Colorado Planting Guide: What to Grow and When

Colorado gardening operates by rules that differ fundamentally from anything a standard planting guide will tell you. The same Denver gardener who lives at 5,280 feet watches their neighbors in Grand Junction — 90 miles west but 2,700 feet lower — harvest their first tomatoes a full month earlier. The Aspen gardener at 8,000 feet works with a growing season of barely 90 days and a July frost risk that would be unthinkable at the same latitude in Kansas. In Colorado, elevation is not a footnote to climate. It is the climate.

This Colorado planting guide covers all four of the state’s primary gardening regions — the Front Range corridor, the Eastern Plains, the mountains, and the Western Slope — with specific frost dates by city, a month-by-month planting calendar, and the varieties that reliably perform across Colorado’s demanding conditions. Whether you garden in Denver’s compressed 140-day season or in Aspen’s tight 90-day window, the timing and plant selection here give you a realistic framework for a productive garden every year.

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The single most important concept for every Colorado gardener is that your last spring frost date is a starting line, not a guarantee. Unlike gardeners in the Midwest or South who watch the calendar and plant accordingly, Colorado gardeners must watch both the calendar and the forecast. A May 15 average last frost date for Denver means frosts can still occur in early June in anomalous years — and they do, often with spectacular and demoralizing results for gardeners who transplanted their tomatoes too early.

Colorado’s Climate Zones: Know Your Elevation First

Colorado spans USDA hardiness zones 3b through 7a, an unusually wide range for a single state. Unlike states where zone differences reflect latitude (warmer to the south, colder to the north), Colorado’s zones run primarily on elevation. A garden at 9,000 feet in Breckenridge is zone 3b, colder in winter than most of Alaska’s populated areas. Grand Junction at 4,600 feet on the Western Slope is zone 7a, comparable to parts of Oregon and the Mid-Atlantic region.

Four distinct gardening regions define Colorado’s growing conditions:

• Front Range Corridor (4,500–6,500 feet elevation) — Denver, Boulder, Fort Collins, Loveland, Colorado Springs, Pueblo — Zones 5a–6b. This is where most of Colorado’s population gardens. Last frost dates range from late April (Pueblo, zone 6a) to mid-May (Fort Collins, zone 5b) and the growing season runs 130–165 days depending on specific location. Late-season hail and intense UV radiation are defining challenges.
• Eastern Plains (3,500–5,500 feet elevation) — Greeley, Pueblo East, La Junta, Lamar — Zones 5b–6b. Warmer summers than the Front Range with less dramatic elevation change, but windier and prone to severe weather. Similar frost dates to Front Range cities at comparable elevation.
• Mountain Communities (7,000–11,000 feet elevation) — Aspen, Telluride, Breckenridge, Steamboat Springs, Glenwood Springs — Zones 3b–5b. Growing seasons of 80–120 days demand short-season variety selection and frost protection strategies. Late June frosts and early September frosts are a genuine risk.
• Western Slope (4,500–5,500 feet elevation, lower valleys) — Grand Junction, Montrose, Delta — Zones 6b–7a. Colorado’s warmest, most forgiving growing conditions. Grand Junction’s average last frost of early April and mild winters make it the closest Colorado comes to traditional extended-season vegetable gardening. Fruit production — peaches, apricots, pears — is commercially viable here in ways impossible elsewhere in the state.

Understanding how climate zones affect plant selection and growing season timing is fundamental to any Colorado planting strategy — and in Colorado, the zone number alone does not tell the full story. Microclimate matters enormously: a south-facing slope at 7,500 feet may perform like a zone 5b garden while a north-facing canyon at the same elevation behaves like zone 4a.

Colorado Frost Dates by City

The frost dates below use Colorado State University Extension historical probability data at 50% likelihood. For frost-sensitive transplants, planning around the 10% or 20% frost probability dates (typically 1–2 weeks later than the 50% dates) provides a more conservative but realistic safety margin in Colorado’s variable spring climate.

Source: Colorado State University Extension frost probability data. Dates indicate 50% probability. Highly variable at high elevations — plan conservatively and monitor forecasts weekly in May and early June.

