The link didn't work for me, but I looked it up. Grayanotoxin.
The rhododendron is the WA state flower, but rhododendrons don't grow well at all in eastern WA...too dry and hot. They grow well on the west side of the Cascades, and they grow really well in the rain forests on the peninsula. Lotsa hippies over on the peninsula, so maybe they're into that sort of thing.
Are azaleas a problem, too?