Colorado's growing calendar is defined by two hard limits: a last spring frost that lingers into May at most Front Range elevations, and a first fall frost that arrives as early as late September. Denver's average season runs roughly 150 days, but gardens above 6,000 feet may see frost in June and again in August โ leaving a window too short for many warm-season crops.
Cool-season vegetables โ those that germinate in cold soil, tolerate frost, and actually prefer 50โ65ยฐF growing temperatures โ are the natural fit. The real challenge is fitting each crop's full maturity window inside that season, which means precise sow dates, transplant timing, and sometimes choosing faster varieties over classic ones.
Colorado's high-altitude conditions offer hidden advantages: intense UV and low humidity suppress fungal diseases that plague humid-climate gardens, cold autumn nights sweeten root vegetables dramatically, and frost-hardened greens develop richer flavor. The key is leaning into the climate rather than fighting it.
At a glance
| Crop | Type | Days to harvest | Sun | Heat | Frost | Level |
|---|---|---|---|---|---|---|
| Pea | Cool | 55โ70 days | Full | โ | โ | Easy |
| Broccoli | Cool | 60โ90 days | Full | โ | โ | Moderate |
| Cabbage | Cool | 60โ100 days | Full | โ | โ | Moderate |
| Cauliflower | Cool | 60โ100 days | Full | โ | โ | Hard |
| Kale | Cool | 50โ65 days | Full | โ | โ | Easy |
| Lettuce | Cool | 30โ60 days | Part | โ | โ | Easy |
| Spinach | Cool | 35โ50 days | Part | โ | โ | Easy |
| Swiss chard | Cool | 50โ60 days | Full | โ | โ | Easy |
| Carrot | Cool | 60โ80 days | Full | โ | โ | Moderate |
| Beet | Cool | 50โ70 days | Full | โ | โ | Easy |
| Radish | Cool | 25โ35 days | Part | โ | โ | Easy |
| Onion | Cool | 90โ120 days | Full | โ | โ | Moderate |
| Garlic | Cool | 240โ270 days | Full | โ | โ | Easy |
| Strawberry | Cool | 90โ110 days | Full | โ | โ | Moderate |
Why each one works
Pea
Cool-season 55โ70 daysPeas thrive in Colorado's cool, variable springs and are one of the first seeds you can put in the ground โ direct-sow as soon as soil is workable, typically late March to mid-April, since established plants handle light frost without damage. Their 55โ70-day window fits neatly before summer heat triggers vine dieback. Try 'Sugar Snap' for reliable production at altitude; sow a second small batch two weeks after the first to stagger harvest.
Full pea growing guide โBroccoli
Cool-season 60โ90 daysBroccoli performs best in Colorado when heads develop during cool weather, which makes fall planting often more successful than spring. Transplant into the garden in early-to-mid July and heads will mature in September's ideal temperatures without bolting risk. 'Belstar' and 'Calabrese' are solid choices; set transplants out rather than direct-sowing to guarantee you finish inside the season.
Full broccoli growing guide โCabbage
Cool-season 60โ100 daysCabbage's frost tolerance makes it a reliable two-season crop in Colorado โ start transplants indoors in late February for spring planting, or set out new transplants in July for fall. Fall-harvested heads that experience light frost actually sweeten noticeably, turning a timing constraint into a flavor advantage. Choose shorter-season varieties like 'Stonehead' (70 days) to keep fall crops safely ahead of hard freeze.
Full cabbage growing guide โCauliflower
Cool-season 60โ100 daysCauliflower is Colorado's most demanding brassica โ it needs consistently cool temperatures and is less forgiving of both late spring cold snaps and summer heat spikes than its relatives. Target fall production exclusively by transplanting in early July, so curds develop through September. Blanch heads by gathering outer leaves over the developing curd to prevent sun exposure and yellowing, which is especially important under Colorado's intense UV.
Full cauliflower growing guide โKale
Cool-season 50โ65 daysKale is arguably the most Colorado-adapted vegetable on this list โ it tolerates hard frost, produces through summer with adequate water, and intensifies in sweetness after temperatures drop below 28ยฐF. Plant transplants 4โ6 weeks before last frost for a full season of harvest, and leave established plants in the ground well into November. 'Red Russian' and 'Lacinato' are especially cold-hardy; harvest outer leaves regularly to keep plants productive.
Full kale growing guide โLettuce
Cool-season 30โ60 daysLettuce is a spring and fall crop in Colorado โ it bolts quickly once daytime temperatures push past 80ยฐF, which eliminates most of summer. Direct-sow or transplant in late March through early May, then sow again in August for fall harvest that runs through October. For any planting that risks heat, use shade cloth (30โ40% density) to extend the window, and choose heat-tolerant varieties like 'Jericho' or 'Nevada' as summer approaches.
Full lettuce growing guide โSpinach
Cool-season 35โ50 daysSpinach germinates in soil as cold as 35ยฐF, making it Colorado's earliest possible food crop โ direct-sow in late March or even early April and it will be ready in 35โ50 days. It bolts reliably when days lengthen and heat arrives, so treat it as a spring-only crop except for a second sowing in August for fall. 'Tyee' and 'Space' are bolt-resistant varieties that extend your harvest window by a week or two at each end.
