Watermelon plants can’t tolerate cold, so never rush them into the ground; wait until the danger of frost is over and the soil temperature reads at least 60°F. Transplanting is more reliable than direct seeding and usually yields earlier crops. I start my plants inside about a month in advance by sowing them into 4-inch-diameter pots containing a sterile potting mix and watering well. Seeds can be presoaked in warm water or carefully filed to speed germination, which occurs best at ambient temperatures between 75° and 85°F. Once sprouts emerge, I use ordinary grow lights to provide 15 hours of light daily. The seedlings are thinned to three per pot, watered as needed, and fed with diluted fish emulsion once or twice.
For a bumper crop of watermelons, I recommend laying down black plastic mulch. Unsustainable as it may be, in this case plastic is useful: It heats the soil, producing earlier and more abundant harvests. Wide spacing is also key to bigger harvests—and bigger fruits. I cut holes in the plastic 2 feet in diameter, at 12-to-15-foot intervals, and place several plants in each opening. I keep them under spun-polyester row covers for several weeks to protect against the elements, removing the covers once plants begin to sprawl and then flower, setting fruit with the aid of bees.
Primary prevention is how I deal with the pests and diseases that attack watermelons. It includes vigilance, disposal of crop debris, rotation, mulches, wide spacing, those spun-polyester covers, and judicious watering. The most likely animal pests to look out for early in the season are rodents that unearth seeds or young plants. Defend ripening fruits from hungry deer, raccoons, and crows with fencing, trapping, and scarecrows. Root-knot nematodes (roundworms that inhabit the soil) produce root galls on infected plants; stunting, yellowing leaves, wilting, and poor yields follow. Both nematodes and fusarium wilt—a disease produced by soil fungus that causes wilt, yellowing of leaves, and lesions or cracking on vines—can be prevented with crop rotation and good field sanitation.
The earliest watermelons will start ripening about 3 months after transplanting. It’s time to pick when the “ground spot” on the underbelly becomes yellow, or the tendril nearest the stem withers, and the melon acquires a weighty feel and a dull rind.
Harvested watermelons should be stored at 55° to 60°F and are best eaten within 3 weeks postharvest. When the fruit is ripe and ready, so are the seeds. If you save and share your seeds with others for replanting, you’ll do your part to keep a rich heritage alive.
Six Heirloom Watermelons Worth Growing
‘Blacktail Mountain’. Small (12 to 18 pounds) and round with dark green rind; dense scarlet flesh is very sweet, juicy, and crunchy. Bred for early maturity and cold tolerance, but widely adapted.
‘Golden Midget’. An oval miniature watermelon (averages 3 to 5 pounds) with a penciled yellow rind and salmon-pink flesh. Resembles papaya; tastes as good as it looks. Early variety with a compact habit.
‘Moon and Stars’ (yellow-fleshed). A cosmic melon. Foliage and fruits are speckled with yellow markings resembling moons and stars. Medium-sized (18 to 24 pounds). Sweet, orangey-yellow flesh. Totally wonderful. Late maturity.
‘Orangeglo’. Sugary firm-fleshed melon with orange flesh. Large (24 to 32 pounds) elongated fruits with dark green stripes. Very vigorous and productive main crop variety.
‘Petite Yellow’. A yellow-fleshed seeded watermelon. Small round icebox size (averages 5 pounds) has high sugars and holds together better than other yellow sorts. Matures early; suitable for short-season areas.
‘Sweet Siberian’. A melon with origins in Russia. Blocky dark green icebox type (averages 10 pounds) with apricot-colored flesh and light brown seeds. Sweet, crunchy; somewhat fibrous.