When to plant watermelon in St. Louis, Missouri
April 15 – April 29
Transplant outdoors · typical St. Louis yearClimate basis: NOAA station St Louis Sci Ctr, MO (USC00237452), 4 mi (7 km) from downtown St. Louis (ZCTA 63101 centroid), elevation 515 ft, on the 1991–2020 climate normals. Transplant watermelon outdoors from approximately the dates above, anchored to the median last spring 32°F freeze (April 1). One year in ten still freezes after April 18.
The St. Louis year
- indoors under lights
- in the garden
- frost-free season
- 1-in-10 years still freeze to Apr 18
Every method, with its sources
Start seeds indoors
Mar 18 – Mar 25Iowa State: start seeds indoors 3–4 weeks before the anticipated outdoor planting date (Missouri G6570: 3–5 weeks). Anchored to the median last spring freeze (Apr 1). Iowa State University Extension and Outreach · University of Missouri Extension
Transplant outdoors
Apr 15 – Apr 29Same outdoor timing as direct seeding, using pot-grown transplants. Anchored to the median last spring freeze (Apr 1). Iowa State University Extension and Outreach · Purdue University Extension
Direct-sow outdoors
Apr 15 – Apr 29Iowa State: plant after the danger of frost is past once soil reaches 60–70°F; Purdue's very-tender melon class (at least two weeks after frost) is the conservative reading used. Anchored to the median last spring freeze (Apr 1). Iowa State University Extension and Outreach · Purdue University Extension
Variety assumption: Assumes an average variety (about 80–90 days from seed).
These windows describe typical years (1991–2020 climate normals), not this year's forecast. Freezes happen outside them — especially in frost pockets, valleys, and urban edges. Watch the forecast before committing tender plants.
Crop rule sources (3)
- Iowa State University Extension and Outreach: Growing Watermelon in the Home Garden (accessed 2026-07-24)"Plant melons after the danger of frost is past and soil temperatures have warmed to 60 to 70°F." "Start seeds indoors 3 to 4 weeks before the anticipated outdoor planting date."
- University of Missouri Extension: Starting Plants Indoors from Seeds (G6570) (accessed 2026-07-24)Weeks needed to grow vegetable transplants indoors (Table 2).
- Purdue University Extension: Indiana Vegetable Planting Calendar (HO-186-W) (accessed 2026-07-24)Hardiness classes: hardy = plant 4-6 wk before last spring frost; semi-hardy = 2-4 wk before; tender = after average last frost; very tender = at least 2 wk after. Table 2: fall planting at least 4-8 wk before first fall frost.