, meadows, pastures, waste places.
Like the Chickweed, this plant has accompanied the European emigrant to every part of the world, its seeds mingled with those of better plants. Where abundant it is not only a troublesome weed, but also dangerous, for it is poisonous, all parts of it having "pronounced diuretic and narcotic properties," and it is said to be especially injurious to horses, sometimes fatally so. When growing in pastures cattle usually reject it and it is left to reproduce itself.
Stems six to eighteen inches long, smooth, four-angled, weak, and slender, some prostrate and some ascending, branched and spreading. Leaves opposite or sometimes in whorls of three, or those near the top sometimes alternate, a quarter inch to a half-inch long, ovate, entire, sessile or slightly clasping, black-dotted on the under side. Flowers lifted on very slender, almost hairlike, axillary peduncles; the calyx has five narrow, keeled, and pointed lobes, united at base and persistent; corolla usually bright scarlet but sometimes salmon-color, occasionally white, about a third of an inch broad, wheel-shaped, five-lobed, with each lobe minutely fringed at the tip with fine, glandular hairs; the five stamens have hairy filaments. The flowers open only in the brightest sunshine, closing quickly if clouds obscure the light and usually by four o'clock in the afternoon even in good weather. Capsule membranaceous and one-celled, nearly globular, smooth, many-seeded, opening by a circular line near the top which falls off like a lid; the pedicels are recurved as they ripen and the small, dark, triangular, finely pitted seeds are emptied out. Seed matures within three weeks from the time of bloom and retains its vitality in the soil for at least two years. (Fig. 217.)
Fig. 217.- Scarlet Pimpernel (Anagallis ar-vensis). X 1/2.
In gardens and fields, early and persistent cultivation is necessary in order to prevent the weed from ripening and distributing its seeds. Badly infested pastures and meadows should be put under cultivation; thick patches should be burned over for the purpose of destroying seeds on the surface before breaking the ground.