Question: I'm worried that a weed that grows on my property may be horse nettle, and I'd like to know more about what this plant can do and how to identify it. I know it's poisonous for horses, but how much do they have to ingest for it to be harmful? Is there a cumulative effect?
Answer: Horse nettle (Solanum carolinense), also referred to as Carolina horsenettle or bull nettle, is a member of the nightshade family and is found in most of the contiguous United States but especially in the Central and Eastern states.
The plant grows up to two feet tall, with an erect, branching structure; the leaves are alternate and can grow to four to six inches long, with irregular wavy or lobed margins. The leaves and stems are often covered with fine hairs and prickly spines. The flowers, which appear at the top of the plant from June through August, are three-quarters to one inch across and range from light purple, blue to white. The plant produces round, tomato-like berries that are half an inch in diameter and change from green to yellow as they ripen.
Horse nettle, like many plants in the nightshade family, contains solanine, a glycoalkaloid that irritates the oral and gastric mucosa and affects the autonomic nervous system, which controls various internal organs. The glycoalkaloids act on the digestive system to cause excessive salivation, colic and diarrhea or constipation. These signs may be followed by depression, weakness, depressed respiration, dilated pupils, collapse and death if horse nettle is eaten in large amounts.
All parts of the plant are poisonous, but toxicity varies depending on growing conditions. The glycoalkaloid levels are higher in the fall than in the spring, and green, unripe berries are more toxic than ripe or dried berries. Toxicity is reduced (but not eliminated) when the plant is dried.
Any livestock---including cattle, sheep, goats and pigs as well as horses---may be poisoned after eating large quantities of horse nettle. Horses tend to avoid the plant because it is distasteful, and they are unlikely to eat enough to cause serious problems unless the weed is rampant in their pasture or they have no other suitable forage. The amount of horse nettle it takes to produce a toxic effect varies, depending on how concentrated the solanine isin the plant, and how much is eaten. However, it generally takes a pound or more to cause poisoning. A single mouthful or a few berries will have little effect on a mature horse. The glycoalkaloids act rapidly once they are absorbed from the intestinal tract, but the effects are not cumulative.
Eradicating established horse nettle is difficult. The plant propagates from seeds, and it also spreads through an extensive underground root system. Mowing the plants before they produce seeds will slow them down but won't eliminate them. And because they can grow back from even small portions of their rhizomatous roots, they are difficult to control with herbicides or by pulling them up manually.
Contact your local extension agent to identify the plant and for tips on the best strategies to control it in your area.
Anthony P. Knight, BVSc, MS, DACVIM
College of Veterinary Medicine & Biomedical Sciences
Colorado State University
Fort Collins, Colorado