May is peak spring migration and the great green-up across Colorado, as the leaf-out and birdsong climb the mountains week by week. Warblers and tanagers pour through the cottonwoods, the foothills blaze with golden banner and the first columbines, and the Front Range gardens finally open for the warm-season planting once the late-May frost passes.
Week by week in May
April 30 – May 6
- Spring migration builds in the cottonwoods as western tanagers, Bullock's orioles, and yellow warblers pour in.
- The Eta Aquariid meteors, debris from Halley's Comet, peak with their best showing low in the southeast before dawn.
- Golden banner and the first Rocky Mountain columbine open in the lower foothill aspen groves.
- Local asparagus is the signature crop as the Front Range outdoor markets begin to open.
May 7–13
- Spring migration peaks in the river cottonwoods with tanagers, grosbeaks, orioles, and a rush of warblers.
- Weidemeyer's admiral patrols the foothill aspens and willow-lined streams.
- The foothill meadows blaze with scarlet paintbrush, silvery lupine, blue flax, and wallflower.
- Watch the forecast — a late freeze or snow can still strike Denver well into mid-May, so hold tender crops.
May 14–20
- After the mid-May average last frost passes, set out tomatoes, peppers, squash, beans, and corn along the Front Range.
- Broad-tailed and migrating rufous and calliope hummingbirds work the foothills, and lazuli buntings sing from the scrub.
- The first monarchs reach Colorado's river corridors to lay eggs on emerging showy milkweed.
- Strawberries, green garlic, and the first peas and spring turnips join the abundant market greens.
May 21–27
- Plains migrant traps and shelterbelts can be alive with western tanagers, buntings, and eastern strays.
- The shimmering pale-green aspen leaf-out climbs steadily from the foothills into the montane zone.
- The first glacier lilies appear at the edges of melting snowbanks in the lower subalpine.
- Mulch deeply and run drip irrigation to set the garden up for the intense, dry Colorado summer ahead.
May 28 – June 3
- Montane breeding birds settle in as the dawn chorus fills the ponderosa, lodgepole, and spruce-fir forests.
- Plains cottonwoods release drifts of cottony seed that pile along the curbs and fences of river towns.
- Spring azures, Melissa blues, and Western pine elfins flutter over the foothill slopes.
- Sweet sugar snap peas, garlic scapes, and the first summer squash arrive at the markets.
Birds This Month
May is the peak of spring migration in Colorado, and the cottonwood corridors of the South Platte and Arkansas rivers, the Front Range foothill canyons, and the plains reservoir woodlots fill with color. Migrant and breeding songbirds arrive in a rush — western tanagers, black-headed grosbeaks, lazuli buntings, Bullock's orioles, yellow, Wilson's, and MacGillivray's warblers, warbling vireos, and a sweep of flycatchers. The famous migrant traps of the eastern plains — isolated groves like those at Bonny and the Pawnee shelterbelts — can be alive with eastern strays.
The high country awakens as the season climbs. Broad-tailed and the migrating rufous and calliope hummingbirds work the foothills, green-tailed and spotted towhees sing from the scrub, and breeding birds move up into the montane forests as the snow recedes. On the eastern plains the state bird, the lark bunting, sings in fluttering song-flights over the short grass, and Cassin's, grasshopper, and lark sparrows set up territories. Mountain plovers and burrowing owls nest on the prairie-dog towns of the Pawnee National Grassland.
Wetlands and reservoirs stay busy with late shorebirds, American avocets, black-necked stilts, Wilson's phalaropes, and breeding grebes, ibis, and terns.
This month's tip: walk a river cottonwood grove at dawn in mid-May — the migration peak crams warblers, tanagers, grosbeaks, and orioles into the green canopy, and an early start finds the most before the day heats up.
What's Blooming
May is the great green-up and the first big bloom across Colorado's foothills, with the wildflower wave climbing the slopes as the snow melts upward. The foothill meadows and ponderosa parks of the Front Range fill with golden banner, chiming bells (Mertensia), spiderwort, wallflower, blue flax, and the first scarlet paintbrush and silvery lupine. The earliest Rocky Mountain columbine, the state flower, opens in the lower foothill aspen groves toward the end of the month.
On the plains, the scarlet globemallow, prickly poppy, locoweeds, and the cream spikes of soapweed yucca bloom, and the sandsage prairie around the Great Sand Dunes greens and flowers. The foothill shrublands carry the white of boulder raspberry, wax currant, and the last chokecherry and serviceberry. As the month ends, the first glacier lilies appear at the edges of melting snowbanks in the lower subalpine, the leading edge of the alpine show still building toward its July peak high above.
Garden This Month
May is the pivotal month in the Colorado garden, the moment the warm-season planting finally arrives along the Front Range. The hard rule all spring has been to wait, and now the wait ends: once the average last frost passes in mid-to-late May — and only after checking the forecast, since a late freeze or snow can still strike Denver well into the month — set out tomatoes, peppers, eggplant, squash, beans, cucumbers, and corn, and direct-sow the warm-season crops into the warming soil. In the mountains, hold these tender crops under cover into June.
This is also the month to set the garden up to survive the Colorado summer, which is defined by intense high-altitude sun, low humidity, and chronic dryness. Mulch every bed deeply to hold soil moisture and moderate temperature, install or check drip irrigation, and water consistently as the heat builds. Keep succession-sowing lettuce, spinach, radishes, and beans, harden off and set out warm-season flowers, and protect tender new transplants from the fierce midday sun and the hail that Front Range storms can deliver in late spring.
