September is the great month of fall migration in Iowa — monarchs funnel south through the state, warblers and hawks stream along the river corridors, and the prairie blazes with goldenrod and aster. The harvest peaks, the first frost threatens the north, and the air turns crisp.
Week by week in September
September 3–9
- Fall warbler migration peaks in the wooded river corridors, the 'confusing fall warblers' moving through in their subtler plumages.
- The great monarch migration streams south, following the river valleys and prairie corridors toward central Mexico.
- Asters and goldenrods take over the prairie — New England aster's deep purple beside drifts of yellow goldenrod.
September 10–16
- Broad-winged hawks kettle overhead on warm afternoons, and the river valleys funnel migrating raptors south.
- The big bluestem and Indian grass flush deep red and gold, the prairie at its most photogenic, studded with aster and goldenrod.
- The apple harvest is in full swing, with many Iowa orchard varieties at their peak and the first fresh cider appearing.
September 17–23
- The fall equinox near September 22 evens day and night, and the lengthening nights bring back real darkness for stargazing.
- The first dark-eyed juncos and white-throated sparrows arrive, and American white pelicans stage on the reservoirs before heading south.
- Plant garlic for next summer's crop and set out spring bulbs while the soil is still warm enough for roots to establish.
September 24–30
- The front edge of the fall snow goose migration reaches the western river bottoms, and the first big flocks of geese return to the marshes.
- The nut harvest is in full swing — black walnuts thud down, hickory nuts and bur oak acorns drop, feeding deer, turkeys, and squirrels.
- The last stragglers of the monarch migration pass through, fueling at the final asters and goldenrods before the freezes.
Birds This Month
September is peak fall migration and one of the richest birding months in Iowa. Warblers stream south through the wooded river corridors in their subtler fall plumages — a 'confusing fall warbler' challenge — alongside vireos, thrushes, and flycatchers. Broad-winged hawks kettle overhead on warm days, and the river valleys funnel migrating raptors, including sharp-shinned and Cooper's hawks, ospreys, and bald eagles. Nighthawks swoop over in evening flocks early in the month.
Waterfowl and shorebird movement builds, and the first sparrows — white-throated, white-crowned, and others — arrive in weedy edges. Ruby-throated hummingbirds finish passing through, and by late month the early dark-eyed juncos may reappear in the north. American white pelicans stage in large flocks on the reservoirs before heading south.
This month's tip: watch the sky on warm, sunny afternoons after a cold front — rising thermals carry kettles of migrating hawks and, on the same days, drifting streams of monarch butterflies, both following the river corridors south.
What's Blooming
September is the season of the asters and goldenrods on the Iowa prairie — the grand finale of the prairie bloom. Goldenrods of many species drench the grasslands and roadsides in yellow, joined by the blues and purples of New England aster, smooth aster, sky-blue aster, and the white heath aster. Sawtooth and prairie sunflowers, rough blazing star, and the last ironweed add to the late color.
The towering prairie grasses — big bluestem ('turkeyfoot'), Indian grass, and switchgrass — reach their full height and flush deep red, copper, and gold, their seed heads catching the low autumn light. This is the prairie at its most photogenic, a sea of warm grass color studded with aster and goldenrod, alive with the last bees and migrating monarchs fueling up on the nectar before continuing south.
Garden This Month
September turns the Iowa garden toward harvest, preservation, and putting the beds to bed. The first frost arrives in late September in the north and early October in the central and southern counties, so watch the forecast and be ready to cover tomatoes, peppers, and basil on the first cold nights to stretch the season. Bring in winter squash and pumpkins as their rinds harden and stems dry, dig potatoes, and pick the fall greens, carrots, and beets that actually sweeten with cool weather.
This is the prime month to plant garlic for next summer's crop, set out spring-flowering bulbs, and divide and move perennials while the soil is still warm enough for roots to establish. Sow a cover crop on cleared beds, plant cool-season cover or a final quick crop of spinach and lettuce, and start the fall cleanup — though leaving some seed heads and stems standing benefits overwintering birds and beneficial insects.
