What plants cause hives on horses?

* Stinging nettle *(Urtica dioica)*: This common weed has tiny, needle-like hairs that can cause a burning, itchy rash when they come into contact with skin.

* Poison ivy *(Toxicodendron radicans)*: This climbing vine has three leaflets that are typically green in the summer and red in the fall. The leaves contain a chemical called urushiol, which can cause an allergic reaction in some people.

* Poison oak *(Toxicodendron diversilobum)*: This shrub or small tree has three leaflets that are similar to poison ivy, but they are typically more rounded. Poison oak also contains urushiol.

* Sumac *(Rhus spp.)*: Several species of sumac can cause allergic reactions in some people. The leaves of these plants are typically compound, with five to 13 leaflets. The leaflets are often serrated and can be either green or red.

* Giant hogweed *(Heracleum mantegazzianum)*: This tall, robust plant has large, lobed leaves and white flowers. The sap of this plant can cause severe burns and blisters when it comes into contact with skin.