Dracunculus vulgaris probably does enjoy good drainage, but it grew for many years in a poorly drained spot in my garden in the Cascade foothills (annual rainfall 50+ inches) where it also got runoff from a paved area. It does better, and flowers regularly (not necessarily a plus unless you can tolerate the smell of carrion) in a sunny raised bed. I saw it in the wild in considerable shade on old terraces in very rocky soil. I see no reason why it should not succeed in Seattle if given just a slightly elevated position. Jane McGary Portland, OR