Generally, anything that creeps or drops tons of seeds!
Cannas. Oh I love the leaves, and the flowers, and put them where their creeping nature won't bother other plants (totally alone in spots from which they can't escape,) but they get totally infested every summer with leaf roller caterpillars, which I feel compelled to battle, manually (I don't have any garden chems,) which I don't doing mind at all, but get a lot of extra mosquito bites, and quite sweaty, doing it.
Dahlias, no, they're never going to stand up on their own...