Full spinach growing guide โSwiss chard
Cool-season 50โ60 daysSwiss chard is one of the few vegetables that bridges Colorado's entire outdoor season โ it tolerates spring and fall frosts while handling midsummer heat that shuts down spinach and lettuce. Sow or transplant in late April and harvest continuously by cutting outer leaves through hard freeze in October or November. 'Fordhook Giant' is reliably productive; cut plants back to 3 inches in late July if heat causes stress and they will regrow vigorously into fall.
Full swiss chard growing guide โCarrot
Cool-season 60โ80 daysColorado's dry soil and intense fall sunshine produce exceptionally sweet carrots โ roots exposed to temperatures below 50ยฐF convert starches to sugars, and harvesting after early frost delivers the best flavor. Direct-sow in late April once soil reaches 50ยฐF, and keep the seed bed consistently moist during the notoriously slow germination phase (cover with burlap until sprouts emerge). Choose 'Chantenay' or 'Danvers' types if your soil is rocky or heavy clay.
Full carrot growing guide โBeet
Cool-season 50โ70 daysBeets tolerate both spring cold and moderate summer heat, making them flexible enough for two Colorado plantings โ direct-sow in early April for summer harvest and again in late July for fall. Like carrots, their sweetness improves dramatically after exposure to cool autumn temperatures. Soak seed clusters overnight before planting to speed germination, and thin to 3-inch spacing early; crowded beets stay small regardless of season length.
Full beet growing guide โRadish
Cool-season 25โ35 daysRadishes mature so quickly โ 25โ35 days โ that they function as a calendar filler between other plantings rather than a primary crop in Colorado. Sow every two weeks from early April through May, then again from mid-August through September; summer heat causes pithy texture and rapid bolting, making July plantings largely wasted. Fall-sown daikon types like 'Miyashige' grow slowly enough to produce substantial roots in October's cool conditions.
Full radish growing guide โOnion
Cool-season 90โ120 daysOnions need Colorado's full season and are most reliable as transplants or sets rather than direct-sown seed, since starting from seed extends the time requirement beyond what most gardens allow. Plant sets or transplants in early to mid-April and expect harvest in late August; choose intermediate-day varieties like 'Candy' or 'Superstar' that are calibrated to Colorado's day-length range for reliable bulb development. Cure harvested bulbs in a warm, dry, ventilated spot for two to three weeks before storage.
Full onion growing guide โGarlic
Cool-season 240โ270 daysGarlic sidesteps Colorado's short summer season entirely by growing over winter โ plant individual cloves in October, two to four weeks before hard ground freeze, and harvest the following July after foliage tips brown. Mulch beds with 4โ6 inches of straw after planting to moderate soil temperature swings; remove mulch gradually in March as shoots emerge. Hardneck varieties like 'Chesnok Red' or 'German Red' overwinter reliably in zone 5 and produce large, flavorful bulbs better suited to Colorado winters than softneck types.
Full garlic growing guide โStrawberry
Cool-season 90โ110 daysStrawberries are perennial in Colorado's zone 5 when given proper winter mulch โ plant bare-root crowns in April, mulch with 4โ6 inches of straw after the ground freezes in November, and pull back mulch gradually in spring. June-bearing varieties like 'Allstar' or 'Honeoye' concentrate their crop into a reliable two-to-three-week window that fits the season cleanly; protect spring blossoms from late frost with row cover, as flowers are far more frost-sensitive than the foliage. Replace plantings every three years as vigor declines.
Full strawberry growing guide โFrequently asked questions
When should I start seeds indoors for Colorado's spring garden?
Count backward from your last expected frost date โ typically early May at Front Range elevations, later at higher altitudes. Broccoli, cabbage, and cauliflower need 6โ8 weeks of indoor growth, so start them in mid-to-late February. Lettuce and kale need only 4โ5 weeks, putting their indoor start in March.
Can I get two harvests of the same crop in one Colorado season?
Yes, for the faster crops. Radishes, spinach, lettuce, and beets all mature quickly enough for a spring sowing and a separate fall sowing in the same year. Sow the spring batch as early as April, let it run its course, then replant in late July or early August to mature before October's first hard frost.
How does Colorado's altitude affect vegetable gardening beyond the frost dates?
Intense UV at high elevation can sunscald transplants that haven't been hardened off, so spend 7โ10 days transitioning seedlings from indoors to full sun gradually. Dry air and strong sun accelerate soil moisture loss, so drip irrigation or consistent hand-watering matters more than in humid climates. The upside is dramatically lower fungal disease pressure โ powdery mildew and downy mildew are far less common than at lower elevations.
What's the best way to extend the season on both ends in Colorado?
Floating row cover (1.5 oz weight) adds roughly 4โ6ยฐF of frost protection and is the most cost-effective tool for pushing spring planting 2โ3 weeks earlier and keeping fall crops productive 2โ3 weeks longer. Cold frames and low tunnels push that margin further still. For fall specifically, monitor overnight lows from mid-September onward and throw cover over beds when the forecast drops below 28ยฐF โ a single uncovered hard freeze will end the season for most crops.