Poison-oak (Toxicodendron diversilobum) is quite a nasty plant – it grows in abundance in wet and shady areas, and the allergic reactions it causes certainly make it “hikers foe”. Nevertheless, it can be quite beautiful – in spring, it’s leaves are of a vivid, bright green, in late summer and early autumn they turn to a glowing red (making it most easy to spot it). It’s most dangerous in winter, when the bare twigs sticking out of the ground are hardly recognizable as Poison-oak at all.
Los Penasquitos Canyon Preserve has a spectacular array of Poison-oak, especially the vines that grow next to the trees and entangle them are of an incredible size and must be very old – some of them are as thick as a human arm, or even leg, almost tree-like themselves. I made these photos there, on an overcast August morning.