How to Grow Watermelon

What watermelons need

Watermelon is a tropical African vine in the same family as cucumbers, squash and cantaloupe, and it behaves like it: it grows best when average air temperatures run between 70 and 85°F (Clemson), stalls in cold soil, and dies at the first frost. Everything about growing it well comes down to giving it enough heat, space, water and bees.

Pick the sunniest spot you have, with at least 6 hours of direct sun and preferably more. The ideal soil is loose, sandy or loamy and well drained. Iowa State recommends a pH of 5.5 to 7.0; NC State lists neutral soil as the target. Heavy clay can still grow good melons if you work in compost or well-rotted manure, but don’t overdo it: Utah State caps compost at about 1 inch spread over each 100 square feet. Watermelons will not tolerate waterlogged roots, so avoid low spots where rain stands.

Space is the constraint most gardeners underestimate. Vines can spread up to 10 feet wide (NC State), and Clemson’s rule of thumb is 24 square feet per plant. If that is more than you have, grow an icebox or bush type rather than crowding a large variety.

Choosing a variety

The biggest decision is size and season length. Large melons (15 to 30 pounds) need the full season; small icebox types ripen sooner, fit smaller gardens and suit short northern summers. Seedless varieties need a seeded pollenizer planted with them (see pollination below).

Extension services recommend these for their own regions:

• Iowa and the Upper Midwest (Iowa State): seeded large Crimson Sweet, Royal Sweet, Sangria, Black Tail Mountain and Yellow Baby; seeded small Sugar Baby, Cathay Belle, Jade Star, Starlight, New Orchid, Sweet Beauty and Yellow Doll; seedless Cooperstown, Crunchy Red, Gypsy, Millionaire, Sweet Gem, Amarillo and Orange Crisp.
• Utah and the Intermountain West (Utah State): Crimson Sweet and Mirage for large red melons, Mickylee and Minilee as 10-15 pound icebox types, Golden Crown and Yellow Baby for yellow flesh.
• South Carolina (Clemson): seeded Crimson Sweet, Royal Sweet, Top Gun, Sentinel and Sangria; seedless Liberty, Majestic, Obsession and Fascination.
• Florida (UF/IFAS): Jubilee (also sold as Florida Giant), Crimson Sweet and Charleston Grey 133 for large melons; Sugar Baby and Mickeylee for small ones.
• North Carolina (NC State): Crimson Sweet, Sugar Baby, Starlight, Mini Love (3-5 pounds), Sunshine (yellow flesh) and the seedless Farmers Wonderful.

Crimson Sweet appears on almost every list, from Utah to Florida, which makes it a safe first choice nearly anywhere with a long enough summer. In the South, disease resistance matters as much as size, so UF advises choosing resistant varieties.

When to plant

Plant after the last frost, once the soil has warmed. Sources set the bar slightly differently: Clemson wants 60 to 65°F at a 4-inch depth, Iowa State says 60 to 70°F, and Utah State waits for 65°F. A soil thermometer settles the question better than the calendar. Melon seed rots or comes up unevenly in cold, wet ground, and Iowa State points out that planting a week later into warm soil beats losing a stand to damping-off.

Use the dates for your zone above as your planting window, and see when to plant watermelon for timing by region, including how much earlier you can go with plastic mulch and row covers.

Seeds or transplants

Direct seeding works well where summers are long. Transplants buy you about two weeks: Clemson and Utah State both say transplanted melons ripen roughly two weeks before seeded ones, and Utah State recommends them for short-season areas.

Watermelon roots resent disturbance, so start seeds in plantable containers such as peat pots. Iowa State sows 3 or 4 seeds per pot 3 to 4 weeks before the outdoor planting date, thins to two seedlings, and sets them out at 1 or 2 true leaves. Utah State allows about 6 weeks and plants out at 2 to 3 mature leaves with a well-developed root system.

Seedless seed is fussier. Clemson recommends germinating it where the air can be held at 92 to 95°F for about 48 hours, then moving seedlings to about 70°F in full sun, and keeping the mix on the dry side.

Harden transplants for a few days before planting: start them in shade and shelter outdoors, then give them longer periods of direct sun.

Planting and spacing

Watermelon is traditionally grown in hills. Iowa State plants 4 or 5 seeds per hill, 1 inch deep, and later keeps the 2 or 3 strongest plants. Utah State sows 4 to 6 seeds 1 to 2 inches deep and thins to 2 plants once they have two leaves. UF/IFAS lists 1½ to 2 inches for Florida.

