Illinois Nature Guide: September 2026
Data updated: August 2026Sources & method · Corrections
September is the turn into fall in Illinois — fall migration surges, the monarchs stream south, and the prairie blazes gold with goldenrod and purple with asters. The harvest shifts to apples, pumpkins, and winter squash, and the first cool nights signal the season's change.
Week by week in September
September 3–9
- Fall migration peaks — warblers, thrushes, vireos, and sparrows pour through, and broad-winged hawks stream south in spiraling kettles.
- The great monarch migration begins, with monarchs streaming south and fueling on goldenrod and asters.
- Plant garlic and divide perennials in the cooling soil, and sow a final round of fast fall greens.
September 10–16
- The prairie's gold-and-purple finale peaks — goldenrods and New England, smooth, and sky-blue asters together.
- Chimney swifts swirl into chimney roosts by the hundreds at dusk, and the year's biggest raptor flights continue.
- Apples, pumpkins, and winter squash arrive as the markets pivot from summer abundance to fall bounty.
September 17–23
- The autumn equinox falls around the 22nd, the nights now longer than the days, with Pegasus and Andromeda climbing the east.
- Monarchs pour south after cold fronts, forming overnight roosts in trees along the lakeshore and river corridors.
- Black walnut and green ash yellow and drop early, and the nut harvest of acorns and hickories is in full swing.
September 24–30
- The first waterfowl trickle back to the river backwaters, and bugling sandhill cranes begin streaming south overhead.
- Pumpkin patches and pick-your-own apple orchards are in full festival swing — Illinois leads the nation in pumpkin production.
- The first sugar maples blush orange, especially in the cooler north and the Shawnee hills.
Birds This Month
September is one of the top birding months in Illinois as fall migration peaks. The warbler wave returns in force — trickier to identify in muted fall plumage — and the Chicago lakefront at Montrose Point again concentrates migrants, joined now by thrushes, vireos, flycatchers, sparrows, and the year's biggest push of raptors. Broad-winged hawks stream south in spiraling 'kettles' on warm afternoons, and sharp-shinned hawks, kestrels, and ospreys follow the ridges and shoreline.
The grasslands and wetlands stay busy: shorebirds continue at Emiquon, great egrets stage in big numbers, and the first waterfowl trickle back. Common nighthawks course over towns at dusk in loose southbound flocks, chimney swifts swirl into chimney roosts by the hundreds, and ruby-throated hummingbirds peak early in the month before thinning out — keep feeders up for stragglers into October. A dawn walk anywhere with cover can turn up a remarkable variety of migrants.
What's Blooming
September is the fall climax of the Illinois prairie, painted in gold and purple. The goldenrods — stiff, showy, Canada, and many more — wash the grasslands and roadsides in gold, while the asters take over the cool end of the spectrum: New England aster in deep purple, smooth and sky-blue asters, and the white heath and frost asters. Together they make the prairie's last great display and a crucial late nectar source for migrating monarchs and bees.
The tall sunflowers — sawtooth, Maximilian, and woodland sunflower — still tower, ironweed and joe-pye weed linger in the wet swales, and bottle gentian tucks its closed blue flowers into the grass. The big bluestem and Indian grass are flowering and beginning to take on their bronze and copper autumn tints. Walk Midewin, Nachusa, or a restored prairie now for the goldenrod-and-aster finale before the killing frosts.
Garden This Month
September is the harvest-and-transition month in Illinois gardens. The summer crops finish strong — tomatoes, peppers, eggplant, beans, and the last squash — while the fall garden hits its stride: lettuce, spinach, arugula, radishes, turnips, broccoli, cabbage, kale, and other greens thrive in the cooling weather, often sweeter after the first light chill. Keep an eye on the forecast, since the first frost can reach the northern third of the state by late month.
This is the time to plant garlic for next summer, divide and transplant perennials in the cooling soil, and sow cover crops in emptying beds. It's also the best window to seed or overseed a cool-season lawn and to plant trees and shrubs while the soil is still warm but the air has cooled. Begin pulling spent plants, composting healthy debris, and saving seed. Spring bulbs — daffodils, tulips, crocus — can go in as the month ends.
Zone 5b (Chicago metro & northern Illinois): the first frost can arrive late in the month or in early October — harvest tomatoes and peppers ahead of it, keep covers ready, and plant garlic and spring bulbs as the soil cools.
Zone 6a (central Illinois): frost holds off a bit longer — keep harvesting the summer crops, sow a final round of fast greens, and start planting garlic and cool-season transplants for fall.
