Poison Oak

Poison oak and poison ivy – my bane!!

Poison oak (Toxicodendron diversilobum) is extremely variable in appearance – sometimes resembling a woody shrub and forming dense thickets and other times more like a climbing vine snaking up a tree trunk. It was, and in some cases still is, considered a subspecies of the Eastern poison ivy. T. diversilobum is only found on the Pacific Coast of the United States and Canada below about 4,000 to 5,000 feet.

Most everyone has been warned to avoid plants with three leaflets. Good advice, except there are many plants with three leaflets other than poison oak. The leaves are glossy: bronze when first opened, bright green in the summer and red in the fall.  Poison oak leaves can have serrated, lobed or almost smooth margins. The central leaflet, of the three, has a stalk while the two lateral leaflets are sessile (stalkless). Clusters of greenish-white flowers are in the leaf axes. Male and female flowers can occur on different plants or on the same plant. Some flowers are also bisexual. Greenish-white berries develop from the fertilized flowers. Poison oak is deciduous, losing its leaves in the fall leaving bare, leafless stems. Reproduction is by creeping rootstalks or seeds.

Contact with any part of the plant, even the leafless stems in the winter, can cause a blistery, itchy, weeping rash (contact dermatitis)  in about 80% of the population. Humans are not born with an allergy to poison oak, but become sensitized, usually as a child. Adults who claim to not have a reaction to poison oak can still develop sensitivity upon exposure. Deer and other animals can eat poison oak leaves without any apparent ill effect. Birds nest and find shelter in the plant. Only primates (humans, apes, chimpanzees, etc.) seem to have a reaction to poison oak.

The sensitizing agent is a volatile oil, urushiol. Urushiol is the name for a lacquer made from the sap of the Japanese lacquer tree. (Interestingly, Japanese lacquer can cause a contact dermatitis in some people.) This oil is in the resin canals, associated with the phloem (food conducting tissue), of the plant. Only when the plant is bruised, damaged or attacked by insects does the urushiol get on the poison oak surface. Burning poison oak can volatilize urushiol. Attached to smoke particles the oil can cause sensitivity in the lungs of anyone who breathes the smoke.

I was sensitized as a very young child and have an extreme reaction to poison oak/ivy. As a result, when I am in poison oak territory I must stay on trails and be very careful. Even with care and an acute awareness of poison oak I still seem to get myself into trouble! My most recent episode occurred in the winter when I was climbing a rocky cliff and grabbed some “branches” to steady myself – you guessed it! And who can see poison oak in the dark?

These pictures were taken along the North Umpqua Trail in Oregon and along the Lower Pit River  below Lake Britten in California.

“Leaves of three, leave them be; berries white, poisonous sight.”

Gallery | This entry was posted in Noxious Weeds, Shrubs, Wildflowers and tagged , , , , , . Bookmark the permalink.

2 Responses to Poison Oak

  1. Lin says:

    Rob’s, TOO !!!

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