An important nuance for Front Range gardeners: Denver’s location at the base of the Rocky Mountain foothills creates a “cold air drainage” effect on still, clear nights. Cold air flows downhill and pools in low-lying areas, meaning a gardener in a river bottom or valley floor location may experience frosts two to three weeks later in spring than a neighbor on the same street at a slightly higher elevation. Knowing your specific microclimate is worth more than any regional frost date table for planning early-season planting.

Spring Planting in Colorado

Spring is the most challenging season for Colorado gardeners because the window between “safe to plant” and “too hot for cool-season crops” can compress to just a few weeks, especially at lower elevations where summer arrives quickly. Cold-season crops should go in as early as possible, and warm-season crops require patience past the average last frost date.

March–April: Cool-Season Crops Outdoors

Front Range and Eastern Plains gardeners can begin direct-sowing cold-tolerant crops outdoors in March once soil is workable. Colorado State University Extension considers spinach, kale, arugula, radishes, and peas appropriate for outdoor sowing in March — they tolerate overnight temperatures into the mid-20s°F, particularly when protected with row cover.

• Direct sow spinach, arugula, lettuce, radishes, and peas as soon as soil can be worked (often mid-to-late March for Denver and Boulder). Use row cover to buffer cold snaps and extend the planting window.
• Transplant broccoli, cauliflower, cabbage, and kale outdoors from mid-to-late April for Front Range gardens after hardening off transplants started indoors six to eight weeks before.
• Start warm-season transplants indoors: tomatoes, peppers, and eggplant need six to eight weeks of indoor time before transplanting, so a May 10 last frost date means starting indoors by mid-March. Starting too early produces leggy, root-bound transplants that underperform at planting time.
• Western Slope gardeners (Grand Junction, Montrose) can transplant warm-season crops outdoors in mid-to-late April after the last frost, an entire month ahead of Front Range neighbors at similar latitude.

May: The Critical Transition Month

May is the most consequential month in Colorado’s spring gardening calendar. In most Front Range gardens, this is when the warm-season transplanting window opens — but it requires careful judgment rather than a fixed calendar date.

• Do not transplant frost-sensitive crops (tomatoes, peppers, cucumbers, basil) before May 10 in Denver and Boulder, or before May 15–20 in Fort Collins and Colorado Springs. These dates represent the average last frost, and individual years vary by two to three weeks in either direction.
• Monitor the 10-day forecast continuously through May. A Colorado May weekend forecast showing overnight lows of 42°F can drop to 30°F after a late-season cold front, and transplants set out without frost protection will not survive.
• Direct sow beans, squash, cucumbers, and corn outdoors after May 15 in Denver-area gardens, when soil temperature consistently reads 60°F or above at 4-inch depth. Germination in cold soil is unreliable and leads to rot. An inexpensive soil thermometer is one of the most practical tools any Colorado vegetable gardener can own.
• Plant sunflowers, zinnias, and marigolds directly in May. These warm-season annuals establish quickly and marigolds serve double duty as companion plants alongside vegetables, helping suppress aphids and whiteflies while adding pollinator traffic to the garden.

June: Mountain Season Opens

For mountain communities above 7,000 feet, June is when the outdoor growing season finally becomes practical. Even here, the calendar is not a guarantee — Steamboat Springs and Aspen gardeners watch their last frost averages in early June but monitor forecasts through the month.

• Transplant warm-season crops in mountain gardens only after the average last frost date for your location, and keep row cover available for the two weeks following transplant. An unexpected early June frost is not rare in Colorado mountain communities.
• Use only short-season varieties in mountain gardens: tomatoes with 55–65 days to maturity, bush beans with 50–55 days, and fast-maturing squash (45–55 days). Standard-season varieties requiring 75+ days to maturity often fail to produce before the first fall frost arrives in September.
• Direct sow cool-season crops alongside warm-season transplants in mountain gardens — Aspen and Steamboat Springs gardeners can grow lettuce, kale, and spinach simultaneously with tomatoes and squash, since summer temperatures rarely push past 80°F at altitude.

