Colorado Nature Guide: September 2026
Data updated: August 2026Sources & method · Corrections
September is the golden climax of the Colorado year, when the quaking aspen sweep the high country in waves of gold and orange and the elk bugle through the frosty mountain mornings. Hawks stream south along the Front Range, the sandhill cranes begin returning to the San Luis Valley, and the harvest of chiles, melons, and apples peaks across the warm valleys.
Week by week in September
September 3–9
- Swainson's hawks gather into kettles of hundreds on the Front Range, riding thermals south.
- Rabbitbrush glows gold along the Front Range roadsides, humming with bees and butterflies.
- The Pueblo green chile harvest is in full swing, with roasting chiles smoking at the stands.
- The Gambel oak scrub of the foothills begins turning rusty-red and bronze.
September 10–16
- The high-country aspen begin to turn, gold sweeping down from the highest groves through the montane slopes.
- Songbird migration peaks in the cottonwoods as warblers, vireos, and the last hummingbirds move through.
- Watch the forecast for the first frost and keep frost cloth ready to extend the tomato and pepper harvest.
- The Andromeda Galaxy hangs nearly overhead from a dark site in the crisp post-monsoon air.
September 17–23
- Aspen color builds toward its peak around Kebler Pass, the Maroon Bells, and Rocky Mountain National Park.
- Elk bugle through the frosty mountain mornings as the high-country breeders drift downslope.
- Western Slope apples and pears from Paonia and Cedaredge join the green chiles and winter squash.
- Plant garlic and spring bulbs and divide perennials while the soil is still warm.
September 24–30
- The aspen gold peaks across the high country, one of America's great fall-color displays.
- Sandhill cranes build in the San Luis Valley, and the first wintering raptors return to the plains.
- The last rabbitbrush glows gold along the foothills before the killing frosts finish the season.
- The southbound monarch trail continues through the river corridors as fresh mourning cloaks settle in to overwinter.
Birds This Month
September is a tremendous migration month in Colorado. The Front Range becomes a raptor highway as Swainson's hawks gather into kettles of hundreds, sometimes thousands, on their way to Argentina, joined by streaming broad-winged, red-tailed, and Cooper's hawks, American kestrels, and turkey vultures; the hawkwatches along the foothills can be spectacular. Songbird migration peaks in the cottonwood corridors and foothill canyons, with warblers, vireos, tanagers, flycatchers, and the last broad-tailed and rufous hummingbirds moving through.
The wetlands and reservoirs fill with southbound waterfowl and the tail of the shorebird passage, and the marquee fall event begins: sandhill cranes start returning to stage in the San Luis Valley around Monte Vista, building toward the great October concentration. White-faced ibis and Franklin's gulls flock on the plains.
In the mountains, the high-country breeders drift downslope ahead of winter, and the foothill Gambel oak and chokecherry thickets draw band-tailed pigeons, flickers, and waxwings to the ripe acorns and berries. On the plains, the state bird lark bunting and the grassland sparrows form flocks for departure.
This month's tip: find a Front Range foothill ridge on a sunny day with a light north wind to watch the Swainson's hawk migration — the birds ride the thermals south in great swirling kettles, an unforgettable September spectacle.
What's Blooming
September's flowers belong to the plains and lower foothills as the high country fades toward winter. The roadsides, ditch banks, and foothill slopes of the Front Range turn gold with blooming rabbitbrush (chamisa), one of the great late-season nectar sources, humming with bees and butterflies, alongside late sunflowers, asters, gayfeather, Maximilian sunflower, and broom snakeweed. The sandsage prairie and the Great Sand Dunes margins carry their last prairie sunflowers and blanketflower.
In the mountains, only the hardiest late bloomers linger — the deep-blue fringed and Parry's gentians in the high wet meadows, a few late asters, and the silvery seedheads of old-man-of-the-mountain and the alpine cushion plants gone to seed. But the real spectacle now is foliage, not flower: the tundra turns to russet and the willows to gold as the high country shifts decisively into autumn. Walk a Front Range foothill or plains roadside this month for the last rich nectar of the rabbitbrush bloom before the frosts close the season.
Garden This Month
September is the great transition in the Colorado garden, the moment the first fall frost looms and the season's end comes into view. Along the Front Range, the average first frost runs from late September into October, so watch the forecast closely and keep frost cloth ready — a light early frost can be dodged to extend the tomato and pepper harvest for weeks of bonus warm days, since Colorado's autumns are often long, bright, and mild after the first cold snap. Harvest the cool-season fall crops — broccoli, cabbage, kale, spinach, lettuce, carrots, and beets — which the cool nights sweeten.
This is also planting time for next year. Get garlic in the ground for a strong summer harvest, plant spring-flowering bulbs and cool-season cover crops on emptied beds, and divide and plant perennials and woody plants while the soil is still warm and the roots can establish before winter. Keep watering trees, shrubs, and beds deeply through the dry fall — the moisture going into winter matters enormously here. In the mountains, the killing frost has arrived, so harvest everything tender and put the high-country garden to bed.
Zone 4b (mountain towns and high foothills): the season is ending. Hard frosts are routine now and a killing freeze is certain this month, so harvest everything tender, pull spent plants, and protect any late greens under cover. Plant garlic for next year, clean up the beds, and mulch perennials and overwintering crops against the long, hard mountain winter ahead.
