Horsenettle is a persistent perennial that reproduces by seed and by its extensive root system. It is not a true nettle but belongs to the Nightshade (Solanaceae) family, which includes potatoes, tomatoes, and eggplant.
The stems are either simple or multi-branched and grow 1 to 4 feet (30 to 120 cm) tall. They are covered with prickly hairs about 1/8 inch (3 mm) long and a scattering of sharp yellow or white spines 1/4 to 1/2 inch ( 6 to 12 mm) long.
Leaves are alternately arranged, green, 1 to 7 inches (2.5 to 18 cm) long, and about half as wide. Margins are coarsely lobed, with one to four pairs of large pointed teeth per leaf. Leaf surfaces are rough, covered with tiny hairs, and have long spines on the midribs, veins, and petioles.
Horsenettle blossoms from May until September, with most of the flowers blooming thirty days after the plants emerge. The star-shaped blossoms are about 1 inch (2.5 cm) across and look like potato flowers. The five petals are pointed and may be blue, violet, or white. The male and female flower parts together form a bright yellow cone-like cluster in the center of each blossom.
Fruits are set about thirty days after flowering. They look like miniature tomatoes—smooth, juicy, round, and 3/8 to 3/4 inch (9 to 18 mm) in diameter. Green at first, they tum yellow when ripe and become wrinkled after drying. Inside the fruit, a foul-smelling pulp surrounds numerous flat, round, yellow seeds 1/16 to 1/8 inch (1.5 to 3 mm) across. The average number of seeds per fruit is about 85, and one plant may produce as many as 100 fruits.
Most of the seeds are dormant when shed from the plant. This dormancy is broken by an overwintering period, during which the gelatinous substance surrounding the seed washes off. Alternating temperatures from 68 to 86 F (20 to 30 C) are necessary for germination. Seedlings emerge best in light-textured, well-drained soil. They can emerge from as deep as 4 inches (10 cm), but optimum depth is between 1/2 inch and 2 inches (1.2 and 5 cm). Seeds are rarely carried far from the parent plant because birds avoid the poisonous berries, and the berries are too heavy to be windborne. However, seeds may be spread throughout the field by grazing animals, although livestock generally avoid eating horsenettle.
Horsenettle is a persistent weed because of its extensive perennial root system. The taproot often reaches 8 feet (2.5 m) into the soil. Roots in the upper 18 inches (45 cm) can extend 4 feet (120 cm) horizontally from the main plant. Horsenettle spreads more quickly in cultivated land than in undisturbed areas because tillage distributes pieces of root throughout fields. New plants can emerge from rootstocks buried 12 inches (30 cm) below the soil surface, and pieces of root less than 1/4 inch long can produce a new plant. Buried root fragments have remained viable for ten years, sprouting when uncovered. No amount of disking and plowing seems to cut horsenettle roots small enough or bury them deep enough to suppress this determined weed.