Biological Sciences, Santa Barbara City College

Biology 100: Concepts of Biology

Toxicodendron diversilobum    Poison Oak

Poison Oak is highly variable in form and color. It may be found both as a shrub and a vine. The leaves vary from red to green. It has erect stems, leaves in threes, small greenish flowers, and smooth seeds about 1/4 inch across. It is often lush in coastal canyons, but sparse in the mountain woodland. It is deciduous, and often loses its leaves in late summer, leaving it hard to recognize. Without leaves, the erect branches give a clue to its identity.

This is probably the most notorious plant in our local flora because of the skin rash that it causes in most (but not all) people who contact it. Unfortunately, it is also one of the commonest plants throughout our area. Simple rules like "Leaves of three, let it be" do NOT work to distinguish it. (But they do keep people away from the blackberries!) This is a plant you just have to learn to recognize in all its guises. This is part of living in California.


Poison-oak is usually a shrub, though it sometimes becomes a vine that grows high into the oak trees attached by air-roots. The leaves DO come in threes. They are shiny, without prickers (unlike Blackberry), and the middle leaf has a distinct stalk (unlike Squaw Bush and Clematis). It is harder to identify Poison Oak in the winter, when it loses its leaves and looks like erect bare sticks coming from the ground.

All parts of the plant contain the oil Urushiol which causes the skin reaction. Some people are highly sensitive, but others barely so. "I don't get it" is a great example of famous last words. It is an allergy, not a poison, and its effects can vary with age. It is just an accident of our physiology that some humans are sensitive to it. Treat it like any allergy.

Deer eat the leaves and wood rats make nests with the branches. It is one of the most important food plants for wildlife in our area.

Poison-Oak can contaminate both smoke and mud. It is a serious problem for fire-fighters! Many people got bad cases digging out after the '95 floods.

Poison-Oak cures are legion. Most of them act as astringents that dry out the skin. Many locals recommend washing with an infusion of Mugwort (Artemesia Douglasiana), but this might be an urban legend. Another choice is Ethyl Alcohol from the hardware store. For severe cases, a shot of cortisone does the trick. When all else fails, take a hot shower and scratch all over. It doesn't do any good, but it feels so good!

text abridged from

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