I am terribly allergic to poison oak (Toxicodendron diversilobum). A few days after touching this plant, my skin breaks out in oozing, itchy blisters that take weeks to heal. To me, poison oak is a scourge, a bane. I think twice about going anywhere that it grows in profusion.
If you happen to be among the 15 to 30% of the population that doesn’t react to the toxin in poison oak, then you probably don’t think of this plant as any more evil than the average shrub. Consider yourself lucky.
What does poison oak look like?
Most of us know the apothegm, ‘Leaves of three, let it be.” Poison oak and its close relatives have leaves that come three to a stem. Poison oak is not closely related to oak trees, but its leaves look remarkably similar to those of the oak trees it often grows near or on. Poison oak leaves have lobed margins like those of Oregon White Oak (Quercus garryana). They are glossy and bright green when they are new in early summer, then become dull and brown or red-orange in autumn. The individual leaflets (each of the three ‘leaves’ is really a leaflet) are usually about four inches long.
One remarkable thing about poison oak plants is that they can look very different. Some of them grow into bushes, others grow into vines that climb up trees. Some are slender stalks, standing alone in open places. Which growth form an individual plant assumes depends on its local environment. For example, if a poison oak plant begins its life at the base of an oak tree, it is more likely to grow into a climbing vine that snakes up the tree.
Small berries form on poison oak in the summer, starting out green but ending up creamy white or yellowish in the autumn. The flesh of the berries isn’t toxic, but the skin is, so don’t eat them!
Because of its variable leaf shape and growth forms, poison oak can be tricky to identify. Look at lots of photos of poison oak to get familiar with its many guises.
Where does poison oak grow?
The range of poison oak spans the west coast from British Columbia to Baja California. Poison oak is found in oak woodlands, Douglas fir forests, and disturbed sites. In these habitats, poison oak can be very common and is an important component of the ecosystem.
Poison oak grows in both sunny and shady places where water is available in winter and spring, but where summer and fall are usually warm and dry. North-facing slopes and stream gullies provide some of the best habitat for poison oak.
The poison: Urushiol
Poison oak produces an oil called urushiol, which flows through the plant’s stems and leaves. Many people get dermatitis when urushiol contacts their skin. Even a minute amount of the oil is enough to cause a bad reaction in people who are sensitive to it. Even in winter the bare stems of poison oak contain enough urushiol to cause dermatitis.
I imagine this oil is a chemical defense used by the plant to deter certain herbivores. But some deer, birds, rodents, and insects go right ahead and eat the leaves, stems, and berries of poison oak, seemingly with no ill effects.
I wonder which animals (besides humans) avoid poison oak because of urushiol. Any ideas?