Vegetable · Cucurbitaceae
Watermelon
Citrullus lanatus
Give it full sun, warm soil, and room to sprawl, and the watermelon rewards you with sweet, heavy fruit by late summer.
Full sun
Every 7–14 days
68–95°F · 20–35°C
70–90 days
Overview
Watermelon is a sprawling annual vine from the gourd family, the same tribe as cucumbers and squash. It runs flat along the ground rather than climbing, so a single plant can spread 5 to 10 feet (1.5 to 3 m) wide while staying only ankle high. Behind that humble habit is a crop that needs real heat and a long warm spell to swell its fruit and load it with sugar.
Each plant carries separate male and female flowers, so bees have to ferry pollen between them for fruit to form. That detail matters most with the seedless types, which are sterile and set nothing unless a seeded variety grows close by to pollinate them. Get the pollination right and the melons follow.
This is a warm-season crop with no patience for cold. It wants full sun, deep and well-drained sandy loam, and steady moisture while the vines run and the fruit fills out. Ease back on the water as the melons near ripeness and the flavor concentrates into something a supermarket melon rarely matches.
Care
The short version: full sun, warm and well-drained soil, and 1 to 2 inches of water a week while the vines grow and set fruit, then less as the melons ripen. Feed lightly with a low-nitrogen mix and give each plant plenty of room to sprawl.
Light
Watermelon wants full sun, eight or more hours of it, and no real shade. The vines turn sunlight into sugar, so the brighter the spot, the sweeter and heavier the fruit.
Water
Give about 1 to 2 inches (2.5 to 5 cm) of water a week as deep soaks every 7 to 14 days, keeping the soil evenly moist while the vines grow and set fruit. Ease off as the melons ripen so the sugars concentrate, and water at the base to keep disease down.
Soil
Plant in deep, well-drained sandy loam worked full of organic matter. A pH of 6.0 to 6.8 suits it best, and the loose, warm ground lets the long roots run and drains fast after rain.
Temperature
It thrives in real heat, roughly 68°F to 95°F (20°C to 35°C), and won't tolerate frost. Cold soil stalls the seed and chilly spells stall the fruit, so wait for warmth before planting.
Humidity
Average outdoor humidity around 40% to 70% is fine, with no number to fuss over. What matters more is airflow, since a damp, crowded canopy is how mildew and blight get started.
Feeding
Work a low-nitrogen balanced fertilizer such as 5-10-10 into the bed before planting, then side-dress before the vines run and again after bloom. Go easy on the nitrogen, or you'll grow leafy vines at the expense of fruit.
Pruning
Watermelon usually needs no pruning, so let the vines run where they like. Cut away any dead or diseased foliage, and if you want bigger melons, hold each vine to a few fruit so the plant pours its energy into those.
Repotting
This is a ground crop, not a container plant, so repotting rarely comes up. If you do grow one in a pot, move it up only when it's badly crowded, with roots matting at the base and the vines wilting between waterings.
Size & spacing
The vines stay low, only 4 to 8 inches (10 to 20 cm) tall, but they sprawl 5 to 10 feet (1.5 to 3 m) across. Give each plant that much room so the runners have space and air can move through to keep disease at bay.
When to plant
Watermelon lives and dies by warmth. Start seed indoors a few weeks before the last frost or sow straight into warm soil once the cold has passed, then pick ripe fruit from summer into early fall.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: 2 to 4 weeks before the last spring frost in biodegradable pots
- Plant: After all danger of frost has passed and soil is at least 65°F (18°C)
- Harvest: Summer to early fall
Hardiness
USDA hardiness zones: 3–11
Temperature: down to -40°C
Troubleshooting
Most watermelon trouble shows up on the leaves or the developing fruit, and the cause is usually water, weather, or pollination. Match what you see below and step in early, while there's still time to turn it around.
Vines yellowing and wilting one runner at a time
This is Fusarium wilt, a soil fungus that clogs the plant's plumbing from the inside, and there's no cure once it sets in. Split a lower stem and you'll see brown streaking, with the trouble worse in warm soil. Pull and destroy the infected plants, never compost them, and next time reach for resistant or grafted varieties and rotate well away from other cucurbits.
Fix:
- Remove and destroy infected plants
- Do not compost diseased vines
Sunken brown rot at the bottom of the fruit
This is blossom end rot, and it traces back to swings between wet and dry soil rather than a shortage of calcium in the ground. When the soil dries out and then floods, the plant can't carry calcium into the fruit. Water deeply on a steady schedule, mulch to even out moisture, and pull off any melons already marked so the vine puts its energy elsewhere.
Fix:
- Keep soil evenly moist with deep, regular watering
- Mulch to buffer soil moisture
- Avoid heavy nitrogen fertilizer that outpaces calcium uptake
- Remove affected fruit so the plant redirects energy
Striped or spotted beetles chewing the leaves and flowers
These are cucumber beetles, and beyond the holes they chew, they can pass along bacterial wilt, though watermelon shrugs it off better than cucumber or muskmelon. Handpick the beetles or cover young plants with a floating row cover until they flower. If a wilted stem oozes a stringy thread when you cut and press it, that vine has the wilt and needs to come out.
