Hemp dogbane is a member of the Dogbane family (Apocynaceae). A perennial broadleaved weed, it reproduces by seed and by its spreading root system. The seeds are extremely small and germinate best from a depth of 3/8 inch (9 mm), but in fine-textured soil may emerge from nearly 2 inches (5 cm) deep. Most seedlings emerge in spring, but shoots from the rhizomes may appear throughout the growing season, even into early fall. Seeds remain viable up to two years in soil. Germination may occur at any temperature between 50° and 104°F (10° and 40°C), but the optimum range for seedling development is between 68° and 77°F (20° and 25°C).
The plant emerges as a single stem but soon divides to form a bushy plant 1 to 5 feet (30 to !50 cm) tall. Stems are smooth, erect, and red to green, often turning entirely red in autumn. When broken, the stems release a milky white sap.
Leaves are arranged in opposite pairs and have slightly pointed tips and a narrow oval shape. They grow 2 to 5 inches (5 to 13 cm) long and ¾ to 1¾ inches (2 to 4 cm) wide. Margins and upper leaf surfaces are smooth; lower surfaces may be slightly downy or entirely smooth. The upper surface is a dull dark-green and has a prominent network of white veins.
Flowers appear in mid- to late summer. They form in round clusters called "cymes," about 2 inches across, in which the central flowers open first. Each urn-shaped flower is less than ¼ inch across and consists of five greenish-white petals.
Some of the flowers become fertilized and in late summer develop into pairs of long, slender green pods called follicles. The pods are sharply pointed, 4 to 8 inches (10 to 20 cm) long and only about ¼ inch (3 mm) in diameter. When dry, they split lengthwise, each half twisting in a spiral and releasing the tiny flat seeds. Attached to each seed is a pappus, or tuft of silky hairs, that is easily carried away by the wind. Each plant produces an average of 20 pods, with about 200 seeds in each pod.
The weed's persistence is due to its perennial root system rather than its seeds. Ten-day-old seedlings already have perennial capabilities; they can resprout if cut off at ground level. Roots grow laterally and send up new shoots, so one hemp dogbane plant can quickly tum into a large patch. In fields under cultivation, tillage chops the roots into small pieces. A root segment less than one inch long with a single bud can produce a new plant.