While I have a rooftop area approximately five foot square where I need to retain at least some spare space for access and hanging out the washing... so "A LOT" is relative! ;-)

(And given that every drop of water --and compost-- has to be carried up by hand and it needs watering at least twice a day in hot weather, it's about as much as I can handle...)

I generally go for things that self-seed and survive in poor soil, but every so often someone gives me a new packet of seed or a bird drops a new species in; I had some exotics last year from what must I think have been an American packet of 'wildflower' seed, like the cowcockle (now successfully into its second generation) and coreopsis, which doesn't seem to have produced viable seed.