Western poison oak Toxicodendron diversilobum
IDENTIFICATION — As with their close relative, poison ivy, “Leaves of three, let it be” is good advice. Eastern and Western poison oak both have three-leaflet leaves, with red-tinged new foliage and green-white fruit in late summer. Both species grow as small shrubs or vines up to 20 ft. long.
All parts of poison oak contain an oil called “urushiol.” About 90 percent of humans are allergic to this compound, and it causes severe itching and blistering on skin. The more you’re exposed to poison oak, the more likely you are to become sensitive to it, so don’t assume that you’re immune just because you’ve never reacted before.
FAVORITE CONDITIONS — Eastern poison oak is found mainly in the southeastern quarter of the United States, from Florida into Kansas. Western poison oak is found in California, Oregon and Washington and British Columbia. Both species are common in fence rows, waste land, stream banks and wild areas — and unfortunately, also in gardens.
CONTROL — It’s important to remove or kill the entire plant when you’re dealing with poison oak because it regrows from even small portions of the roots. You can pull seedlings or small plants by hand, but be very careful. Urushiol on gloves or clothing can get on your skin, so you’ll need to wash anything you’re wearing — even shoes. Don’t burn the pulled-up plants because inhaling the smoke can cause throat and lung irritation. It’s not a good idea to put them in the compost pile either, so put the plants in a heavy paper bag and dispose of them with your yard waste.
Using herbicide is often the easiest way to get rid of large, old plants. Systemic herbicides with glyphosate (like Roundup®) will kill poison oak. But it’s most effective when you apply it in late summer, after fruit have formed but before the leaves turn red. You may need to apply it more than once. Because the herbicide will kill any plant it comes in contact with, be careful not to get it on any nearby ornamental plants.