Published on Organic Gardening (http://www.organicgardening.com)
Sow watermelon seed 6 to 8 weeks before the last spring frost. Seeds sprout in 3 to 12 days at 75° to 95°F. Grow seedlings at 70° to 80°F.
Links:
[1] http://www.organicgardening.com/seed-starting-watermelon
[2] http://www.organicgardening.com/learn-and-grow/grow-juicy-sweet-watermelons