Quick answer: 29 native Illinois plants have a recorded bloom range that includes September.
29 species
- Big bluestem Andropogon gerardii · 25 states
- Black-eyed Susan Rudbeckia hirta · 42 states
- Butterflyweed Asclepias tuberosa · 37 states
- Cardinal flower Lobelia cardinalis · 37 states
- Common buttonbush Cephalanthus occidentalis · 23 states
- Common evening primrose Oenothera biennis · 12 states
- Common sneezeweed Helenium autumnale · 13 states
- Compass plant Silphium laciniatum · 11 states
- Culver's root Veronicastrum virginicum · 8 states
- Cup plant Silphium perfoliatum · 17 states
- Eastern purple coneflower Echinacea purpurea · 22 states
- Fall obedient plant Physostegia virginiana · 21 states
- Fireweed Chamerion angustifolium · 28 states
- Grayhead coneflower Ratibida pinnata · 12 states
- Great blue lobelia Lobelia siphilitica · 32 states
- Harebell Campanula rotundifolia · 25 states
- Indiangrass Sorghastrum nutans · 19 states
- Little bluestem Schizachyrium scoparium · 23 states
- Mexican hat Ratibida columnifera · 17 states
- Partridge pea Chamaecrista fasciculata · 13 states
- Partridgeberry Mitchella repens · 17 states
- Prairie blazing star Liatris pycnostachya · 16 states
- Showy tick trefoil Desmodium canadense · 21 states
- Sideoats grama Bouteloua curtipendula · 25 states
- Smooth oxeye Heliopsis helianthoides · 12 states
- Swamp milkweed Asclepias incarnata · 25 states
- Sweet Joe-Pye Eutrochium purpureum · 17 states
- Switchgrass Panicum virgatum · 24 states
- Witch-hazel Hamamelis virginiana · 28 states