I don't know for sure what blooms in September, but I can offer some fall/late fall blooming plant suggestions. Mums (chrysanthemums) can be pinched back earlier in the summer to bloom in fall. Autumn crocus will bloom in the fall as well as ornamental grasses 9like Karl Forester reed grass). Goldenrods, asters, and joe-Pye
weed bloom in the fall, but some consider them to be
weeds. I think in the warmer
zones, naked ladies (also know as belladonna lilies) send up just a stem to bloom. In my
zone 5 garden, morning glories, rudbeckia, echinaceas, calendula, reblooming
roses, hardy hibiscus/
rose of sharon all bloom till frost, as well as more tender plants that need to be taken inside for winter (like brugmansias, callas, dahlias, tropical hibiscus, mandevilla etc.) The Chinese lanterns (Physalis alkekangi) are long done blooming, but in September, the puffy
seed pods that are left behind turn a bright orange and add a splash of colour. They can be dried because they keep their colour even after they're dried.