Spacing advice varies with the system:

• Iowa State: hills 2 to 3 feet apart in rows 6 to 8 feet apart.
• Clemson: rows 6 to 8 feet apart, with plants 4 feet apart in 6-foot rows or 3 feet apart in 8-foot rows.
• Utah State: seeded mounds 4 feet apart, or transplants 2 feet apart through black plastic in rows 4 to 6 feet apart.

Crowding has a cost beyond tangled vines. Clemson lists close planting, along with excess nitrogen, as a main cause of big vines with few melons. When setting transplants, Iowa State waters each one in with about 2 cups of starter solution, made by dissolving 2 tablespoons of a 10-10-10 fertilizer in a gallon of water or following the label of a water-soluble product.

Black plastic and row covers

Black plastic mulch is the single most useful tool for watermelon outside the Deep South. It warms the soil, holds moisture, smothers weeds and keeps fruit off damp ground, which Clemson notes reduces fruit rot. Lay it over moist soil on a calm day; plastic stretched over dry soil delays growth (Iowa State). Bury the edges in furrows 2 to 3 inches deep and cut planting holes just large enough for seeds or transplants. Water can’t get through it, so run drip line underneath.

Row covers add more earliness. Utah State says that with plastic mulch plus row covers you can plant 2 to 3 weeks before the last frost. Covers trap heat fast, though: ventilate them once air temperatures pass 80°F, and remove them when plants begin to flower or temperatures exceed 90°F, both to prevent heat damage and to let bees in.

Hold off on straw, grass clippings or other organic mulch until the soil is warmer than 75°F (Utah State). Laid too early, it keeps the soil cold.

Fertilizing

Test your soil if you can; every extension source prefers it to guessing. Without a test, the published rates are close to one another:

• Iowa State: 1 to 2 pounds of 10-10-10 per 100 square feet before planting, using the lower rate on sandy loam and the higher on clay. Use the lower rate under plastic.
• Clemson: 5-10-10 at 30 pounds per 1,000 square feet (about 3 pounds per 100 square feet).

Then side-dress with nitrogen. Clemson does it just before the vines start to run, with 1 pound of 34-0-0 or 2 pounds of calcium nitrate per 100 feet of row, and again after bloom when fruit is developing. Utah State side-dresses once the runners form, with 3 to 4 tablespoons of 21-0-0 sprinkled around each plant and watered in.

More is not better. Excess nitrogen produces rampant vines with few fruit, interferes with calcium uptake (leading to blossom-end rot) and, during ripening, can cause white heart, the pale, hard streaks in the flesh.

Watering

Watermelon fruit is about 92% water, and the plant needs steady moisture from germination to harvest. Dry soil at germination gives patchy emergence; drought at bloom causes poor fruit set and misshapen melons; stress near harvest shrinks fruit and makes vines decline quickly (Iowa State).

In dry weather, Iowa State applies 1 to 2 inches every 7 to 10 days, and Utah State recommends 1 to 2 inches a week, applied deeply and infrequently. Clemson adds a useful detail: most watermelon roots sit in the top 12 inches of soil, so water beyond that depth is wasted along with the nutrients it carries. On sandy soil, that may mean several short drip cycles rather than one long soak. Soil type doesn’t change the total water a crop needs, only how often you apply it.

Drip irrigation is the best method because it keeps leaves dry. If you must water overhead, do it early in the morning so the foliage dries quickly.

As the fruit nears maturity, cut back. Iowa State halves each application. Heavy rain or irrigation in the final week or two dilutes sugar and can split fruit.

Pollination and seedless melons

Watermelons produce separate male and female flowers on the same vine. Female flowers have a tiny melon-shaped swelling at the base, and each one is open for only a single day. Watermelon pollen is heavy and sticky, so wind doesn’t move it; bees do.

A female flower needs roughly 500 to 1,000 pollen grains. Clemson estimates that takes about 8 bee visits for a seeded variety and about 21 for a seedless one, because bees also carry useless pollen from the seedless male flowers. More visits mean larger, better-shaped fruit (Iowa State). Cool, rainy or windy weather keeps bees home, and cold can stop flowers from producing pollen at all.

