September is the turn of the Wyoming year — the aspens igniting gold across the Tetons, Snowies, and Bighorns, the elk bugling through the mountains, raptors streaming south along the ridges, and the first hard frosts ending the garden. It is one of the most beautiful and dramatic months in the state.
Week by week in September
September 3–9
- Raptors stream south along the mountain fronts — Swainson's hawks, kestrels, and Golden Eagles riding the ridge thermals.
- The quaking aspens begin to ignite gold across the Tetons, Snowy Range, and Bighorns, the signature Wyoming autumn spectacle.
- The killing frost reaches the high valleys — harvest tomatoes and squash ahead of the freeze and dig potatoes as the tops die back.
September 10–16
- Rabbitbrush bursts into full gold across the basins, humming with the last bees and butterflies against the turning aspen.
- Big staging flocks of Sandhill Cranes bugle out of Jackson Hole and the Bear River as they gather to leave for the south.
- The Andromeda Galaxy glows naked-eye from a dark basin, the most distant thing visible to the unaided eye, high in the autumn sky.
September 17–23
- Aspen gold peaks on the mountain slopes as bull elk bugle and thrash saplings through the turning groves.
- Songbirds pour through the cottonwoods in fall plumage — warblers, vireos, tanagers, and waves of white-crowned and Lincoln's sparrows.
- Plant garlic and spring bulbs late in the month, and let the frost-sweetened kale and roots keep producing under cover.
September 24–30
- The fall harvest fills the tables with potatoes, winter squash, pumpkins, apples, fresh cider, and frost-sweetened greens.
- The last painted ladies and sulphurs feed at the rabbitbrush as hard frosts in the basins shut activity down through the month.
- Bears strip the fruiting chokecherry and serviceberry thickets, fattening for hibernation, as robins and waxwings join the feast.
Birds This Month
September is prime fall migration in Wyoming. Raptors stream south along the mountain fronts — Swainson's, red-tailed, and ferruginous hawks, American kestrels, prairie falcons, Cooper's and sharp-shinned hawks, and Golden Eagles riding the ridge thermals, with the Bighorns and other ranges making fine hawk-watch country. Songbirds pour through the cottonwoods in fall plumage: yellow-rumped, Wilson's, orange-crowned, and Townsend's warblers, warbling vireos, western tanagers, flycatchers, and waves of white-crowned, Lincoln's, and chipping sparrows.
Waterfowl and cranes build on the wetlands — returning ducks, tundra swans, and big staging flocks of Sandhill Cranes bugling out of Jackson Hole and the Bear River as they gather to leave. Shorebird migration finishes on the drying flats of Seedskadee NWR and the reservoirs. In the mountains Clark's nutcrackers cache pine seed against winter, the high breeders are gone, and the rut begins — bull elk bugling across the parks of Yellowstone, Grand Teton, and the National Elk Refuge meadows.
This month's tip: combine birding with the fall color — a ridge hawk-watch or a crane-staging marsh against the gold aspens is the quintessential Wyoming September. Mornings are best before the wind, and listen at dawn and dusk in the high parks for bugling elk.
What's Blooming
September is the close of Wyoming's wildflower year, dominated by the late-season golds. Rabbitbrush bursts into full brilliant yellow across the sagebrush flats, roadsides, and basins — the signature bloom of the high-desert autumn, humming with the last bees and butterflies. With it the late asters in purple and white, goldenrod, gumweed, broom snakeweed, and the last wild sunflowers hold along the disturbed ground and ditches. In the mountains a few hardy gentians and asters linger in sheltered meadows after the early frosts, and fireweed finishes in a blaze of red foliage and drifting seed cotton. Most of the spring and summer flowers have gone to seed — the silvery plumes of grasses and the dried heads of paintbrush, lupine, and balsamroot standing through the curing landscape. The rabbitbrush gold against the turning aspen is one of the great color pairings of the Wyoming fall.
Garden This Month
September brings the killing frost to most of Wyoming and ends the warm-season garden. Across the high valleys the first hard freeze has usually struck by early in the month, and even the warmer basins frost by late September, so the priority is harvest: bring in tomatoes, peppers, squash, and beans ahead of the freeze, ripen green tomatoes on a windowsill or in paper, and dig the potatoes after the tops die back. Pull and cure onions, garlic, and winter squash, and lift carrots, beets, and turnips for storage.
The cold-hardy crops carry on. Kale, chard, spinach, lettuce, and root crops sweeten with the frost and keep producing under row cover and in cold frames well into fall. As beds empty, begin the cleanup — pull spent plants, compost healthy debris, and plant garlic and spring bulbs late in the month for next year. Sow cover crops or mulch bare ground against the wind, and leave seed heads on native perennials for the birds and for winter structure.
Zone 3b (Jackson Hole, high valleys): the killing frost has usually arrived by early September here, ending the warm-season garden — harvest everything tender, then turn to cleanup and putting beds away. The hardy greens and roots can keep producing under cover well into the month.
