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The Scourge of Poison Oak

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.

Poison oak (Toxicodendron diversilobum)

Poison oak, growing in an oak savanna habitat

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?

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11 comments… add one

  • DaveOnFidalgo July 29, 2011, 11:37 AM

    I may be one of those immune to the toxin. I traipsed through a patch of Poison Ivy when I lived in Iowa, but nothing happened. I don’t plan to test the theory, though. I have seen the lesions and it’s wicked.

    • Ivan Phillipsen August 4, 2011, 8:40 PM

      Yeah, it really is bad stuff. I have never actually come in contact with the plant knowingly. I just develop huge, nasty lesions a week after frolicking around in poison oak country.

  • Chris Janousek July 31, 2011, 11:14 AM

    Thankfully, poison oak is not present in the coast range of Oregon where salal, huckle/salmon/blackberries, ferns and other smaller plants tend to make up the understory. I’ve had too many experiences with it in the drier coastal habitats of California, though!

    • Ivan Phillipsen August 4, 2011, 8:41 PM

      Yes, that’s true: poison oak seems to be rare in the Coast Range of Oregon. I haven’t seen it in the Olympic Mountains, either. I suspect those areas are a little too damp for poison oak.

      Thanks, Chris!

  • jill i August 1, 2011, 11:40 AM

    I wonder if I’m one of the lucky few or I’ve just never encountered it. It’s never seemed to have been an issue wherever I’ve hiked in the NW, but when I lived down in the Sierra foothills just outside Yosemite, I was told to be very careful. Went on a hike with a park biologist and she said even when there are no leaves in the late winter it can be toxic if you come in contact with the stems. So I bought poison oak treatment lotion but never needed it as I recall.

    • Ivan Phillipsen August 4, 2011, 8:44 PM

      I sure wish I was one of the lucky ones. I end up getting poison oak without even realizing that I touched the plant. By the time I get a rash, it’s too late to use the lotion.

  • Mike B. August 4, 2011, 2:52 PM

    It’s everywhere in the Camassia Natural Area. My kids and I walked through a huge patch of it once. We kept our hands in our pockets and I washed our clothes 4 or 5 times. I even threw our shoes out! We never got a rash. Probably just lucky- I had a terrible reaction to Poison Ivy as a kid.

    • Ivan Phillipsen August 4, 2011, 8:45 PM

      It sounds like you got lucky for sure. I’ll have to keep an eye out for PO when I finally get my butt over to Camassia. Thanks for the heads up, Mike!

  • Susan W. March 16, 2012, 11:50 AM

    I’ve lived on the central Oregon coast for 30+ years and have hiked, mushroomed, generally climbed and snooped all over areas west of the coast range mountains and have never had an encounter with poison oak or poison ivy. In fact, I was told when we moved here that none existed on the coast. However I have friends that insist that we do and say that they see it all the time and have had skin reactions. I’m wondering what it is that they are reacting to if infact it does not grow here.

    • Ivan Phillipsen March 20, 2012, 1:18 PM

      Thanks for the comment, Susan. Poison oak tends to grow in drier areas and you would probably agree that the coast isn’t very dry. According to data from the Oregon Flora Project, poison oak is rare on the western side of the Coast Range north of Port Orford. There are more records of it growing near the coast south of there.

      • Susan W. March 24, 2012, 2:32 PM

        Thanks for the reply Ivan. I think some of my friends are getting into nettles or something of that sort and thinking it’s poison oak. That we do have plenty of!

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