Colorado’s high altitude and cool nights require tomato planting to be timed carefully for a successful harvest. Knowing exactly when to plant tomatoes in Colorado can mean the difference between a bumper crop and a frost-killed disaster. This guide will walk you through every step, from soil temps to last frost dates, so you can get your tomatoes in the ground at the perfect moment.
Tomatoes are warm-season plants that hate cold weather. In Colorado, the growing season is short, so timing is everything. If you plant too early, a late frost can kill your seedlings. Plant too late, and your tomatoes won’t ripen before the first fall frost.
Let’s break it down simply. You need to know your local frost dates, soil temperature, and how to harden off your plants. We’ll cover all of that here.
When To Plant Tomatoes In Colorado
The short answer: Most of Colorado should plant tomatoes outdoors between mid-May and early June. But this varies by elevation and microclimate. Denver, Colorado Springs, and the Front Range typically aim for the last week of May. Mountain towns like Leadville or Breckenridge may need to wait until mid-June or even early July.
Here’s the key rule: Wait until after your area’s average last frost date. Then, check the soil temperature. It should be at least 60°F (16°C) for tomatoes to thrive. Cold soil stunts growth and invites disease.
Understanding Colorado’s Frost Dates
Colorado’s weather is unpredictable. A warm April can trick you into planting early, only to have a May snowstorm. Always check your local frost date. You can find this online from the Colorado State University Extension or the National Weather Service.
General guidelines for Colorado:
- Denver/Front Range: Last frost around May 5-15. Plant tomatoes after May 15-20.
- Colorado Springs: Last frost around May 10-20. Plant after May 20.
- Pueblo/Southern CO: Last frost around April 25-May 5. Plant after May 5-10.
- Mountain towns (8,000+ ft): Last frost as late as June 10-20. Plant after June 10.
- Western Slope (Grand Junction): Last frost around April 20-30. Plant after May 1-5.
These are averages. A cold snap can happen anytime. Always watch the 10-day forecast before planting.
Soil Temperature Matters More Than Air Temperature
Air temperature can be warm, but cold soil will shock your tomatoes. Use a soil thermometer. Insert it 4 inches deep in the morning. If it’s below 60°F, wait. Ideal soil temp is 65-70°F.
To warm soil faster:
- Cover beds with black plastic or landscape fabric a week before planting.
- Raised beds warm up quicker than ground soil.
- Plant in a sunny spot that gets at least 8 hours of direct light.
Starting Tomatoes From Seed Indoors
If you grow from seed, start indoors 6-8 weeks before your last frost date. For Denver, that means starting seeds around late March to early April. Use seed-starting mix and keep them under grow lights or a sunny window.
Steps for seed starting:
- Fill small pots or trays with moist seed-starting mix.
- Plant seeds 1/4 inch deep.
- Keep soil moist and warm (70-80°F).
- Once seedlings appear, move to bright light.
- Thin to one strong seedling per pot.
- Fertilize with a weak liquid fertilizer after true leaves appear.
Hardening off is critical. About 7-10 days before planting outdoors, start moving seedlings outside for a few hours each day. Increase time gradually. Protect from wind and direct sun at first. This prevents transplant shock.
Buying Transplants: What To Look For
If you buy plants from a nursery, choose stocky, dark green plants with thick stems. Avoid tall, leggy plants or ones with yellow leaves. Look for plants that are not yet flowering or fruiting—they’ll transplant better.
When to buy: Don’t buy too early. Wait until you’re ready to plant within a week. Keep them in a sheltered spot if you buy before the weather is settled.
Planting Day: Step-By-Step
Choose a cloudy day or plant in the late afternoon to reduce transplant stress. Follow these steps:
- Dig a hole deeper than the root ball. Remove the bottom leaves and bury the stem up to the top leaves. Roots will grow from the buried stem, making a stronger plant.
- Add a handful of compost or slow-release tomato fertilizer to the hole.
- Water the hole before placing the plant.
- Set the plant in, fill with soil, and firm gently.
- Water thoroughly after planting.
- Add a tomato cage or stake right away to avoid damaging roots later.
- Mulch around the base with straw or shredded leaves to retain moisture and keep soil warm.
Protecting Tomatoes From Late Frosts
Even after planting, a late frost can hit. Be prepared to cover your plants. Use row covers, old sheets, or frost blankets. Remove covers in the morning when temps rise above 40°F.
For extra protection, use cloches (plastic jugs with bottoms cut off) or water-filled walls that absorb heat during the day and release it at night.
Varieties That Work Best In Colorado
Not all tomatoes are equal in Colorado’s short season. Choose varieties that mature in 60-80 days. Here are top picks:
- Early Girl (50-60 days) – reliable, classic red slicer.
- Celebrity (70 days) – disease-resistant, good for canning.
- Sun Gold (55-65 days) – sweet cherry tomato, very productive.
- Roma (75 days) – great for sauces, paste tomatoes.
- San Marzano (80 days) – classic paste tomato, needs warm spot.
- Glacier (55 days) – bred for cool climates, perfect for mountains.
- Sub-Arctic Plenty (50-55 days) – super early, handles cold.
