Three Common Toxic Forest Plants

Rhus Family - Poison Ivy, Oak, Sumac

Poison Ivy. Ed Reschke, Oxford Scientific, Getty Images

Poison oak, ivy, and sumac are toxic plants that commonly grow in the forests of the United States and Canada. They are members of the plant genus called Rhus. These plants are similar in their appearance and growth and cause itchy skin blisters (dermatitis) on humans. Each Rhus "family" member produces a similar toxin and respond alike to treatment of the skin irritations they inflict.

The plants are easy to identify and most commonly seen along fence rows, open and cut-over forest lands, stream banks, and up tree trunks.

In the fall, this species' leaves turn brilliant red and become a tempting target to touch. Many people who are unfamiliar with the plant gather the colorful leaves in the fall and many times suffer acute allergic reactions.

Hospitalization may be required to deal with the problem in cases of severe reactions.


Both poison oak and poison ivy are readily identified by their leaves. Shiny when young, the leaves grow in groups of three. Leaflets range from a half-inch to two inches long. Flowers are greenish white, about one-quarter of an inch across and are borne in clusters on a slender stem. The fruits are white, berry-like, glossy and dry when ripe, about one-sixth of an inch in diameter in poison ivy and a little larger in poison oak.

Poison sumac is a little different in that it resembles non-poisonoua sumac's bark and leaves and lives in a similar habitat. The leaves are approximately twelve inches long with 7 to 13 leaflets on a long red petiole.

The fruit is also berry-like but smooth.

All parts of these plants except the pollen are poisonous year round. The best way to prevent problems with Rhus is to avoid contact with the plant.


Poison Ivy, Poison Oak and Poison Sumac contain an oily substance, urushiol, which causes irritation and blistering of the skin.

Human reactions vary from extreme susceptibility to near immunity. Just becoming older can have a tempering effect on the severity of the rash and the condition is most serious on children and young adults.

After identifying the plant you should not touch any portion, including the roots and berries. However, in many cases contact is unavoidable and you have to assume that some exposure has occurred.

When working around poison oak or poison ivy, wash exposed skin as soon as possible with soap and cold water, or a special soap such as "TechNu," specifically designed to remove urushiol. Follow washing by liberally applying rubbing alcohol or a solution of alcohol and water in equal proportions to dissolve all the unabsorbed poison. There is also a "blocking agent" called Ivy Block that is applied before coming in contact with the plant.

I actually keep a current prescription of hydrocortisone cream USP, 2.5%. Applying it as needed reduces the severity of itch and quickly clears the rash.

Contaminated clothing can carry the poisonous oil for years. Poisoning may occur even after laundering. If you are extremely sensitive, dry cleaning of clothing is recommended. Do not wash contaminated clothing with other clothing.

Take care to rinse out the washing machine thoroughly after laundering contaminated clothing.

The ultra-violet part of sunlight is thought to help break down urushiol. You actually may want to dry or expose your clothes to the sun.


Plant experts indicate three ways to control poison oak or ivy on your property . Pull or dig out the entire plant, browsers grazing infested areas, or by using chemical control. Burning is not recommended as inhaling dust and ash from the smoke can result in poisoning of the lungs and that can require hospitalization. Burning does not kill the plant.

To hand pull or dig poison oak, careful precautions must be used. People with known sensitivities should not attempt hand pulling. Wear protective clothing and gloves even if you are less sensitive to the sap.

Pull plants when the soil is wet and loose. Make sure to remove all roots of the plant. Cut all vines growing up trees. Pull as much of the vine away from the tree as possible. Mowing has little effect unless mowing is repeated frequently.

Grazing animals will only keep poison oak or ivy at bay if it is continued consistently for several years.

Chemical control options include spraying the leaves with brush killer (glyphosate is one) in the late spring or early summer after the plants are in full leaf. Spray only according to label directions on a calm day. Selective herbicides can remove poison oak and ivy without killing grass, but those herbicides can damage valuable plants. Use care.