The Total Avoidance Option

 People who have had serious reactions to poison-oak should consider avoiding the plant completely. Their blood relatives may also be very sensitive. The following comments on the Range of Poison-Oak are offered for their consideration: 

• In Mexico poison-oak grows only in the Baja. However, there are two poison-ivies in the Sierra Orientale and Occidentale of mainland Mexico. There are also other less familiar plants, of other genera but in the same plant family, Anacardiaceae, that cause the same rash. Some of these, for example the Pepper Tree, are beautiful and cultivated as far north as San Diego, CA.
• Poison-oak does not grow where precipitation is less than ten inches per year. That keeps the plant out of Southeastern California, and further east. However, there are poison-ivies in the mountains of Arizona and New Mexico.
• Combined altitude and dryness have kept the Anacardiaceae out of the Great Basin. For example the High Sierra, California east of the Sierra, Northeastern California and Nevada are safe.
• At the Columbia River Gorge poison-oak and Rydberg poison-ivy intergrade for about fifty miles. To the West there is only poison-oak. East of the Gorge there are only ivies.
• .From south to north, poison-oak’s range is from northern Baja California at thirty degrees latitude, to Vancouver Island and adjacent British Columbia just above the forty-ninth parallel. At thirty degrees it grows to 4860 feet. At forty-nine degrees it only grows at sea level. North of that there are no Anacardiaceae up the Inland Passage, in Alaska or in the Yukon. There is, however, a sensitizing liverwort that grows among the lichens on the bark of trees in British Columbia and the panhandle of Alaska.
• It is known that Rydberg Poison Ivy, a plant closely related to poison-oak, grows 109 meters lower for each degree of latitude across its range. I have not found similar published observations for Poison-Oak.
• However, assuming a similar relationship it is predicted that at the US border with Mexico poison-oak grows to an altitude of 4235 feet. At the latitude of Morro Bay, thirty-five degrees, poison-oak may grow only below 3610 feet. At the Oregon California border it may not grow above 1750 feet.