Zone 4a (high basins, Lander, Cody): first frost comes this month — harvest tomatoes, peppers, and squash before a hard freeze, ripen green tomatoes indoors, and dig potatoes and roots. Cold-hardy greens, kale, and root crops carry on.
Zone 4b (warmer basin towns, Torrington area): the longest season — cover for the first light frosts to extend the harvest, keep picking tomatoes and peppers, and sow a final crop of spinach and lettuce under cover for fall.
What's at the Farmers Market
September markets in Wyoming hold the abundant fall harvest as the season tips toward its close. The tables carry the storage crops coming in — potatoes, onions, winter squash, pumpkins, carrots, beets, turnips, cabbage, and the last sweet corn and tomatoes from the warmer basins — alongside kale, chard, and greens that the cool nights and early frosts make especially sweet. The state's grass-fed beef, lamb, and bison come into their fall sales as ranchers bring stock to market.
Look for the new crop of Wyoming honey, apples from the warmer valleys, fresh-pressed cider from small orchards, and jars of preserves and chokecherry jelly from the late-summer harvest. Cure winter squash and pumpkins somewhere cool and dry, store potatoes, onions, and roots cool, dark, and ventilated, and keep the tender greens cold and use them quickly.
Night Sky This Month
September's lengthening nights and the autumnal equinox bring back longer darkness over Wyoming's exceptional skies, with comfortable evening temperatures before the deep cold. The Red Desert and southwest basins, and the backcountry of Yellowstone and Grand Teton, remain premier dark-sky destinations, with town-edge sites near Lander, Pinedale, and Saratoga easily showing the Milky Way as the late star parties of the season gather.
The summer Milky Way still arches overhead through Cygnus and Sagittarius in the early evening, while the autumn sky climbs the east: the Great Square of Pegasus, the chained figure of Andromeda, and the Andromeda Galaxy (M31) — at 2.5 million light-years the most distant thing visible to the naked eye, an easy glow from a dark Wyoming basin. Cassiopeia's W rides high in the northeast. No major meteor shower peaks in September, but the long, dark, transparent nights are superb for the Andromeda and Triangulum galaxies and the Milky Way clusters of Cassiopeia.
Exact planet positions shift year to year — the printable Wyoming night-sky guide lists this season's planet visibility and the darkest viewing sites near you.
Butterflies & Pollinators
September winds down Wyoming's butterfly year, concentrated on the late nectar and the migrants. The brilliant gold of blooming rabbitbrush across the sagebrush flats becomes the season's great butterfly magnet, drawing the last painted ladies, West Coast ladies, red admirals, clouded and orange sulphurs, and lingering blues and skippers to feed on warm afternoons. Monarchs from the valley milkweed drift south in their generation-spanning migration, and the mourning cloaks, Milbert's tortoiseshells, and California tortoiseshells of the new brood feed up before settling into hibernation for the long winter ahead. Asters and goldenrod add to the late spread. The high country is finished — frost has shut down the alpine parnassians and fritillaries. As the first hard frosts reach the basins, activity drops sharply through the month, and by late September only a few hardy hibernators fly on the warmest days, the closing of the season.
What to plant for them is in the Wyoming native plants guide
Trees This Month
September is the great color month in Wyoming's high country. The quaking aspens ignite across the mountain slopes, turning the Tetons, Snowy Range, Bighorns, Wind Rivers, and Black Hills foothills to sheets of brilliant gold — the signature autumn spectacle of the state, drawing leaf-watchers to the canyons and passes. In the river bottoms the plains cottonwoods begin their own turn, the gallery groves along the Green, Snake, North Platte, and Bighorn going golden week by week. The willows and aspens along the streams add yellow and orange.
The conifers hold their green backdrop, the dark lodgepole pine, Douglas-fir, and spruce-fir forests setting off the gold. At treeline Clark's nutcrackers finish caching whitebark and limber pine seed for winter. The fruiting chokecherry, serviceberry, hawthorn, and mountain-ash are stripped by bears fattening for hibernation and by robins and waxwings, and bull elk bugle and thrash saplings through the turning aspen groves. It is the most beautiful weeks of the Wyoming tree year.
Weather & Daylight
Across Wyoming, a typical September day averages a high near 70°F and a low near 40°F, with about 1.3 inches of precipitation — statewide averages from the 1991–2020 U.S. climate normals.
Summer eases into fall as afternoons cool to the 70s, nights turn crisp, and rain picks back up modestly.
| Average high | 70°F |
|---|---|
| Average low | 40°F |
| Precipitation | 1.3 in |
| Sunrise (mid-month) | 6:48 MT |
| Sunset (mid-month) | 19:21 MT |
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 (MT).
Next month's version of this page, by email
One short note when Wyoming's next month turns — what's arriving, blooming, and worth a night outside.
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The complete Wyoming birding, native-plant, and night-sky references — or the whole year, week by week, in one book.
Same month elsewhere: September in Alabama · September in Arizona · September in Arkansas