For high elevation, stick with determinate or semi-determinate varieties that ripen all at once. Indeterminate types can work but need a long, warm season.
Common Mistakes To Avoid
Even experienced gardeners make errors. Here are the biggest ones:
- Planting too early. Patience pays off.
- Not hardening off. This causes leaf burn and stunted growth.
- Planting in cold soil. Use a thermometer.
- Overwatering. Tomatoes like consistent moisture, not soggy feet.
- Skipping support. Cages or stakes prevent broken branches.
- Ignoring wind. Colorado wind can snap stems. Use windbreaks or plant near a fence.
When To Plant In Different Colorado Regions
Let’s get specific. Here’s a breakdown by region:
Front Range (Denver, Boulder, Fort Collins)
Last frost mid-May. Plant tomatoes around May 15-25. Use black plastic to warm soil. Choose early-mid season varieties.
Colorado Springs & Pueblo
Last frost late May for Springs, early May for Pueblo. Plant in late May for Springs, early May for Pueblo. Watch for hail—cover plants if needed.
Western Slope (Grand Junction, Montrose)
Warmer climate. Last frost late April. Plant in early May. You can grow longer-season varieties like Brandywine or Cherokee Purple.
Mountain Towns (Leadville, Breckenridge, Aspen)
Very short season. Last frost mid-June to early July. Plant after June 10. Use cold frames or high tunnels. Stick with ultra-early varieties like Glacier or Sub-Arctic Plenty. Consider growing in containers that can be moved indoors.
Eastern Plains (Limon, Burlington)
Similar to Front Range but windier. Plant mid-May to early June. Use windbreaks and sturdy cages.
How To Extend Your Growing Season
Colorado’s season is short, but you can stretch it. Use these techniques:
- Use row covers or low tunnels in spring and fall.
- Plant in raised beds or containers that warm faster.
- Use reflective mulch to increase light and warmth.
- Prune suckers to focus energy on ripening fruit.
- Pick green tomatoes before first fall frost and ripen indoors.
Signs Your Tomatoes Are Ready To Plant
Before you put plants in the ground, check these signs:
- Nighttime temperatures consistently above 50°F.
- Soil temperature at 60°F or higher.
- No frost in the 10-day forecast.
- Plants are hardened off (have spent 7+ days outside).
- Stems are thick and leaves are dark green.
If any of these are missing, wait. A few extra days can save your crop.
What If You Plant Too Early?
If you jumped the gun and a frost is coming, act fast. Cover plants with blankets or row covers. Water the soil before the frost—wet soil holds heat better. Use cloches or milk jugs for individual plants. If the frost is severe, you may lose leaves, but the stem might survive. Trim damaged leaves and hope for regrowth.
If plants are stunted from cold, they may never fully recover. It’s often better to replant with new seedlings if the damage is bad.
Fertilizing After Planting
Tomatoes are heavy feeders. About 2 weeks after planting, start fertilizing with a balanced fertilizer (10-10-10) or one higher in phosphorus (like 5-10-10) to encourage flowers and fruit. Avoid too much nitrogen, which gives lush leaves but few tomatoes.
Fertilize every 2-3 weeks during the growing season. Water deeply before and after applying.
Watering Tips For Colorado’s Dry Climate
Colorado is dry, so consistent watering is crucial. Water deeply 2-3 times a week, more in hot weather. Use drip irrigation or soaker hoses to keep leaves dry and prevent disease. Mulch helps retain moisture.
Signs of underwatering: wilting leaves, cracked fruit. Overwatering: yellow leaves, root rot. Stick your finger in the soil—if dry 2 inches down, it’s time to water.
Harvesting: When To Pick
Tomatoes are ready when they are fully colored and slightly soft to the touch. Pick them at the “breaker stage” (first blush of color) to ripen indoors if frost threatens. Store at room temperature, not in the fridge.
In Colorado, you’ll likely harvest from late July through September, depending on planting date and variety.
Frequently Asked Questions
Can I Plant Tomatoes In Colorado In April?
Only in very warm areas like Grand Junction or Pueblo, and only if you use protection like row covers. For most of Colorado, April is too early. Wait until May or June.
What Is The Best Month To Plant Tomatoes In Colorado?
May is the best month for most areas, specifically the last two weeks. Mountain regions should wait until June.
How Late Can I Plant Tomatoes In Colorado?
You can plant as late as early July, but you’ll need early-maturing varieties (under 70 days) and may need to protect from early fall frosts. Late June is the cutoff for most areas.
Should I Start Tomatoes From Seed Or Buy Plants In Colorado?
Starting from seed gives you more variety, but it takes effort. Buying transplants is easier and more reliable for beginners. If you start seeds, do so indoors 6-8 weeks before planting.
Do Tomatoes Need Full Sun In Colorado?
Yes, at least 8 hours of direct sun daily. Colorado’s intense sun can sometimes scorch leaves, so some afternoon shade in very hot areas (like the Western Slope) can help.
Remember, the key to a great tomato harvest in Colorado is patience. Wait for warm soil and stable weather. Use the right varieties for your elevation. Protect from frost and wind. With these tips, you’ll be enjoying homegrown tomatoes by late summer.
Now go check your soil temperature and get ready to plant. Your Colorado garden is waiting.