Zone 7a (far southern Illinois / 'Little Egypt'): warm weather lingers well into fall — extend the summer harvest, plant a full fall garden of greens and brassicas, and set out garlic late in the month.
What's at the Farmers Market
September markets pivot from summer abundance to the bounty of fall. The last tomatoes, peppers, eggplant, sweet corn, melons, and beans of summer overlap with the arriving fall harvest: apples in many varieties, winter squash, pumpkins (Illinois leads the nation in production), sweet potatoes, cabbage, broccoli, and the first cool-season greens. Southern Illinois peaches finish early in the month.
Pumpkin and squash stands and pick-your-own apple orchards are the season's centerpiece, and cider begins to appear. Cut flowers shift to sunflowers, mums, and dahlias. Choose apples that are firm and heavy with no soft spots and store them cold; pick pumpkins and winter squash with a hard rind and a dry, intact stem and keep them somewhere cool and dry, not the fridge; and ripen the last tomatoes on the counter. The markets are full and colorful as the seasons change hands.
Night Sky This Month
September brings the autumn equinox around the 22nd, when the nights grow longer than the days and the dark-sky window expands. The Summer Triangle still rides high after dusk, but the autumn constellations move in: the Great Square of Pegasus climbs in the east, the 'W' of Cassiopeia stands high in the northeast, and the faint smudge of the Andromeda Galaxy — the most distant object visible to the naked eye — can be glimpsed beside it from a dark site.
There's no major meteor shower this month, but the lengthening, cooling nights make for comfortable viewing, and the Milky Way still arches overhead in early evening. The dark skies of the Shawnee National Forest in far southern Illinois remain the state's best window on the autumn sky, far from the Chicago light dome.
The printable Illinois night-sky guide lists this year's planet positions and the exact equinox timing for your location.
Butterflies & Pollinators
September is the month of the great monarch migration in Illinois. The final summer generation — the long-lived 'super generation' — streams south through the state toward central Mexico, and on good days, especially after a cold front, monarchs pour past in loose rivers, stopping to fuel up on the abundant goldenrod, asters, and blazing star. Communal overnight roosts can form in trees, especially along the Lake Michigan shoreline and the river corridors that funnel the migration.
Other late-season butterflies are still active: painted ladies and red admirals migrate south too, sometimes in large numbers, and common buckeyes, clouded and orange sulphurs, cabbage whites, and the last fritillaries and swallowtails work the fall flowers. Planting late-blooming native nectar plants like goldenrod and aster is one of the most valuable things a garden can offer the southbound monarchs this month.
What to plant for them is in the Illinois native plants guide
Trees This Month
September is when fall color begins to creep into the Illinois woods, starting with the early-turning species. Black gum (tupelo) and sumac flame scarlet, sassafras turns orange, gold, and red, green ash and black walnut yellow and begin dropping their leaves early, and the first sugar maples blush orange, especially in the cooler north and in the Shawnee hills.
The nut harvest is in full swing: oak acorns, shagbark hickory nuts, and black walnuts drop and feed the squirrels, deer, and jays stocking up for winter, and the Osage orange drops its big green 'hedge apples' along old fencerows. The great mass of the canopy — the white and bur oaks — is still green, holding its color for October's peak. By late month the woods are unmistakably turning, the green giving way patch by patch to the coming blaze of autumn.
The Illinois native plants guide covers what grows with them
Weather & Daylight
Across Illinois, a typical September day averages a high near 78°F and a low near 55°F, with about 3.4 inches of precipitation — statewide averages from the 1991–2020 U.S. climate normals.
Heat begins to ease, with highs near 78F and noticeably crisper nights dipping into the mid-50s.
| Average high | 78°F |
|---|---|
| Average low | 55°F |
| Precipitation | 3.4 in |
| Sunrise (mid-month) | 6:36 CT |
| Sunset (mid-month) | 19:06 CT |
Figures are statewide averages over the 1991–2020 U.S. climate normals; local conditions vary with elevation and distance from the coast. Sunrise and sunset are computed for a midstate point on the 15th (CT).
Next month's version of this page, by email
One short note when Illinois's next month turns — what's arriving, blooming, and worth a night outside.
Go deeper with the Illinois guides
The complete Illinois birding, native-plant, and night-sky references — or the whole year, week by week, in one book.
Same month elsewhere: September in Indiana · September in Iowa · September in Kansas