Watermelon (warm-season vine crop) is classed very tender in the cited extension guidance. The timing rules below are relative to your local freeze dates — enter a location to turn them into calendar dates.
The rules this site applies
Method
Frost-relative window
Rule & reconciliation
Start seeds indoors
2–1 weeks before the median last spring freeze
Iowa State: start seeds indoors 3–4 weeks before the anticipated outdoor planting date (Missouri G6570: 3–5 weeks).
Transplant outdoors
2–4 weeks after the median last spring freeze
Same outdoor timing as direct seeding, using pot-grown transplants.
Direct-sow outdoors
2–4 weeks after the median last spring freeze
Iowa 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.
Seed germination soil temperature: optimum 95°F (optimum range 70–95°F); minimum 60°F, maximum 105°F. Purdue HO-186-W Table 3. Cold soil below the minimum rots seed — a reason windows and soil readings both matter.
Variety assumption: Assumes an average variety (about 80–90 days from seed).
"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."
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.