During this dry time of year, if you see a green plant with oak-like leaves in groups of three emerging from the tan grasses of Solano County, chances are you are looking at Toxicodendron diversilobum, or Poison Oak. This “weed” as it is commonly classified, is native to all parts of California below elevations of 5,000 feet. It seems particularly prolific even in this drought year. In my microclimate it never completely died back during our warmer than average winter, so now I routinely see six-foot high plants along rural Fairfield roads. Several references indicate this plant can grow in upright or vining form to 14 feet.

Poison Oak seems to be a very flexible plant, having adapted to both full sun as well as the dappled shade areas below native oak trees, which are not close relatives. Toxicodendron belongs to the Anacardiacae family, while oaks, Quercus, belong to the family Fagaceae.

I had always wondered if Poison Oak was related to Poison Ivy and Poison Sumac, since they look quite different and grow in very different parts of the country. Turns out that, yes, they are all members of the Sumac family, and all irritate our skin because of the same chemical compound. Urushiol is a resin that all parts of the Poison Oak plant can yield. Leaves, stems and roots all emit urushiol, which binds with the proteins found on human skin, and can cause irritation quickly. Since it takes 10 to 15 minutes for that binding action to occur, the best way to avoid skin irritation resulting from contact with Poison Oak is to thoroughly wash the area with lots of soap and warm water. Urushiol can adhere to our clothing, boots, tools and tool handles, as well as the fur of our pets, and then transfer easily to us. Wash any potentially contaminated clothing with hot water and detergent before you touch it further. Wash off tools and their handles if they could have come in contact with Toxicodendron diversilobum so you won't suffer the next time you go to use them.

Keep in mind that Poison Oak is deciduous, and its leaves do turn a beautiful red color in autumn. Don't think that all danger has passed once those pretty leaves have dropped. The stems and roots produce the urushiol also and should be avoided.

There are two bits of good news about Poison Oak that I can share. First, some lucky people are immune to urushiol and do not ever develop the usual immune response, a rash. Second, deer are happy to eat Poison Oak, which does not affect them negatively. If only they would eat more of it! I would be happy to share with them.