Summer Gardening in Colorado: Managing Altitude and Hail

Colorado summers are genuinely pleasant compared to most of the US: Front Range temperatures typically top out at 90–95°F rather than the 100°F+ of the Plains and Southwest, and cool nights in the 55–65°F range give gardens a nightly recovery period that warm-climate gardeners never experience. The genuine summer challenges in Colorado are not heat — they are UV radiation, hail, and water management.

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UV Radiation at Altitude

At Denver’s elevation of 5,280 feet, UV radiation is approximately 25% more intense than at sea level. At 8,000 feet in Aspen, it is roughly 35% more intense. This has real consequences for plant physiology: some heat-tolerant varieties that perform well at sea level show sunscald and bleaching in Colorado’s high-altitude UV environment, particularly on the south and west faces of fruits like peppers and tomatoes. Colorado State University Extension recommends choosing varieties with dense foliage cover or using light shade cloth on the south and southwest sides of tomato and pepper cages to prevent fruit sunscald during July and August.

Hail: Colorado’s Signature Gardening Hazard

Colorado sits in one of the most hail-prone corridors in North America. Denver averages 7–9 hail events per year, many occurring from May through August during late-afternoon thunderstorm activity. A single storm can strip a mature vegetable garden to bare stems in minutes — a devastating experience that has caused many Colorado gardeners to abandon outdoor growing entirely.

The most effective response is hail cloth, which has become widely available and has transformed what is possible in Colorado vegetable gardens. A permanent hail cloth structure over raised beds, using a frame of hoops or a simple cattle panel arch, can deflect all but the largest hail events while allowing light, air, and water to pass through. Colorado gardeners who install hail cloth structures report dramatically more consistent harvests year over year compared to uncovered gardens. It is worth the one-time investment and is the single biggest protective measure available to Colorado vegetable gardeners.

Temperature Swings and Cool Nights

Colorado summers are characterized by a diurnal temperature range that is exceptional compared to most of the US — a swing of 30–40°F between daily high and low temperatures is common throughout the summer, even in July. This is not a challenge but an advantage: cool nights slow respiration, concentrate sugars in fruit, and reduce water stress on plants that have absorbed intense afternoon sun. Colorado tomatoes, grown slowly with cool nights, develop richer flavor than those grown in consistently warm climates. The same pattern benefits corn, peppers, and melons when the season is long enough to ripen them.

Managing this effectively means understanding that Colorado’s “short season” is relative. At the Front Range, 140–165 days of growing time is sufficient for almost any standard garden vegetable — the challenge is not days but timing. Starting transplants at the right date indoors, hardening off properly, and getting them in the ground on the correct schedule produces excellent results. Review a comprehensive seasonal planting calendar to understand how Colorado’s timing fits within the broader US planting picture and to identify any succession sowing opportunities within your season.

Fall Planting in Colorado: Extended Cool-Season Harvests

Fall is an underutilized season in Colorado gardens. Many gardeners treat summer’s end as the end of production, but cool-season crops planted in August and September produce some of their best harvests in the cooler weeks of September and October. Kale and Brussels sprouts genuinely improve after light frosts, which trigger the conversion of starches to sugars. Spinach and arugula sown in late August are growing vigorously in October when the spring sowings would have already bolted. Fall planting extends the productive season by four to six weeks at almost no additional effort.

August: Fall Planting Begins

Count back from your expected first fall frost date to plan fall planting. For Denver (first frost averaging October 7), an August 1 planting date of fast-maturing kale (50 days) allows full maturity before frost, with harvests continuing through multiple frost events as kale tolerates temperatures into the mid-20s°F.

• Direct sow kale, spinach, arugula, and lettuce from early August onwards for Front Range and Eastern Plains gardens. These crops germinate well in summer soil temperatures and establish before the season cools.
• Sow radishes, turnips, and beets in mid-to-late August for fall harvests. Beets planted in August in Denver routinely reach harvestable size by early October.
• Start broccoli and kale transplants in August for setting out in September. CSU Extension recommends transplanting fall brassicas rather than direct sowing for more reliable results in Colorado’s variable August weather.

