When to Plant Tomatoes in Jefferson County, CO
This month in Jefferson County, Colorado
May is a pivotal month for Jefferson County, Colorado gardens. Focus on these tasks first and you'll set up the rest of the season for success.
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Harden off and plant tomatoes
Bring a watering can to the bed. Each transplant gets a drink the moment it's in the ground, not ten minutes later.
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Seed tomatoes outdoors
Succession planting is the secret here. Put in a row now, another in 2 weeks, a third in 4.
Tomatoes are the most popular home garden crop, available in thousands of varieties from tiny cherries to massive beefsteaks. They are warm-season plants needing full sun.
Jefferson County, Colorado is in USDA Zone 6a. The average last spring frost is May 14 and the first fall frost is September 26, giving you a growing season of approximately 135 days.
At an elevation of 5,376 feet, Jefferson County receives approximately 22.7 inches of rainfall annually with predominantly loam soil. Summer highs average 84°F, so choose short-season varieties of Tomatoes to ensure they mature before fall.
Jefferson County Soil Profile
Soil Type
Loam
Soil pH
6.8-7.7
Drainage
Well Drained
Plant Planting Risk Windows
Percentages indicate frost risk at transplant. The 70% safe window means there is a 30% chance of frost after transplant — suitable for cold-hardy crops or gardeners with frost protection. The 90% safe window is best for tender plants.
Soil Compatibility in Jefferson County
How your county's soil matches Tomatoes's growing requirements.
Soil pH
Your soil pH (6.8–7.7) is more alkaline than Tomatoes prefers (6.0–7.0). Add sulfur or peat moss to lower pH.
Soil Texture
The loam soil in Jefferson County is excellent for Tomatoes — good drainage, moisture retention, and nutrient holding capacity.
Organic Matter
Organic matter is moderate (2.6%). Annual compost additions will help Tomatoes.
How to Plant Tomatoes
Succession Planting Tomatoes
Sow every 6.9 weeks. Last sowing by Jul 03 to harvest before frost.
Plant Water Budget
Water stress score is 8/10 — consider drought-tolerant varieties and mulching
Monthly Watering Guide for Tomatoes
Tomatoes needs approximately 1.2 inches of water per week (5.2" per month). Here's how your county's rainfall compares month by month.
| Month | Tomatoes Needs | Rainfall | You Supplement | Action |
|---|---|---|---|---|
| Jan | — | 1.8" | 0" | ❄️ Dormant |
| Feb | — | 1.3" | 0" | ❄️ Dormant |
| Mar | — | 1.9" | 0" | ❄️ Dormant |
| Apr | — | 2.7" | 0" | ❄️ Dormant |
| May | 5.2" | 2.6" | 2.6" | 🚿 Regular watering |
| Jun | 5.2" | 1.8" | 3.4" | 🚿 Regular watering |
| Jul | 5.2" | 1.9" | 3.3" | 🚿 Regular watering |
| Aug | 5.2" | 2.4" | 2.8" | 🚿 Regular watering |
| Sep | 5.2" | 1.7" | 3.5" | 🚿 Regular watering |
| Oct | — | 1.8" | 0" | ❄️ Dormant |
| Nov | — | 1.4" | 0" | ❄️ Dormant |
| Dec | — | 1.4" | 0" | ❄️ Dormant |
Water needs are for active growing months only (May–Sep in Jefferson County). Supplement amounts are based on average rainfall — actual needs vary with temperature, soil, and mulching.
Tomatoes Heat Requirements (GDD)
What are Growing Degree Days (GDD)?
Growing Degree Days measure the total warmth your plants receive during the growing season. Think of it as a "heat bank" — every day above 50°F deposits warmth that helps your plants grow.
Each plant needs a certain amount of accumulated heat to mature. If your county provides more GDD than the plant needs, it's a great fit. If it's close, you may want to choose faster-maturing varieties or start seeds indoors to get a head start.
Tomatoes Planting Timeline — Jefferson County, CO
Tomatoes Planting Calendar
| Activity | When | Date Range |
|---|---|---|
| Start Indoors | March 19 | Mar 19 – Apr 2 |
| Transplant Outdoors | May 28 | May 28 – Jun 11 |
| Direct Sow | May 21 | May 21 – Jun 11 |
| Harvest | July 30 | Jul 30 – Oct 8 |
Plant 0.5" deep · 24" apart · Rows 36" apart
Month-by-Month Timeline
| Month | Activities |
|---|---|
| January | — |
| February | — |
| March | Start Indoors |
| April | Start Indoors |
| May | Transplant Outdoors Direct Sow |
| June | Transplant Outdoors Direct Sow |
| July | Harvest |
| August | Harvest |
| September | Harvest |
| October | Harvest |
| November | — |
| December | — |
Growing Conditions
☀️ Sun
Full Sun (6-8+ hours)
💧 Water
1.2"/week · 1-2 times/week
📅 Days to Maturity
60–85 days
🧪 Soil pH
Needs 6–7 · Your soil: too_alkaline
🗺️ USDA Zone
Zone 6a
📆 Growing Season
135 days in Jefferson County
Growing Tips for Tomatoes in Jefferson County
Direct sow Tomatoes outdoors after May 14 in Jefferson County when soil has warmed and frost danger has passed.
Common pests for Tomatoes in this region include tomato hornworm and aphids. Use row covers early in the season and inspect plants weekly.
Jefferson County receives only 23" of rain annually. Tomatoes needs consistent moisture — install drip irrigation or water deeply 2-3 times per week.
General growing tips
Start seeds indoors 6-8 weeks before last frost. Bury transplants deep to encourage rooting along the stem. Provide consistent moisture to prevent blossom end rot and cracking.
Recommended Tomatoes Varieties for Jefferson County
Choose determinate, early-maturing varieties for your short season
Companion Planting
Good Companions
Avoid Planting Near
Check more plant combinations with our Companion Planting Checker →
Isolate 25 ft between varieties for purity. Use open-pollinated varieties for true-to-type seeds.
Tomatoes in Other Locations
Frequently Asked Questions
When should I plant Tomatoes in Jefferson County, CO?
Jefferson County is in Zone 6a with an average last frost of May 14. Plan your Tomatoes planting based on this frost date — see the calendar above for exact timing.
What planting zone is Jefferson County, CO?
Jefferson County, Colorado is in USDA Hardiness Zone 6a. The average last spring frost is May 14 and first fall frost is September 26.
Your Jefferson County Garden Planner — Free
A 24-page printable planner built for Jefferson County (Zone 6a). Planting dates, a month-by-month schedule, harvest log, seed inventory, and succession charts — all dialed in for your exact growing season.