Well, that they grow from seeds just chucked into the ground next to the bed of onions that was still empty in June. It might even have been closer to July before they got planted. I would totally have taken more care to spread them out if I had known they had time to grow AND bloom all summer!
Garden Experts should be shouting
PLANT ZINNIA SEEDS!!
(How did I not know this? :) I'm buying them in bulk next year . . . I wonder if the seed pods would be worth saving? hmm . . .