Ninety frost-free days sounds like a prison sentence for your garden. It’s actually plenty — if you plan like it matters.
Across the Mountain West — high valleys in Colorado, most of Montana, northern Wyoming, central Idaho, and the higher elevations of Utah — gardeners work with growing seasons between 75 and 110 frost-free days. That’s roughly mid-June to mid-September. Meanwhile, garden magazines show off tomato harvests from gardeners in Zone 7 who planted in April and won’t see frost until November. Those articles aren’t for you.
This one is.
The Math That Matters
Your frost-free days aren’t your only growing days. They’re just the days when you can have tender crops outdoors without protection. With planning, your actual productive season can stretch from April through November — even with 90 unprotected days.
Here’s how to think about it:
| Season Segment | Approximate Dates (Zone 4-5, 6,000+ ft) | What’s Growing |
|---|---|---|
| Early spring (protected) | April 1–May 31 | Cool-season starts in cold frames, peas direct-sown under cover |
| Main warm season | June 15–September 15 | Everything — tomatoes, beans, squash, peppers with protection |
| Fall extension | September 15–November 1 | Cold-hardy greens, root crops under cover |
| Winter harvest | November–March | Overwintered garlic, stored root crops, cold frame greens |
That’s potentially 7 months of food production from a climate most people write off as too cold for gardening.
Strategy 1: Start Indoors Early and Harden Off Slowly
Indoor seed starting isn’t a nice-to-have in short-season climates. It’s the foundation of your entire warm-season harvest.
The timeline for a June 15 transplant date:
| Crop | Weeks Before Transplant | Indoor Start Date |
|---|---|---|
| Peppers, eggplant | 10–12 weeks | March 20–April 1 |
| Tomatoes | 6–8 weeks | April 15–May 1 |
| Broccoli, cauliflower, cabbage | 6–8 weeks | April 15–May 1 |
| Squash, cucumbers | 3–4 weeks | May 15–25 |
Hardening off matters more at altitude. Mountain West UV is intense. A seedling that goes from your windowsill to full mountain sun without transition will bleach and wilt within hours. Harden off over 7–10 days:
- Days 1–3: Shaded area outdoors, 2–3 hours, then back inside
- Days 4–5: Morning sun only, 4–5 hours
- Days 6–7: Full sun, 6–8 hours, bring in at night
- Days 8–10: Leave out overnight if temps stay above 45°F
Pro Tip: A cold frame or unheated hoop house is the ideal hardening-off station. It buffers wind and temperature while letting plants acclimate to outdoor UV gradually.
Strategy 2: Choose Varieties by Days-to-Maturity, Not by Flavor Reviews
In a 90-day season, every day counts. That beautiful heirloom Brandywine tomato (80 days listed, realistically 95+ at altitude)? It’s a gamble you’ll probably lose. Save it for a hoop house.
The short-season variety rule: Listed maturity + 14 days altitude penalty must be less than your frost-free days minus 7 days of safety margin.
For a 90-day season: Listed maturity should not exceed 69 days for warm-season crops planted as transplants.
Short-Season All-Stars
Tomatoes (all under 65 days listed):
- Stupice (60 days) — Cold-tolerant, productive, decent flavor
- Glacier (55 days) — One of the earliest. Semi-determinate, good fresh eating.
- 4th of July (49 days) — Lives up to its name. Medium-sized fruit.
- Sub Arctic Plenty (45 days) — Tiny plant, small fruit, but absurdly early
- Siletz (52 days) — Oregon-bred for cool climates. Larger fruit than most early varieties.
- Moskvich (60 days) — Russian heirloom. Rich flavor for an early tomato.
Peppers (under 70 days):
- Ace (50 days) — Green bell that actually ripens in short seasons
- King of the North (68 days) — Large bell, bred for northern gardens
- Early Jalapeño (65 days) — The only jalapeño worth planting above 6,000 feet
- Shishito (60 days) — Thin-walled, fast-maturing, incredible grilled
Beans (under 55 days):
- Provider (50 days) — Cold soil tolerant. Can direct sow slightly earlier than other beans.
- Contender (49 days) — Disease-resistant, reliable
- Dragon Tongue (55 days) — Gorgeous wax bean. Dual-purpose fresh or dry.
Squash (under 55 days):
- Black Beauty Zucchini (50 days) — The standard for a reason
- Ronde de Nice (52 days) — French round zucchini. Pick at baseball size.
- Patty Pan/Sunburst (52 days) — Scallop squash. Charming and fast.
Cucumbers (under 55 days):
- Marketmore 76 (58 days) — Pushing it, but works with an early transplant start
- Spacemaster (56 days) — Compact plants, good for containerss
- Pick a Bushel (50 days) — Pickling cucumber that produces fast
Strategy 3: Use Season Extenders on Both Ends
The difference between a 90-day gardener and a successful short-season gardener is usually 4–8 weeks of protection at the margins.
Spring Extension (Gaining 2–4 Weeks)
Wall O’ Waters — Those red teepee-shaped water-filled season extenders look ridiculous and work beautifully. Set them around tomato and pepper transplants 2–4 weeks before your last frost date. Water absorbs daytime heat and releases it overnight, keeping the interior 10–20°F warmer than outside air. At altitude, this is the difference between a June 1 and June 15 transplant date — or the equivalent of 200–300 additional growing degree days.
