At least a hundred different plants, poisonous to animals, livestock or people, grow in Ohio. Some are only toxic to some animals species, while others affect only humans. Sometimes, the whole plant is poisonous, while in other cases only some parts, such as berries, are toxic. Some of the poisonous plants were introduced as ornamentals to North America and then escaped into the surrounding environment. Poison hemlock (Conium maculatum) is an example of such a plant. It is highly toxic and grows in U.S. Department of Agriculture plant hardiness zones 3 through 10. It has spread all over Ohio, growing in fields and pastures. Other poisonous plants are native to Ohio.
Poison ivy (Toxicodendron radicans or Rhus radicans) is widespread throughout Ohio. You can recognize poison ivy by its shiny leaves that are composed of three leaflets. Each mature leaflet is 2 to 4 inches long and has smooth, lobed or toothed margins. Poison ivy grows as a vine, but sometimes becomes shrub-like. It proliferates in USDA zones 5 through 10 and can be found along roads, in fields, at the base of trees and along the edges of wooded areas. Poison ivy propagates mostly by seed: it produces seeds within small, white berries that are spread by birds and animals. Poison ivy releases an oil called "urushiol" when bruised or crushed, which causes severe swelling, blistering and itching upon contact with the skin of susceptible people. Even the inhaled smoke from burning poison ivy can produce an extremely painful reaction in the lungs. All parts of the poison ivy plant contain urushiol and cause an allergic reaction.
Horsenettle (Solanum carolinense) is a 1- to 4-foot-tall weed that grows in all counties of Ohio. It prefers sandy or gravelly soils and grows in fields, pastures and along roadsides. Horsenettle belongs to the nightshade family of plants and bears yellow fruits that look like small yellow tomatoes. It grows in USDA zones 4 through 10. Horsenettle, if eaten in large quantities, is poisonous to livestock. It contains an alkaloid compound that becomes activated in the stomach when the plant is eaten. Young children may be attracted to the yellow berries, which can lead to death upon digestion.
Wild four-o'clock (Mirabilis nyctaginea) is becoming more widespread in Ohio. It grows in USDA zones 3 through 10 in pastures, gardens and waste landscapes. Wild four o'clock also invades no-till crop fields. It is a bushy perennial, 1- to 4-foot-tall plant with a tap root, swollen joints and heart-shaped leaves. The seeds and roots of wild four-o'clock are toxic, because they contain an alkaloid that can cause gastrointestinal distress. Some animals appear to be able to eat the weed without ill effects.
Jimsonweed (Datura stramonium), also called Jamestown weed or thorn apple, is a 2- to 5-foot-tall annual plant. It thrives in USDA zones 7 through 11, but will also grow down to zone 3. Jimsonweed prefers rich soil and cultivated fields, but will also grow in wastelands. Jimsonweed contains a toxic alkaloid and all parts of the plant are poisonous. Normally, animals avoid eating Jimsonweed because of its unpleasant odor and taste, but hungry animals will eat it. Convulsions, coma and death in farm animals are preceded by early signs of poisoning, such as rapid pulse and nervousness.