Watermelons are slow growers. It takes 70 to 90 days to grow watermelons after sowing the seed. Watermelons can be planted from late spring through early summer when soil temperatures reach 70° F or higher. The warmer the soil, the faster it will grow. For example, it takes around three days to sprout in 90°F conditions and about ten days in 70° F. A healthy watermelon plant will produce two to four fruits each plant.