When to plant watermelons in Iowa
Iowa’s average last spring frost is April 25, and the first fall frost typically arrives around October 8 — about 166 frost-free days (USDA zones 4b, 5a, 5b). Watermelon needs 70–100 days to mature, which fits comfortably within Iowa's growing season.
When to plant watermelons in Iowa: the short answer
For a spring planting, plant out May 2 – May 19, with seeds started indoors Mar 28 – Apr 11. 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 Iowa, step by step
- Start seeds indoors (Mar 28 – Apr 11). 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 2 – May 19). 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 18 – Aug 17). 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 Iowa’s average frost dates. More in the full watermelon growing guide.
Watermelon planting dates by USDA zone in Iowa
Iowa spans USDA zones 4b, 5a, 5b. 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 Iowa ask
When is the best time to plant watermelons in Iowa?
Plant watermelon transplants outdoors in Iowa between May 2 and May 19. That timing is set by Iowa's average last spring frost of April 25.
When should I start watermelon seeds indoors in Iowa?
Start seeds indoors between March 28 and April 11, about 3 weeks before the last frost, so the seedlings are ready to plant out from May 2.
What is the latest I can plant watermelons in Iowa?
About July 30. Watermelons need at least 70 days to mature, and frost around October 8 ends the season, so later plantings risk being cut short.
When will watermelons planted in Iowa be ready to harvest?
Planted on time, watermelons are usually ready from about July 18 to August 17, 70-100 days after planting.
When is the last frost in Iowa?
On average around April 25, with the first fall frost around October 8 - about 166 frost-free days. Your town may differ by two weeks or more.