Poison oak (Toxicodendron diversilobum) is the most widespread shrub in California occurring in a wide selection of habitats and soil types from mixed evergreen forests, woodlands, riparian areas, to chaparral. It reproduces from fresh seed, seed banks (with or without fire), adventitious roots, and is easily propagated with cuttings. Ecologically, poison oak is one of the most adaptive, tolerant, and productive plants around.
Poison oak is not really poisonous (more on that later) and not really an oak even though its leaves resemble true oaks. Poison oak is a member of the Cashew family, including edible cashews and mangos - two of this writer’s favorite foods. It provides palatable browse for wildlife, particularly for black-tailed deer, with high crude protein values, especially in the spring. Birds also readily and hastily eat the seeds in the fall. If poison oak didn’t cause human beings so much irritation, literally, it might be the number one recommended shrub for restoration projects.
Hey, let’s cut this poor ole’ plant some slack. We’re all trying to make a living. Over 400 million years ago, plants growing in fresh and salt water began taking hold on land. Since then, plants have been evolving and fine tuning ways to reproduce and disperse, survive drying out from wind or lack of water, and survive hungry attacks by insects and animals. Rapid diversification of insects occurring around the same time, also led to a boom in a variety of plant protection mechanisms.
Poison oak developed a protection mechanism in its sap, or resin, through a substance called urushiol. Urushiol is not really a poison, but rather, a potent allergen. Urushiol is produced internally in resin canals located in the inner fibro-vascular system of stems, roots, and leaves. The resin is easily transported to the surface where it is readily brushed off on to skin. The resin can be washed off with soap and water, but 50% of the urushiol is absorbed within 10 minutes, so time is of the essence. Once urushiol has been absorbed by the skin it is recognized by special immune cells which are found among the skin cells and act as sentinels. These immune cells then migrate to nearby lymph nodes where they present the urushiol to T-lymphocytes, thus, recruiting them to the skin. Once in the skin, the T-lymphocytes cause rash by triggering the gathering of other immune response cellular substances, histamine and heparin, which cause collateral cellular damage to the skin and utter annoyance to its owner.
All that itching aside, poison oak is the ultimate provider and survivor. It competently vegetates watersheds, provides nutritious food and habitat for wildlife and birds, recolonizes readily after fire or other disturbance, and protects itself from insect and even human attack. I don’t know many plants that can do that. So let’s hear it for poison oak. California has a state tree, California redwood, California has a state grass, purple needlegrass, how about a state shrub?
This article is part of a series about natural life on the Lost Coast, sponsored by the Lost Coast Interpretive Association, which may be contacted at email@example.com.
Jennifer Wheeler is a botanist for the BLM Arcata Field Office with a passion for managing grassland and coastal dune landscapes as well as for eradicating invasive weeds, particularly French and Scotch broom. She resides in McKinleyville with her family and her 2,000 square foot garden and hopes to someday clear 1,000 pounds of produce if she can successfully convince the gophers to let her.
PHOTO BY JENNIFER WHEELER, BLM ARCATA FIELD OFFICE