The watermelon is a tender, warm-weather annual. Watermelons along with muskmelons and cantaloupes are sometimes called summer melons. Sow watermelon seed in the garden or set out transplants 3 to 4 weeks after the last average frost date in spring. Start watermelon seed indoors about 6 weeks before transplanting seedlings into the garden. Start seed indoors in 4-inch or larger biodegradable peat or paper pots that can be set wholly into the garden so as not to disturb the roots. Watermelons require 65 to 90 frost-free days to reach harvest.