Colorado · zones 5a–7a
When to plant tomatoes in Colorado.
Sow or transplant between May 16 to Jul 4 in a typical year, keyed to Colorado's median last frost around May 9. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Tomatoes dates for a typical Colorado season.
1 · Start indoors
Apr 4 to May 23
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
May 16 to Jul 4
Keyed to the ~May 9 last frost. Sow 0.25" deep, 24" apart.
Harvest
~73 days later
~12 lb per plant over ~8 weeks.
Colorado spans more than one climate: Zone 6 is the most common zone, but the state runs 5a to 7a. That's why the planner works from your ZIP code's frost dates, not your zone: mountain and coastal gardens in the same state can be three weeks apart.
Colorado growing notes
Colorado tomatoes fight three things at once: night chill, hail, and thin air.
Pick a variety that sets fruit in cool nights
Tomato pollen loses viability once nighttime temperatures drop below about 55 degrees, and high-elevation Colorado gardens can see nights that cool even in July, which is why season length on paper does not guarantee a full harvest. Stupice, a Czech heirloom bred for short, cool seasons, and Fourth of July, ready in roughly 49 days from transplant, both keep setting fruit through the cold snaps that stall slower slicing types.
Early Girl splits the difference for gardeners who want a full-size slicer without betting the whole season on it. Whatever variety goes in, wait until soil at 2 inches holds steady in the 60s before transplanting, and consider a wall-of-water sleeve or similar season-extending cover for the first 2 to 3 weeks; it buys 8 to 10 degrees of night protection while the plant establishes roots.
Cage for hail before it happens, not after
A mature tomato plant loses leaves and cracks fruit under hail much the way a strawberry bed does, and Colorado's storm season runs May through August, overlapping almost the entire fruiting window. A sturdy welded-wire cage, not the flimsy cone type sold in most garden aisles, holds its shape under hail impact and keeps branches from snapping outward under the weight of ice.
For a bed of any size, keep a stack of old bedsheets or frost blankets within reach from June onward; throwing a cover over cages ahead of a severe thunderstorm warning takes 5 minutes and can save a season's fruit set. Plants that lose more than a third of their leaf canopy to hail in one storm often need 2 to 3 weeks to resume normal fruit sizing.
Dry air, uneven watering, and blotchy fruit
Colorado's alkaline clay and low humidity combine to create a specific watering trap: the soil surface looks dry within hours of a soak, tempting a gardener to water again before the root zone actually needs it, and that swing between soaked and parched is what triggers blotchy, sunken patches on the fruit's blossom end. Mulching 2 to 3 inches deep keeps the surface from lying about what is happening 4 inches down.
Aim for consistent moisture rather than a fixed schedule: a soil probe or even a wooden skewer pushed in and pulled out tells the truth in seconds, damp dirt clinging to it means hold off, a clean dry skewer means water now. Tomato hornworm is easiest to spot by the black droppings it leaves on lower leaves rather than by trying to see the caterpillar itself, which is camouflaged almost perfectly against the vine.
In Colorado: watch for
The pests that find tomatoes here.
The short season here, about 143 frost-free days, keeps tomato, slicing in front of tomato hornworm only briefly. One early row cover usually carries a planting through the worst of it.
Big green caterpillars strip leaves and leave black droppings.
Organic fix: Handpick at dusk; spray Bt on the young ones.
Clusters on new growth, sticky leaves, curling tips.
Organic fix: Blast with water, then insecticidal soap or neem.
Dark spots with concentric rings, starting on the lowest, oldest leaves.
Organic fix: Remove affected leaves, mulch to stop soil splash, space for airflow.
Greasy, water-soaked blotches in cool, wet spells. No rings, and not bottom-up.
Organic fix: Pull infected plants fast, keep leaves dry, use neem as a preventive.
Septoria leaf spot
Many small tan spots with dark brown edges, low leaves first, no rings inside.
Organic fix: Strip the lowest leaves, mulch against soil splash, stake for airflow.
Your season math: Colorado gives tomatoes about 143 frost-free days, and it needs roughly 73 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting tomatoes in Colorado: questions
When should I plant tomatoes in Colorado?+
In a typical Colorado year, sow tomatoes or set out transplants between May 16 to Jul 4, keyed to the state's median last frost around May 9. Your exact dates shift with your ZIP code and microclimate.
When do I start tomatoes seeds indoors in Colorado?+
Start seeds inside about 6 weeks before set-out: roughly Apr 4 to May 23 for a typical Colorado season.
What USDA zone is Colorado?+
Colorado spans zones 5a–7a, mostly zone 6. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many tomatoes plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.