Warning sign in Seattle's Seward Park
A while back I was on a geology field trip looking at the Crescent Formation on the Olympic Peninsula. We made a stop at Lake Crescent. While scrambling up a slope of rock rock fragments to reach a cliff face I was surprised to see a patch of poison oak. I was thinking we were practically in the rain forest being on the Olympic Peninsula. Until that time I did not know poison oak was present in western Washington. I managed not to get a rash from that encounter and then saw the warning sign after I came back down the slope.
Poison oak is not common in western Washington but is present at a few droughty hot spots in western Washington. Our dry summers allow for it to thrive particularly on south facing rock slopes or areas that get hotter in the summer than other parts of western Washington. Besides Lake Crescent, I have seen patches on the south facing slope above Commencement Bay north of Tacoma and at the southern tips of both the Bolton Peninsula and the Toandos Peninsula on the northern part of Hood Canal. The southern tip of the Toandos is called Oak Head and I have not seen Oak trees there, but have seen plenty of thick poison oak patches.
The biggest poison oak patches I have seen are in the clear cuts near Brinnon on the west side of Hood Canal. After thrashing through some rough woody brush for several minutes I realized I was not in my usual brushy environment - I had been smashing through a thicket of poison oak. The Brinnon area has very dry and warm summer weather, and in areas of thin soil over bedrock or glacial till, poison oak thrives.