SOWING Sow outdoors 4 weeks after the average last frost date and harvest in 70 days. Plant four to six seeds 1 to 2 inches deep in 8- to 12-inch-tall mounds spaced 4 feet apart. Water the seeds well and keep the soil moist, but not wet, until they emerge. Thin your watermelon seedlings to two plants per mound when their first true leaves appear.