The Old Farmer's Almanac, the Burpee seed company and Bonnie Plants all recommend waiting to plant watermelon seeds outdoors until the soil reaches a temperature of at least 70 degrees Fahrenheit. Waiting for the soil to warm up ensures more reliable germination.
Watermelon seeds can be planted indoors a month before transferring them to the garden. The young watermelon vines are very tender, and they should not be transplanted outdoors until two weeks after the last frost date for the region. Provide the watermelon vines with 1 to 2 inches of water per week while they are growing. It takes 70 to 90 days for watermelon plants to reach maturity.