Zone 5b (Front Range cities — Denver, Boulder, Fort Collins, Colorado Springs): the first fall frost approaches (typically late September into October). Harvest the cool-season fall crops, keep frost cloth ready to protect tomatoes and peppers and extend the harvest through the early light frosts, and plant garlic late in the month. Sow cover crops on emptied beds and keep watering until the ground freezes.
Zone 6a (warmest Front Range and lower Western Slope — Grand Junction area): the mild long season runs on. Keep harvesting the abundant fall crops, sow more cool-season greens and overwintering crops, and plant garlic. Frost comes later here, so the warm valleys keep producing well into autumn — keep water steady and beds mulched.
What's at the Farmers Market
September is a peak harvest month at Colorado's markets, rich with the state's signature fall crops. The Pueblo green chile harvest is in full swing, and the smoky smell of roasting chiles fills the air at stands across the Front Range and the Arkansas Valley. The last of the Palisade peaches, the Rocky Ford melons, and the Olathe corn overlap with the first big wave of fall fruit and storage crops.
The autumn harvest is in full flood — tomatoes, peppers, winter squash and pumpkins, Western Slope apples and pears from Paonia and Cedaredge, fresh San Luis Valley potatoes, onions, carrots, beets, and the first pinto beans of the new crop. Colorado pantry staples continue: local honey, eggs, grass-fed beef, bison, and lamb.
For selection and storage: choose firm, glossy chiles and freeze roasted ones whole in their skins; pick winter squash and pumpkins with hard rinds and intact stems and cure them in a warm spot before storing cool and dry; refrigerate apples to hold them but ripen hard pears at room temperature; and keep new potatoes and onions cool, dark, and airy. This is the heart of Colorado's preserving and chile-roasting season.
Night Sky This Month
September brings crisp, increasingly long nights to Colorado's superb dark skies, and the post-monsoon air clears to give some of the steadiest seeing of the year. The certified dark-sky sites are ideal now — Great Sand Dunes National Park beneath the Sangre de Cristos, the dark-sky town of Westcliffe-Silver Cliff and its Smokey Jack Observatory, Black Canyon of the Gunnison, Dinosaur National Monument, and Jackson Lake State Park on the plains. The cooler, drier autumn air is wonderfully transparent.
The sky is in transition. The Summer Triangle still rides high overhead in the evening with the Milky Way arching through it, but the great square of Pegasus and the autumn constellations — Andromeda, with the naked-eye Andromeda Galaxy high overhead from a dark site, and Cassiopeia's W in the northeast — now climb in the east. The equinox brings night and day to balance. There is no major shower this month, making September a fine deep-sky month for galaxies and the rich star fields of the late-summer Milky Way.
Because planet positions change each year, check the printable Colorado night-sky guide for this year's specific viewing nights and planet visibility from your latitude. The crisp, clear post-monsoon nights reward an early-evening start; bring warm layers as the high-country nights turn cold.
Butterflies & Pollinators
September winds the Colorado butterfly season down, but the warm, bright early-autumn days keep many species flying, especially on the plains and in the foothills. The blooming rabbitbrush becomes the great late-season magnet, its gold flowers swarming with painted ladies, orange and clouded sulphurs, variegated fritillaries, woodland skippers, and coppers taking the last abundant nectar before the frosts.
The most significant movement is the monarch migration: the southbound generation drifts down through Colorado's river corridors and plains in September, the trailing edge of the western and central populations heading toward overwintering grounds — never the great river of monarchs the plains states see, but a steady passage along the South Platte and Arkansas. The hardy mourning cloak reappears as fresh fall adults that will overwinter, basking on warm foothill rock and bark. The high-country and alpine species are finished now, killed back by the mountain frosts. Leaving rabbitbrush, asters, and native milkweed standing supports both the resident late fliers and the migrating monarchs as the season closes.
What to plant for them is in the Colorado native plants guide
Trees This Month
September is the golden climax of the Colorado tree year, the famous quaking aspen spectacle. As the cold mountain nights deepen, the aspen turn in waves of brilliant gold, orange, and occasional red, sweeping down from the highest groves through the montane slopes — the high country around Kebler Pass, the Maroon Bells, Rocky Mountain National Park, and a hundred mountain roads draws leaf-peepers from across the country to one of America's great fall-color displays, peaking in the last full week of September. The aspen gold against the dark spruce and the blue sky is the signature image of the Colorado autumn.
Lower down, the season follows behind. The Gambel oak scrub of the foothills turns rusty-red and bronze, the cottonwoods along the plains rivers begin to gild, and the rabbitbrush blooms gold across the slopes. The Western Slope orchards finish their apple and pear harvest. The dark conifers — blue spruce, Engelmann spruce, ponderosa, and bristlecone pine — stand as the steady green backdrop that makes the aspen gold blaze all the brighter against the coming winter.
The Colorado native plants guide covers what grows with them
Weather & Daylight
Across Colorado, a typical September day averages a high near 74°F and a low near 44°F, with about 1.5 inches of precipitation — statewide averages from the 1991–2020 U.S. climate normals.
Summer loosens its grip, days cooling into the mid-70s and nights dipping into the 40s with lighter rain.
| Average high | 74°F |
|---|---|
| Average low | 44°F |
| Precipitation | 1.5 in |
| Sunrise (mid-month) | 6:42 MT |
| Sunset (mid-month) | 19:11 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 Colorado's next month turns — what's arriving, blooming, and worth a night outside.
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The complete Colorado birding, native-plant, and night-sky references — or the whole year, week by week, in one book.
Same month elsewhere: September in Connecticut · September in Delaware · September in Washington, D.C.