Black plastic mulch — Lay it 2–3 weeks before planting to warm soil. At 6,000+ feet, cold soil is often the limiting factor more than air temperature. Black plastic can raise soil temperature 5–8°F.
Low tunnels (hoops + row cover) — Wire hoops every 4 feet with Ag-30 row cover draped over them. Provides 4–8°F of frost protection and wind reduction. Leave in place until nighttime temps are consistently above 50°F.
Fall Extension (Gaining 3–6 Weeks)
Heavy row cover (Ag-50 or Ag-70) — For cold-hardy crops, heavier fabric provides 6–10°F of protection. Kale under Ag-50 can survive into late October or November even in Zone 4.
Cold frames — A 4x8-foot cold frame with a recycled window sash on top creates a microclimate that’s 15–20°F warmer than outside on sunny days. Vent on warm afternoons to prevent overheating. Fall lettuce and spinach in a cold frame produce well into December.
Harvest before you lose it — Pick all remaining tomatoes (even green ones) before the first hard frost (below 28°F). Green tomatoes ripen indoors in a paper bag with a banana. You’ll get ripe tomatoes into November from a September harvest.
Strategy 4: Succession Plant Cool-Season Crops
Don’t plant all your lettuce on the same day. With 90 frost-free days, you can get 3–4 succession plantings of greens and radishes:
- Planting 1 (early spring under cover): Late April–mid-May. Peas, spinach, lettuce, radishes in cold frame or under row cover.
- Planting 2 (main season): At last frost date. Direct sow another round of greens plus all warm-season crops.
- Planting 3 (midsummer): July 1–15. Sow fall lettuce, spinach, kale, radishes, and turnips.
- Planting 4 (late summer): August 1–10. Final round of radishes, arugula, and spinach for fall harvest under cover.
This approach gives you continuous salad greens from May through November instead of one big harvest in July.
Strategy 5: Focus on What Short Seasons Do Best
Instead of fighting your climate to grow mediocre warm-season crops, lean into what 90-day seasons produce exceptionally well:
Root vegetables — Carrots, beets, turnips, and parsnips develop complex sweetness in cool temperatures. A Mountain West carrot harvested after a light frost has more sugar than any California carrot you’ll find in a store.
Brassicas — Broccoli, cauliflower, cabbage, Brussels sprouts, and kohlrabi all prefer the cool temperatures that are your default. They’re at home here in ways they never are in Zone 7.
Peas — Sugar snap and snow peas produce for weeks in cool mountain climates. In hot regions, peas are done by June. In the Mountain West, they’ll produce into August if you plant successionally.
Potatoes — A 90-day season is perfect for early and mid-season potatoes. Yukon Gold (65 days), Red Norland (70 days), and Kennebec (80 days) all fit comfortably. Plant 2 weeks before last frost — potato shoots tolerate light frost and the tubers are underground.
Garlic — Plant hardneck varieties in October. They overwinter in the ground, emerge in spring, and harvest in July. Zero frost-free days required during establishment. Colorado and Montana grow world-class garlic.
Strategy 6: Build Heat Into Your Garden Design
Orient raised beds on a north-south axis to maximize sun exposure on both sides. But if you can only have one bed, orient it east-west and place tall crops (tomatoes, corn) on the north side so they don’t shade everything else.
Thermal mass matters. Place large rocks or cinder blocks along the north side of beds. They absorb solar heat all day and radiate it back overnight, reducing frost risk for nearby plants. A south-facing stone wall is the warmest microclimate in your garden — plant your tomatoes there.
Container gardening as heat management. Dark-colored containers on a south-facing patio absorb tremendous heat. A 15-gallon black nursery pot with a tomato plant against a south wall will outperform the same variety in an open garden bed at altitude.
Common Mistakes
- Planting the same varieties as your low-elevation friends. Their 160-day season and your 90-day season require completely different variety lists.
- Waiting for “perfect” weather to transplant. There’s no perfect week. Get transplants out on schedule with protection and move on.
- Neglecting fall gardening. Your fall season is cooler, calmer, and better for greens than your spring. Don’t stop planting in June.
- Refusing to use season extenders. Wall O’ Waters cost $15 for a set of three. A 4x8 cold frame costs $100 in lumber and a recycled window. These investments pay for themselves in the first season.
- Growing too many warm-season crops. In a 90-day season, allocate 60% of your space to cool-season crops and 40% to warm-season. You’ll eat better.
Your Short Season Is Enough
Ninety frost-free days with a plan beats 180 frost-free days without one. Mountain West gardeners who embrace their climate — choosing the right varieties, extending the season intelligently, and leaning into cool-season strengths — grow more food than they expect and eat better than they imagined.
For complete short-season planting plans, variety recommendations by elevation, and week-by-week task calendars, check out The Mountain West Vegetable Gardening Guide from Harvest Home Guides. Because gardening in the mountains shouldn’t require guessing.
📚 Want the complete guide? Mountain West Vegetable Gardening covers everything you need — planting calendars, variety picks, soil strategies, and more — all tailored to your region. Browse the Harvest Home Guides series →