September–October: Garlic and Season Wrap

• Plant garlic from mid-September through mid-October, before the soil freezes in November. Colorado State University Extension recommends hardneck varieties (Rocambole, Purple Stripe, Porcelain types) for Colorado gardens — these are better adapted to Colorado’s cold winters and produce superior cloves for the high-altitude climate than softneck types designed for mild-winter regions.
• Plant spring-blooming bulbs in September and October: tulips, daffodils, and alliums all thrive in Colorado’s cold winters and are not reliably reliable in warmer climates. Colorado’s cold winters provide the vernalization that spring bulbs require without any pre-chilling. Plant before the ground freezes hard, typically by early November in Front Range locations.
• Apply a 3–4 inch layer of compost or wood chip mulch to all beds before frost to insulate soil, preserve structure, and break down over winter so beds are ready to plant immediately in spring.
• Mountain gardeners wrap up outdoor production by mid-to-late September at most elevations above 7,000 feet. Use row covers or cold frames to extend the season through October for cold-hardy greens.

Colorado Month-by-Month Planting Calendar

This calendar uses three reference zones: Front Range (Denver/Boulder/Fort Collins, zones 5a–6b), Mountain (Aspen/Steamboat Springs, zones 3b–5b), and Western Slope (Grand Junction/Montrose, zones 6b–7a).

Top Plants for Colorado Gardens

Colorado’s altitude, intense UV, and wide temperature swings favor plants with specific traits: compact growth, cool-season tolerance, and varieties bred for short-season performance. The table below covers the most reliable performers across Colorado’s primary gardening regions.

Common Colorado Gardening Challenges

Late Spring Frosts and Unpredictable Weather

Colorado’s spring weather is notoriously inconsistent. A warm, sunny spell in late April that reaches 75°F can be followed a week later by a late-season snow event that drops temperatures to 28°F overnight. This is not unusual — it is the pattern. Colorado State University Extension consistently recommends that Front Range gardeners not transplant frost-sensitive crops before their specific last frost date regardless of how warm the preceding weeks have been, and that they keep row cover and frost cloth accessible through Memorial Day weekend.

The practical strategy is to harden off transplants properly (gradual outdoor exposure over one to two weeks) and to invest in quality floating row cover (1.5 oz weight minimum for frost protection) as a permanent gardening tool rather than an emergency measure. Gardens with row cover available show dramatically better survival rates through Colorado’s variable spring than uncovered plots.

Intense Sun and Low Humidity

At Colorado’s altitudes, the combination of high UV radiation and very low humidity (Front Range summer relative humidity averages 30–40%) accelerates soil moisture loss and can cause leaf scorch on plants that perform perfectly at sea level. Dark-colored vegetables like eggplant and purple basil can show sun bleaching on western-facing surfaces. Tomatoes and peppers on the south and west sides of plants often develop sunscald — white or tan patches that indicate UV damage even when air temperatures are not extreme.

Three practices address this most effectively. First, maintain consistent mulch depth of 3–4 inches over all vegetable beds throughout the growing season to minimize soil moisture loss. Second, water deeply and infrequently using drip irrigation — shallow, frequent watering combined with Colorado’s low humidity produces surface salt accumulation and shallow root development. Third, provide afternoon shade (30% shade cloth) on the west side of heat-sensitive crops like lettuce and spinach during July and August to prevent bolting and leaf burn.

Dry Conditions and Water Conservation

Denver receives approximately 14 inches of annual precipitation, most of it arriving as winter snow or summer afternoon thunderstorms that evaporate quickly in low humidity. During the critical June and July growing period, supplemental irrigation is essential — a Front Range vegetable garden typically requires 1–1.5 inches of water per week during peak summer growing when evapotranspiration rates are highest.

Colorado State University Extension recommends drip irrigation as the highest-efficiency option for Colorado vegetable gardens. Overhead sprinklers waste water to evaporation at Colorado’s altitude and keep foliage wet, increasing the risk of powdery mildew and leaf diseases during the humid stretch of monsoon moisture that occasionally reaches the Front Range in August. Drip irrigation also allows gardeners to track exactly how much water their garden receives, which is valuable for adjusting to CSU Extension’s published crop water use estimates.

