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Poison Oak Identification

How to identify poison oak

By: Sandy Feather ©2009
Penn State Extension

Q. A friend of mine says she has poison oak on the hillside behind her house. The man who mows for her told her it was poison oak. It has five leaves and reddish stems. Does poison oak even grow in western Pennsylvania? It was my understanding that we don't have poison oak in this part of the country and my Internet search pretty much backed this up; on the other hand, some sites disagree. What is the best way to get rid of it? Is it true that, because of climate change, poison ivy has become even more potent?

A. There is a great deal of confusion about poison oak in our area - I get questions about it all the time. For clarification, I rely on "The Plants of Pennsylvania" by Ann Fowler Rhoads and Timothy Block, both botanists from the University of Pennsylvania and the Morris Arboretum. USDA’s PLANTS Database is a good website to determine whether or not a plant is native or introduced to an area.

Poison ivy leaves in summer
"Leaves of three, let it be"
Its upward-growth as a vine often makes it look like a shrub or tree.

"The Plants of Pennsylvania" is a 1002-page treatise on all species of plants found in Pennsylvania, native and introduced.  It lists three related species known to cause contact dermatitis: poison ivy (Toxicodendron radicans), giant poison ivy (Toxicodendron rydbergii) and poison sumac (Toxicodendron vernix) as growing in Pennsylvania. Poison oak is not listed as a native or introduced species. Poison oak is also known as Pacific poison oak or western poison oak (Toxicodendron diversilobum). Although it is as common in the Pacific Ocean states as poison ivy is here, it is not known to occur in Pennsylvania. All of these plants can cause an itchy, blistering dermatitis in susceptible individuals because their tissues are suffused with a compound called urushiol.


The old saying, "leaves three, let it be" is mostly correct. Most poison ivy has leaves composed of three leaflets; mature plants can have anywhere from five to seven leaflets. It typically grows as a vine, but can grow as weak-stemmed shrub if there is nothing handy for it to climb. The stems-especially very mature ones-are covered with aerial rootlets that give them a very hairy appearance. Poison ivy has beautiful red fall color and produces white fruits in late summer and early fall when mature.

Colorful poison ivy leaves in fall
Poison ivy has beautiful fall color!

Giant poison ivy is considered a threatened or rare species limited to dry, rocky woods. Its appearance is very similar to common poison ivy.



Poison sumac is a shrub or small tree with an upright growth habit and produces a grayish-white fruit in fall. It has alternate, pinnately compound leaves comprised of seven to thirteen leaflets. Poison sumac is an obligate wetland species, and is “occasionally found in swamps, bogs, fens and marshes” throughout Pennsylvania.

Your friend’s “poison oak” may be a native vine known as Virginia creeper (Parthenocissus quinquefolia), based on the description she gave you. It has palmately compound leaves comprised of five leaflets that turn scarlet in fall. Virginia creeper produces dark purple fruits, also in fall. Although it is often mistaken for poison ivy, it generally does not cause dermatitis.

Most plants-including poison ivy-are likely to grow bigger and faster due to increased CO2 in the atmosphere. A study from Duke University found that exposing poison ivy to higher levels of CO2 resulted in faster, lusher growth due to the fertilization effect of the higher CO2 levels. It also found that higher CO2 levels made urushiol more potent. These findings could have a significant future impact on human health, since more potent urushiol will likely increase the number of people requiring medical treatment for the resulting contact dermatitis. As of now, roughly 80 percent of the population is susceptible to urushiol. An interesting sidebar from the Duke study: “Other species in the Anacardiaceae family, including mango, cashew, and pistachio, also can be allergenic. It is possible that these plants, too, may become more problematic in the future.

Increased vine growth could have severe consequences for our increasingly fragmented and degraded woodlands as well. Tree regeneration is reduced when rampantly growing vines of any kind block sunlight and reduce the saplings’ ability to manufacture carbohydrates via photosynthesis. If they cannot produce enough carbohydrates to sustain themselves, the saplings die.


Poisonous plants


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