Plants that cause hives include sumac, poison ivy and poison oak, according to MedicineNet.com. Patients can get hives through direct contact with the plant or by touching anything that came into contact with the plant. It is also possible to get hives after touching a stinging nettle plant.
Poison oaks, poison ivy and sumac contain urushiol, which is a sap that causes rashes when it comes into contact with the skin, explains MedicineNet.com. These plants cause hives in about 50 percent of Americans who come into contact with them.
For some people, brushing up against the stinging nettle plant causes hives, according to MedicineNet.com. The leaves and stems of the stinging nettle plant have sharp hairs that look like tiny hypodermic needles. When these hairs brush against the skin, the body releases the chemical histamine, which causes hives about 30 to 60 minutes after exposure. The histamine is responsible for the itching, swelling and redness, according to the University of Texas at Austin. Many individuals who have poison ivy allergies may also react when their skin comes into contact with the black sap of the Japanese lacquer tree and oil from the shells of cashew nuts, according to WebMD.
Certain plants, trees and grasses cause hives through their pollen, explains the American College of Allergy, Asthma and Immunology. Individuals who have pollen allergies may get hives if they inhale pollen from Russian thistle, pigweed, tumbleweeds, sage brush and plantain. Inhaling pollen from ash, beech, oak and willow trees may also cause hives.