No beach plums in my neighborhood, but I understand they are common along the East Coast, Cape Cod area for sure, where they apparently have quite a mystique. Grow wild in sandy areas along beaches and roadways -- mature slowly, grow big and gnarled. Some years they are plentiful, some years none to be found. I understand it's best to pick them (for the best-tasting jam, anyway)when they are reddish blue.