Zone 4b (mountain towns and high foothills): still a cool-season game for most of the month — the average last frost runs late May into June up here. Direct-sow peas, spinach, lettuce, radishes, and carrots, set out onions and potatoes, and transplant hardy brassicas. Hold tomatoes and squash under cover until the very end of the month at the earliest, and keep frost protection ready all summer.
Zone 5b (Front Range cities — Denver, Boulder, Fort Collins, Colorado Springs): the big planting month. After the mid-May average last frost passes (and watch the forecast — a late freeze can still strike), set out tomatoes, peppers, eggplant, squash, beans, cucumbers, and corn, and keep succession-sowing greens and beans. Mulch to hold moisture against the dry, sunny heat ahead.
Zone 6a (warmest Front Range and lower Western Slope — Grand Junction area): full warm-season planting is on. Set out all the tender crops early in the month, direct-sow beans, corn, squash, and melons, and get heat-lovers established before the intense Grand Valley summer. Mulch heavily and set up drip irrigation for the dry months ahead.
What's at the Farmers Market
May is when the Colorado outdoor farmers markets begin opening across the Front Range, and the season's first true abundance arrives. The tables overflow with tender greens — spinach, head and leaf lettuces, arugula, kale, chard, salad mixes, and microgreens — alongside radishes, green onions, spring turnips, fresh herbs, and the local asparagus that is one of the month's signature crops.
Spring favorites fill out the stalls: rhubarb, green garlic, the first strawberries in the warmest areas late in the month, and overwintered potatoes and onions. Vegetable, herb, and flower starts are at their peak supply for home gardeners now planting their warm-season gardens. Colorado pantry staples continue: local honey, eggs, grass-fed beef, bison, and lamb, and milled flour.
For selection and storage: choose firm asparagus with tight tips and stand it upright in an inch of water in the refrigerator; keep tender greens dry and loosely bagged; and refrigerate strawberries unwashed and use them quickly. Harden off any transplants you buy before setting them out, since Front Range frost and hail remain possible.
Night Sky This Month
May brings warm, comfortable nights to Colorado's superb dark-sky country, and the observing season opens in earnest. The certified sites are at their most pleasant now — Great Sand Dunes National Park beneath the Sangre de Cristos, the dark-sky community of Westcliffe-Silver Cliff and its observatory, Black Canyon of the Gunnison, Dinosaur National Monument, and Jackson Lake State Park on the plains. The thin, dry high-altitude air keeps the stars sharp and steady on calm nights.
The spring sky stands at its best. Leo drops toward the west while the Big Dipper rides high overhead, its handle arcing down to brilliant orange Arcturus in Boötes and on to blue-white Spica in Virgo. By late evening the Summer Triangle of Vega, Deneb, and Altair begins to climb in the east, hinting at the Milky Way season to come. The modest Eta Aquariid meteor shower, debris from Halley's Comet, peaks in early May with its best meteors low in the southeast before dawn.
Because planet positions and exact shower timing change each year, check the printable Colorado night-sky guide for this year's specific viewing nights and planet visibility from your latitude. Pick a clear, calm night for the steadiest seeing at altitude.
Butterflies & Pollinators
May is a rich butterfly month across Colorado's foothills and lower mountains, with the season climbing the slopes behind the melting snow. The foothill canyons and river corridors fill with the big swallowtails — western tiger swallowtails sailing the cottonwood and willow draws, the great two-tailed swallowtail gliding through the canyons, and the dark Pale and black swallowtails in the meadows. The white-and-black Weidemeyer's admiral patrols the foothill aspens and willows.
The blues, hairstreaks, and elfins are out in force on the foothill slopes — spring azures, Boisduval's and Melissa blues, Western pine elfin, and Sara orangetips — and the meadows fill with checkerspots and the first fritillaries. The painted lady and American lady work gardens and roadsides, and migrant waves continue. The first monarchs reach Colorado's river corridors as the breeding generation moves up from the south to lay eggs on emerging showy milkweed. The alpine specialists, including the Rocky Mountain parnassian and the state insect Colorado hairstreak, still await the high-country thaw.
What to plant for them is in the Colorado native plants guide
Trees This Month
May is the great green wave in Colorado, sweeping up the slopes elevation by elevation. The quaking aspen leaf out in their unmistakable shimmering pale green, climbing from the lower foothills into the montane zone as the weeks pass — one of the loveliest sights of the Colorado spring. The ponderosa pines flush new growth and shed yellow pollen on the warm foothill afternoons, carrying their warm vanilla-butterscotch bark scent, and the Colorado blue spruce along the montane streams pushes soft new blue-green growth at its branch tips.
Down low, the plains cottonwoods are in full leaf and beginning to set their seed, and the Western Slope orchards shift from bloom to fruit-set, the growers' anxious frost watch finally easing. The foothill Gambel oak leafs out late, the last of the foothill woody plants to green up. By month's end the green has reached the subalpine, but the Engelmann spruce and subalpine fir of the high country are only just beginning to stir as the snow finally lets go of the upper slopes.
The Colorado native plants guide covers what grows with them
Weather & Daylight
Across Colorado, a typical May day averages a high near 67°F and a low near 39°F, with about 2.0 inches of precipitation — statewide averages from the 1991–2020 U.S. climate normals.
Comfortably warm by day near 67F, mild evenings in the 30s, and the wettest stretch of the year rolling through.
| Average high | 67°F |
|---|---|
| Average low | 39°F |
| Precipitation | 2.0 in |
| Sunrise (mid-month) | 5:48 MT |
| Sunset (mid-month) | 20:07 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: May in Connecticut · May in Delaware · May in Washington, D.C.