For seedless varieties, plant a seeded pollenizer within 10 feet (Clemson). Seed packets often include pollenizer seed; Iowa State suggests choosing one with a different rind pattern so you know which melons are which. The pollenizer’s fruit is edible too.

Protect the bees. Spray insecticides only when necessary and only early in the morning or in the evening. Clemson advises avoiding sprays between 8 a.m. and 4 p.m.

Regional advice

Cold north and short seasons. Getting the crop finished before fall is the challenge. In central Iowa, the usual planting time is mid-May, a week earlier in southern Iowa and a week later in the north, and Iowa State gives June 20 as the last practical date to sow early-maturing varieties. Use transplants, black plastic and row covers, and choose icebox varieties. Utah State notes that poor flavor is common where the season is very short.

Humid South and Gulf Coast. Heat is plentiful; disease is the problem. Clemson plants in April on the South Carolina coast and from mid-April to mid-June in the Piedmont, and does not recommend fall plantings. Gummy stem blight thrives in warm, humid weather, and powdery and downy mildew, anthracnose and root-knot nematodes are all common. Use drip irrigation, resistant varieties and strict rotation. If you try a second crop on old black plastic, Clemson suggests spraying it white (1 part cheap white latex paint to 7 parts water) so the soil doesn’t overheat in summer. North Florida gets a second window in July and August (UF/IFAS).

Arid West and desert Southwest. In the Phoenix low desert, the University of Arizona schedules seeding from mid-February through March, using varieties of 90 to 120 days, so vines fruit before the worst summer heat. In both the desert and the Intermountain West, steady drip irrigation is critical. Don’t let powdery mildew strip the leaves: Utah State notes that fruit exposed to the sun then ripens prematurely.

Warm-winter zones 9-11. Plant in late winter. UF/IFAS lists December through March for South Florida and January through March for Central Florida, timed so the crop ripens before peak summer heat.

Pests and diseases

Most insect damage hits seedlings and young vines. Cucumber beetles chew leaves and spread bacterial wilt; squash bugs, aphids and mites follow. Iowa State recommends row covers from planting until first bloom, hand-picking and good weed control, and says insecticides work best early in the season when pests are small. Aphids also spread mosaic viruses, which leave mottled, curled leaves and distorted fruit. Remove infected plants, and buy certified virus-free seed.

Leaf diseases such as powdery mildew, downy mildew, Cercospora leaf spot, Alternaria leaf blight and anthracnose favor wet foliage and still air. Water at the base in the morning, don’t work among wet vines, mulch to stop soil splash, and clear away plant debris at season’s end. Fungicides work best at the first sign of symptoms and do little against wilts such as Fusarium, which live inside the stem.

Rotation is the cheapest control. Don’t plant watermelon, cantaloupe, cucumber, squash or pumpkins in the same spot for at least 3 years (Clemson) or 3 to 4 years (Iowa State). Watermelon fruit blotch, a bacterial disease that causes water-soaked or coffee-colored blotches on the rind, arrives on infected seed or transplants and cannot be cured, so buy clean seed.

Common problems and fixes

• Misshapen or lopsided fruit: too few bee visits. Avoid daytime spraying and expect more trouble in cool, wet springs.
• Huge vines, few melons: too much nitrogen or plants spaced too close. Cut back on side-dressing and give each plant its full space next year.
• Rotting blossom end: calcium isn’t reaching the fruit because of uneven watering or excess nitrogen. You rarely need to add calcium; water evenly and don’t overfeed.
• Bland, pale or split fruit: too much water in the last week or two, poorly drained soil, or wilting, diseased vines. Reduce watering as fruit matures.
• Seedlings collapse at the soil line: damping-off in cold, wet soil. Replant once conditions warm up.

Harvesting

Fruit ripens 35 to 45 days after pollination, and Clemson puts most varieties at 85 to 90 days from transplanting to the first ripe fruit. Use several signs together:

• The ground spot turns from greenish white to creamy or buttery yellow. This is easiest to see on dark-skinned varieties.
• The rind loses its gloss and turns dull.
• The curly tendril nearest the fruit browns and dries. In some varieties, it browns 7 to 10 days early, so confirm with the ground spot.
• The melon has reached its expected size.

Cut the fruit with about 2 inches of stem attached. Whole melons keep about a week at room temperature. Iowa State says they last 2 to 3 weeks at 50 to 60°F, and Utah State gives 1 to 2 weeks at 45 to 50°F.