Seedless watermelons produce small, sterile seeds. New plants can't be grown from these, so purchasing specially modified seedless watermelon seeds from a seed supplier is necessary. Most seedless varieties take longer to reach maturity---possibly as long as 85 days. Start seeds for traditional seed melon varieties as well, as these must be interspersed with your seedless melons in order to provide the pollen for them to set fruit.
Start seeds inside three weeks before you plan to transplant them into the garden. Fill individual peat pots with a quality potting mix and moisten the soil evenly throughout.
Sow one seedless watermelon seed per pot to a depth of 1 inch. Cover the pot with a layer of plastic wrap and place in a warm, 80 to 85 F room to germinate.
Remove the plastic wrap once seedlings emerge, approximately seven to 10 days after planting. Place seedlings under grow lights for 12 hours a day and maintain a temperature of 75 to 80 F.
Keep the soil moist at all times, watering as needed to prevent it from drying out. Discard any pots holding ungerminated seeds.
Prepare a well-drained garden bed in full sun after all danger of frost has passed and the soil has warmed to 70 F. Lay a 2-inch layer of compost over the bed and till it in to aid drainage and soil nutrition.
Create small hills spaced 6 feet apart in rows. Space the rows 4 feet apart.
Score the bottom and sides of the peat pot with a sharp knife. Dig two planting holes per hill and plant a seedless watermelon in one hole and a seeded watermelon in the other. Plant so the rim of the peat pot is just beneath the soil surface.