Can Poison Oak Have Five Leaves?

Poison oak is not related to the oak tree, nor is it technically a poison. It received its name from the shape of its leaves, which resemble those of oaks. The number of leaves can vary, depending on the species. Poison oak is related to poison ivy and poison sumac. They all contain a chemical compound called urushiol, which can cause an allergic reaction resulting in itching and a blistered rash that can be severe in especially sensitive people.

Poison oak is a member of the genus Toxicodendrons, which is Greek for "poison tree." Two species of poison oak are native to North America --- T. pubescens and T. diversilobum. "Pubescens" refers to the velvety hairs that cover the plant, while "diversilobum" indicates the wide variety of forms that species can take. The other members of the Toxicodendron genus occurring in the U.S. include two species of poison ivy and one species of poison sumac.

T. pubescens is an erect shrub, generally from 2 to 4 feet tall but it may reach 10 feet. It grows in dry areas in woods, thickets and sandy fields. Atlantic poison oak occurs from New Jersey to Florida and west to Kansas, Oklahoma and Texas. It bears small white flowers in early spring and white fruit in late spring. The leaves resemble those of oak and come in threes. In the fall, they turn a brilliant orange-red.

T. diversilobum can grow as a vine or a shrub and has oak-like leaves occurring usually in three but sometimes five leaflets; they turn bright red in the fall. It bears white flowers and berries. As a shrub, it generally grows from 2 to 6 feet tall but may reach 13 feet. As a vine, it can extend from 10 to 30 and even as long as 100 feet. Pacific poison oak is found west of the Cascade Range in Washington, Oregon and California and from Baja California north to British Columbia. It prefers dry, sandy soil and its habitats include oak woodlands, chaparral and conifer forests.

Urushiol is the chemical compound that is released when any part of poison oak is contacted. (Ref 6) It is contained in the leaves, stem, root and the skin of the berries. While a small proportion of the population is not sensitive to the chemical---about 10 to 15 percent---the other 85 percent are more vulnerable. Of this group, another 10 to 15 percent are very sensitive to the compound. Wildlife and livestock are not affected by poison oak but pets may have an allergic reaction.