If you’re a gardener in Colorado, figuring out when to plant tomatoes is your first step to a great harvest. The best time for planting tomatoes here is a careful balance, as our unique climate presents both opportunity and challenge.
Colorado’s growing season is shorter than many places, with a high risk of frost in spring and fall. Getting your timing right protects your young plants and gives them the longest possible season to produce those delicious fruits. Let’s break down the key dates and factors so you can plan with confidence.
When to Plant Tomatoes in Colorado – Best Time for Planting
For most of Colorado’s Front Range and urban areas, the safe window to transplant tomato seedlings outdoors is from mid-May through early June. The absolute key is watching the weather, not just the calendar.
You must wait until all danger of frost has passed. A light frost will kill or severely damage tomato plants. The average last spring frost date is your guiding star:
- Denver Metro Area: Around May 5-10
- Colorado Springs: Around May 10-15
- Fort Collins: Around May 5-10
- Pueblo: Around April 25 – May 5 (earlier due to lower elevation)
- Mountain Towns (above 7,000 ft): June 1-15 or later
Always verify the specific date for your exact location. A good rule is to plan for 1-2 weeks after the average last frost. This extra buffer accounts for Colorado’s unpredictable spring weather.
Understanding Colorado’s Climate Challenges
Our state isn’t just cold; it’s a mix of factors that affect tomatoes. The intense sun, low humidity, and wide temperature swings are just as important as frost dates.
Elevation is Everything
Your planting schedule is directly tied to how high you live. Higher elevations have shorter growing seasons and cooler nights. For every 1,000 feet gain in elevation, the growing season shortens by about 3-4 days. Gardeners in the mountains must choose fast-maturing varieties and be patient.
Temperature Swings & Sun Scald
Colorado is famous for a 40-degree temperature drop in a single day. Tomatoes need consistent warmth, especially at night. They also can suffer from sun scald on fruits due to our intense, high-altitude sunlight. Using shade cloth during the hottest part of summer afternoons can really help prevent this.
Getting Your Tomatoes Ready: From Seed to Garden
Tomatoes need a long head start indoors in Colorado. Here’s a simple timeline to follow:
- Start Seeds Indoors: Sow tomato seeds 6-8 weeks before your planned transplant date. For a May 15th transplant, start seeds around late March.
- Provide Strong Light: Use grow lights to prevent leggy seedlings. Our winter sunlight through a window is rarely enough.
- Harden Off Plants: This is non-negotiable. 7-10 days before transplanting, gradually expose seedlings to outdoor conditions. Start with a shady, sheltered spot for an hour, increasing time and sun exposure each day.
The Step-by-Step Planting Process
When your hardened-off seedlings and the weather are ready, follow these steps:
- Choose a Sunny Spot: Tomatoes need at least 6-8 hours of direct sun. More is better in Colorado, as long as they’re acclimated.
- Amend the Soil: Our native soil is often clay-heavy. Mix in several inches of compost or well-rotted manure to improve drainage and fertility.
- Plant Deeply: Bury the stem up to the first set of true leaves. Tomato stems will grow roots along the buried section, creating a stronger plant. This is a key tomato-growing tip.
- Water Well & Mulch: Water deeply immediately after planting. Apply 2-3 inches of mulch (straw or wood chips) to conserve moisture and regulate soil temperature.
- Provide Support: Install cages or stakes at planting time to avoid damaging roots later.
Choosing the Right Tomato Varieties for Colorado
Selecting varieties suited to a shorter season with cool nights improves your success. Look for these traits:
- Days to Maturity: Choose varieties with < 75 days for mountains, and < 85 days for Front Range for reliable harvests. You can also try a few longer-season ones as a gamble.
- Cold Tolerance: Some varieties handle cooler nights better.
- Recommended Colorado-Tested Varieties: Early Girl, Celebrity, Siberian, Sun Gold (cherry), Mountain Merit, and Glacier are all excellent choices that have performed well for many local gardeners.
Protecting Your Plants from Late Frosts & Cool Nights
Even after your planting date, be prepared to protect you’re tomatoes. Have these items on hand:
- Frost Blankets or Row Covers: Lightweight fabric that can be draped over plants.
- Wall O’ Waters or Season Starters: These water-filled teepees absorb daytime heat and release it at night.
- Homemade Cloches: Milk jugs with the bottoms cut out work in a pinch for individual plants.
If a surprise frost is forecasted after planting, cover your plants in the evening. Uncover them in the morning once temperatures rise.
FAQ: Your Colorado Tomato Questions Answered
Can I plant tomatoes in early May in Colorado?
It’s risky. While the date may be past the average frost, a late cold snap is common. Early May planting requires vigilant weather watching and having frost protection ready. Mid-May is generally safer for most.
What happens if I plant tomatoes too early?
Tomatoes planted in cold soil (<50°F) will suffer from stunted growth, nutrient deficiency (purple leaves), and are highly susceptible to disease. They won't grow faster; they'll just sit there stressed. A light frost will kill them outright.
Is it better to buy transplants or start from seed?
Starting from seed gives you access to hundreds of disease-resistant and climate-adapted varieties not found in stores. Buying transplants from a local nursery is easier and ensures the plants are already acclimated to our area. Both are great options depending on your time and goals.
When is it to late to plant tomatoes in Colorado?
Aim to have transplants in the ground by mid-June at the latest for the Front Range. This gives them enough time to mature before the first fall frost, which typically arrives in early October. For mountain areas, late planting is very difficult due to the short season.
How do I extend the tomato season in the fall?
As nights cool in September, cover plants with row covers to trap heat. If an early hard frost is predicted, you can harvest mature green tomatoes and ripen them indoors in a paper bag with a banana. This lets you enjoy tomatoes for several more weeks.
Success with tomatoes in Colorado comes from patience and preparation. By respecting the last frost date, choosing the right varieties, and being ready to protect your plants, you’ll set the stage for a productive and rewarding summer in the garden. The taste of a homegrown tomato, ripened under the Colorado sun, is worth the careful wait.