I came across an old thread, which I quote: I acquired a brilliant rash (now developed with blisters characteristic of poison oak exposure) last weekend on the trail to the top of Mt Seymour. Almost certainly due to contact with poison oak, I suspect. On the underside of my right forearm - the one I was using to reach into the shrubbery for huckleberries! I've been hiking the trails in the local mountains for many years and not previously had such a contact, though I'm familiar with (and cautious about) poison ivy & poison oak in other environs. I had bought the received wisdom that poison oak did not occur in the lower mainland (accounting perhaps for its near-omission by Pojar) and so was not on the outlook for it. The above quote was posted in 2004 - any new intelligence on this matter since? Or do I have to go back next weekend to look for it? :-)
Will soon be in its autumn finery, making it easier to spot. You'd think it would at least grow in the drier areas east of Vancouver Island mountains, while most abundant south of my area there are plenty of places where it can be seen here and there in western WA (north of Olympia).
Just double-checked Illustrated Flora of BC. T. diversilobum is said to occur on eastern Vancouver Island, the Gulf Islands and Howe Sound -- I wouldn't consider Mt. Seymour to be Howe Sound, though... This is a northeast reference, Avoiding Rash Decisions: A Guide to Plants You Shouldn’t Touch, but it also mentions giant hogweed -- which is a potential culprit on North Shore hiking trails. The article shares how to differentiate the two types of rashes (by the sounds of it, yours is more likely poison ivy).
Almost certainly urushiol and not psoralens. I've been stung by poison ivy before. Rash from this, which I now assume to have been poison oak, very similar. The nice ref you gave describes the effect pretty well (squares with this and my previous experiences with poison ivy on the prairies). The psoralens need the sun to do the damage - in my case the rash was on the inside of my arm and least exposed to sunlight. I wonder if our exceptional summer may have encouraged growth. In any case, I think the BCPassport tourist info on the web may be a bit over-reassuring in stating that there is no poison oak on the North Shore hikes. Though it did take me 35 years to find some!
I doubt very much that there is any poison oak in the Mount Seymour area. I've been hiking the trails and bushwacking my way around that area for over 45 years and have never seen any. I'm very familiar with its appearance and habitat from many encounters near Roseburg, Oregon, where my mother used to live. It likes a hot, dry environment, which is the opposite of what we have at Mount Seymour, especially at the higher elevations. If poison oak occurs anywhere in the North Shore area, it will be near Howe Sound, in the habitat that supports Arbutus trees. The poison oak areas in Oregon are full of Arbutus.
