When to plant watermelons in Colorado
Colorado’s average last spring frost is May 5, and the first fall frost typically arrives around October 1 — about 149 frost-free days (USDA zones 3b, 4b, 5a, 5b, 6a, 7a). Watermelon needs 70–100 days to mature, which fits comfortably within Colorado's growing season.
Colorado covers a wide range of climates, so your exact frost dates may differ from the state average by two weeks or more. For dates tuned to your town, use the calendar generator.
When to plant watermelons in Colorado: the short answer
For a spring planting, plant out May 12 – May 29, with seeds started indoors Apr 7 – Apr 21. Wait until the soil is at least 70°F and frost is over.
Windows are ~1–2 week bands around each date. Frost dates are a statewide average from NOAA/NCEI normals; your town may differ by two weeks or more. Calendar year 2026.
How to grow watermelons in Colorado, step by step
- Start seeds indoors (Apr 7 – Apr 21). Sow seeds 1 inch deep in seed-starting mix under bright light. They take 4-10 days to sprout, fastest in soil around 85°F. Harden seedlings off for a week before they go outside.
- Prepare the bed. They need at least six to eight hours of direct sun a day and loose soil at least 24 inches deep, with a pH between 5.5 and 6.8. Dig in compost before planting.
- Plant out (May 12 – May 29). Space plants about 36 inches apart in rows 72 inches apart. It is killed by frost, so nothing goes outside until the danger has passed.
- Water and feed. Give it steady, even moisture, about an inch of water a week from rain or the hose.
- Watch for pests and problems. Pests: striped cucumber beetle, aphids, squash vine borer. Diseases and disorders: anthracnose, fusarium wilt, powdery mildew, gummy stem blight.
- Harvest (from about Jul 28 – Aug 27). Watermelon takes 70-100 days to reach harvest. Longest-season cucurbit here - icebox varieties (70-75 days) are the realistic choice north of the mid-Atlantic. Ripe when the curly tendril nearest the fruit is brown and dry and the ground spot is buttery yellow.
Figures come from our crop data; dates use Colorado’s average frost dates. More in the full watermelon growing guide.
Watermelon planting dates by USDA zone in Colorado
Colorado spans USDA zones 3b, 4b, 5a, 5b, 6a, 7a. Colder zones, usually higher or further inland, have a later last spring frost and an earlier first fall frost than the state average, so the watermelon dates shift with them.
| Zone | Last frost | First frost | Season | Start indoors | Plant watermelon |
|---|---|---|---|---|---|
| Zone 3 | May 15 | Sep 15 | 120 days | Apr 17 – May 1 | May 22 – Jun 8 |
| Zone 4 | May 10 | Sep 25 | 135 days | Apr 12 – Apr 26 | May 17 – Jun 3 |
| Zone 5 | Apr 30 | Oct 5 | 160 days | Apr 2 – Apr 16 | May 7 – May 24 |
| Zone 6 | Apr 15 | Oct 15 | 185 days | Mar 18 – Apr 1 | Apr 22 – May 9 |
| Zone 7 | Apr 5 | Oct 25 | 200 days | Mar 8 – Mar 22 | Apr 12 – Apr 29 |
When to plant watermelons in nearby states
Questions gardeners in Colorado ask
When is the best time to plant watermelons in Colorado?
Plant watermelon transplants outdoors in Colorado between May 12 and May 29. That timing is set by Colorado's average last spring frost of May 5; Colorado spans several climates, so cooler and higher parts of the state run later.
When should I start watermelon seeds indoors in Colorado?
Start seeds indoors between April 7 and April 21, about 3 weeks before the last frost, so the seedlings are ready to plant out from May 12.
What is the latest I can plant watermelons in Colorado?
About July 23. Watermelons need at least 70 days to mature, and frost around October 1 ends the season, so later plantings risk being cut short.
When will watermelons planted in Colorado be ready to harvest?
Planted on time, watermelons are usually ready from about July 28 to August 27, 70-100 days after planting.
When is the last frost in Colorado?
On average around May 5, with the first fall frost around October 1 - about 149 frost-free days. Your town may differ by two weeks or more.