In most of the Portland area, I would plant it now, so it can get established before the frost, and have a head start on next summer's heat. Spring planting--just before the average last day of frost--is also an option, but the tree will suffer more in July and August, especially if it is a hot year. Right on the waterfront, you could plant it almost anytime, since the Pacific Ocean is probably the ultimate in "thermal mass". : )