When to plant watermelons in Montana
Montana’s average last spring frost is May 15, and the first fall frost typically arrives around September 15 — about 123 frost-free days (USDA zones 3a, 3b, 4a, 4b, 5a, 5b, 6a). Watermelon needs 70–100 days to mature, which fits comfortably within Montana's growing season.
Montana 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 Montana: the short answer
For a spring planting, plant out May 22 – Jun 8, with seeds started indoors Apr 17 – May 1. 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 Montana, step by step
- Start seeds indoors (Apr 17 – May 1). 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 22 – Jun 8). 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 Aug 7 – Sep 6). 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 Montana’s average frost dates. More in the full watermelon growing guide.
Watermelon planting dates by USDA zone in Montana
Montana spans USDA zones 3a, 3b, 4a, 4b, 5a, 5b, 6a. 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.
When to plant watermelons in nearby states
Questions gardeners in Montana ask
When is the best time to plant watermelons in Montana?
Plant watermelon transplants outdoors in Montana between May 22 and June 8. That timing is set by Montana's average last spring frost of May 15; Montana spans several climates, so cooler and higher parts of the state run later.
When should I start watermelon seeds indoors in Montana?
Start seeds indoors between April 17 and May 1, about 3 weeks before the last frost, so the seedlings are ready to plant out from May 22.
What is the latest I can plant watermelons in Montana?
About July 7. Watermelons need at least 70 days to mature, and frost around September 15 ends the season, so later plantings risk being cut short.
When will watermelons planted in Montana be ready to harvest?
Planted on time, watermelons are usually ready from about August 7 to September 6, 70-100 days after planting.
When is the last frost in Montana?
On average around May 15, with the first fall frost around September 15 - about 123 frost-free days. Your town may differ by two weeks or more.