Zone 4b (far north Iowa): the first frost is imminent — harvest all tender crops, cover or bring in what you can on cold nights, and dig potatoes and cure winter squash. Plant garlic late in the month for next year.
Zone 5a (central Iowa): harvest the main crop ahead of the first frost (typically early October here), keep picking fall greens, and plant garlic and spring bulbs as the soil cools toward month's end.
Zone 5b (southern Iowa): still weeks from frost — keep harvesting tomatoes and peppers, sow a last quick crop of spinach and lettuce, and plant garlic and cool-season greens for fall and overwintering.
What's at the Farmers Market
September markets bridge the summer and fall harvests in Iowa. The last of the summer crops linger — tomatoes, peppers, sweet corn, melons, cucumbers, and summer squash — while the autumn produce arrives in force: winter squash, pumpkins, potatoes, onions, broccoli, cabbage, kale, carrots, and beets. The apple harvest is in full swing, with many Iowa orchard varieties at their peak, and the first cider appears.
Look also for fall raspberries, grapes, the season's honey, hardy mums and cut flowers, and ornamental gourds and cornstalks for the season. Store winter squash and pumpkins in a cool, dry room rather than the fridge after their rinds harden, keep apples cold to hold their crispness, and use the tender last tomatoes quickly before the cool nights soften them.
Night Sky This Month
September balances Iowa's sky between summer and fall, with comfortable nights and steadily earlier darkness. The Summer Triangle still rides high after sunset, but the great square of Pegasus climbs in the east, leading the autumn constellations, and the 'W' of Cassiopeia rises high in the northeast. Following Pegasus and Andromeda upward leads to the Andromeda Galaxy (M31) — at 2.5 million light-years, the most distant object visible to the naked eye, a faint smudge from a dark Iowa sky.
The fall equinox near September 22 evens day and night, and the lengthening nights bring back real darkness. Late in the evening, watch the western sky after dusk from a dark site for the faint zodiacal light, and enjoy the Milky Way still arching overhead before it sinks westward through autumn.
Exact planet positions vary year to year — the printable Iowa night-sky guide gives the current month's details for your location.
Butterflies & Pollinators
September is the month of the great monarch migration through Iowa, one of the most remarkable wildlife events of the state's year. The long-lived migratory generation streams south, following the river valleys and prairie corridors toward central Mexico, sometimes in loose rivers of orange on the days after a cold front. They cluster overnight in sheltered trees and pour onto the late prairie blooms — goldenrod, aster, and blazing star — to refuel for the journey. Watching them gather and move is a signature Iowa autumn experience, and prairie sites like the Loess Hills and Neal Smith NWR are excellent places to witness it.
Other migrants travel south too: painted ladies, red admirals, and common buckeyes — now at peak abundance — drift southward, and clouds of sulphurs work the late flowers and fields. The cooling nights gradually slow butterfly activity, but warm, sunny September afternoons on a blooming prairie can still be alive with wings.
Trees This Month
September begins Iowa's fall color and the autumn nut drop. The earliest turners lead the way: black walnuts and green ash yellow and begin dropping leaves, black gum and sumac flare scarlet at the timber edges, and the Virginia creeper vine reddens on tree trunks and fences. The cottonwoods along the rivers brighten toward gold.
The nut harvest is in full swing: black walnuts thud down in their green husks, shagbark hickory nuts fall, and the bur oak and red oak acorns drop, feeding deer, turkeys, squirrels, and wood ducks — the autumn mast that sustains Iowa's wildlife through fall. By late September the maples and oaks are beginning to color in earnest, and the timber stands poised on the edge of its full autumn display.
Weather & Daylight
Across Iowa, a typical September day averages a high near 75°F and a low near 52°F, with about 3.5 inches of precipitation — statewide averages from the 1991–2020 U.S. climate normals.
Warm days easing into the mid-70s with cooler nights in the low 50s, and rainfall tapering as summer humidity fades.
| Average high | 75°F |
|---|---|
| Average low | 52°F |
| Precipitation | 3.5 in |
| Sunrise (mid-month) | 6:53 CT |
| Sunset (mid-month) | 19:24 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).
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Same month elsewhere: September in Kansas · September in Kentucky · September in Louisiana