Soil Challenges at Altitude

Front Range soils are often alkaline (pH 7.5–8.0) and may contain caliche-like hardpan at depth. Mountain soils tend to be thin, rocky, and acidic depending on the surrounding geology. Neither extreme is ideal for vegetable production without amendment. Colorado State University Extension recommends raised beds filled with a 50/50 mix of compost and native topsoil as the most reliable starting point for Front Range gardeners dealing with alkaline clay, and suggests annual additions of compost (3–4 inches per bed per year) to gradually improve soil structure and nutrient availability.

Iron deficiency is common in alkaline Front Range soils, causing yellow leaves with green veins (interveinal chlorosis) in tomatoes, raspberries, and many ornamentals. Use chelated iron rather than iron sulfate for alkaline soil applications — iron sulfate becomes chemically unavailable above pH 7.5, while chelated forms remain plant-accessible across the pH range typically found in Colorado gardens.

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Frequently Asked Questions About Colorado Gardening

When should I plant tomatoes in Colorado?

For the Front Range (Denver, Boulder, Fort Collins, Colorado Springs), transplant tomatoes outdoors after May 10–15 in most years, but monitor the 10-day forecast carefully. A year with a warm April followed by a cold front in late May can damage tomatoes planted on the average last frost date. Keep frost cloth on hand through Memorial Day as a safety measure. Western Slope gardeners can transplant by April 20–30 in most years. Mountain gardeners should wait until early to mid-June, using only short-season varieties (55–65 days to maturity).

What are the easiest vegetables to grow in Colorado?

Kale, broccoli, spinach, peas, garlic, and zucchini are Colorado’s most reliable performers. Kale is exceptional — it grows from early spring through hard frost, improves in flavor with cold, and can produce for eight months at Front Range elevations. Peas thrive in Colorado’s cool spring temperatures and can go in the ground in March. Zucchini produces prolifically in Colorado summers with minimal management once established. All three work at mountain elevations with only minor timing adjustments.

How do I protect my garden from hail in Colorado?

Install a permanent hail cloth structure over your vegetable beds. Agricultural hail cloth (30–50% UV shade rating with hail protection spec) stretched over a simple hoop or arch frame deflects all but the most extreme hail events while allowing light, water, and air movement. This is the single most effective protection strategy available to Colorado vegetable gardeners, and the investment pays back in even one prevented hail event. Some Front Range gardeners also use floating row cover for smaller hail events — it will not stop large hailstones but buffers minor impacts that damage tender leaf tissue.

Can I garden year-round in Colorado?

Outdoors, year-round production is not practical at most Colorado locations. Front Range gardeners can extend the outdoor season from March through November with row covers and cold frames, giving eight to nine months of production. In January and February, cold frames and low tunnels over kale and spinach can maintain some production in mild winters. Western Slope gardeners have the longest outdoor season, sometimes extending to nine or ten months with season-extension structures. Mountain gardeners above 7,500 feet work with a true five-to-six-month outdoor window. Indoor growing under lights is practical statewide for year-round lettuce and herb production.

Why do my tomatoes have so many blossoms but little fruit in Colorado?

Blossom drop is common in Colorado during temperature swings and is one of the most frequent questions Colorado State University Extension receives from vegetable gardeners. Tomatoes fail to set fruit when nighttime temperatures drop below 55°F or daytime temperatures exceed 95°F — both conditions occur regularly on the Front Range. Cool early-summer nights in June are a common cause of poor initial fruit set even in otherwise healthy plants. The solution is to plant heat-set varieties (Celebrity, Legend, Mountain Magic) that hold fruit more reliably through temperature fluctuations, and to be patient — Front Range tomatoes often produce their best set in late July and August when temperatures stabilize.

Sources

• Colorado State University Extension — Home Horticulture Resources
• Colorado State University Extension — Vegetable Gardening in Colorado (Publication 7.712)