Fix:
- Handpick beetles or use floating row covers until flowering
- Apply an approved insecticide if populations are high
- Remove and destroy any plant confirmed to have bacterial wilt
White powdery patches spreading across the leaves
That talc-like dusting is powdery mildew, a fungus that takes hold when warm days meet a crowded, humid canopy. It won't rub away cleanly, and it works through the oldest leaves first. Open the planting up for airflow, keep water off the foliage, and strip the worst leaves before it spreads across the vine.
Fix:
- Improve air flow and avoid overhead watering
- Remove badly infected leaves
- Apply a labeled fungicide if needed
Melons that fail to set or grow lopsided and small
This points to poor pollination. Watermelon carries separate male and female flowers on the same plant, and bees have to move pollen between them, so plenty of blooms with little fruit means too few visits. Seedless types need a seeded variety planted within about 10 feet (3 m) to act as a pollenizer. Skip insecticide sprays while the plants are in bloom and add flowers nearby to draw the bees in.
Fix:
- Plant a seeded pollenizer within about 10 feet of seedless types
- Avoid spraying insecticides during bloom and while bees are active
- Add nearby flowers to attract pollinators
Dark sunken spots on leaves, stems, and fruit
This is anthracnose or gummy stem blight, two fungi that splash from leaf to leaf in warm, wet weather. Look for tan to dark leaf spots ringed with yellow, sunken patches on the fruit, and a gummy ooze on the stems where gummy stem blight is at work. Switch to drip watering, keep your hands off the vines while they're wet, clear away infected debris, and stay ahead of it with a labeled fungicide if the weather stays damp.
Fix:
- Remove infected debris
- Use drip irrigation and avoid handling plants when wet
- Apply a labeled fungicide on a preventive schedule
Toxicity
Good news for pet owners: watermelon is safe all round. The sweet flesh is a non-toxic treat for people and animals alike, and the only catch is that a large helping of tough rind or a mouthful of seeds may give a pet a brief, mild stomach upset. Serve ripe, deseeded flesh in moderation and there's nothing to worry about.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats Safe
The flesh is safe and most cats just sniff it. A big helping of rind or seeds is the only thing that might bring on a passing bout of loose stool, which settles on its own.
Dogs Safe
Seedless or deseeded flesh makes a fine treat in moderation. Let a dog wolf down the tough rind or a mouthful of seeds, though, and you may see a day of mild stomach upset.
Horses Safe
Horses can have small pieces of flesh with no worry. As with any sweet treat, keep the portions modest so a sudden load of sugar doesn't unsettle the gut.
Rabbits Safe
A thin sliver of flesh now and then is fine. Skip the rind and go light, since the watery, sugary fruit can loosen a rabbit's droppings if it eats too much.
Livestock Safe
Cattle, goats, and sheep eat surplus melons without harm, and growers often feed them culls. Introduce it gradually so a sudden change of feed doesn't upset digestion.
Birds Safe
Backyard hens and pet birds peck the flesh and even the seeds happily, and it doubles as a cooling summer snack. There's nothing here that harms them.
People Safe
Watermelon is a refreshing fruit eaten the world over, seeds and all. The flesh is safe for everyone, including children.
Offer only ripe, rind-free flesh in moderation and remove seeds and rind before sharing with pets.
Varieties
Hundreds of watermelon varieties exist, from tiny icebox melons to giants. These are the ones worth knowing by name.
Crimson Sweet
A popular open-pollinated melon with a striped rind and sweet, crisp red flesh that gardeners reach for again and again.
Sugar Baby
A compact icebox type whose small, round, dark green melons ripen early enough to suit a short season.
Charleston Gray
An old standard that carries large, oblong fruit behind a pale gray green rind.
Jubilee
A big, oblong striped melon grown for heavy yields, with open-pollinated forms like Jubilee II popular in Texas.
Seedless (triploid)
A sterile triploid hybrid that sets fruit only when a seeded diploid variety grows nearby to pollinate it.
How to propagate
Watermelon grows from seed, and that's the only practical way to start it. You can raise transplants indoors or sow straight into the bed once the weather warms.
SeedEasy
Start indoors about 2 to 4 weeks before the last spring frost in peat or paper pots to limit root disturbance · Seeds sprout in about 7 to 10 days at 70 to 90°F (21 to 32°C); transplant after frost once soil is at least 65°F (18°C)
Watermelon hates having its roots disturbed, so start seed in biodegradable peat or paper pots about 2 to 4 weeks before your last frost and plant the whole pot later. Kept at 70°F to 90°F (21°C to 32°C) the seeds break ground in about 7 to 10 days. Harden the seedlings off and set them out once frost is past and the soil holds at least 65°F (18°C).
SeedEasy
Direct sow outdoors after all danger of frost has passed and the soil seed zone is at least 65 to 70°F (18 to 21°C) · Emerges in about 7 to 10 days in warm soil, then thin to the strongest plants
You can also sow straight into the bed once frost has passed and the seed zone reaches 65°F to 70°F (18°C to 21°C). Warm soil brings the seedlings up in about 7 to 10 days, and you then thin to the strongest plants. Direct sowing skips transplant shock, but the crop can ripen up to two weeks later than transplants.
Myths
A watermelon keeps ripening on the counter after you pick it.
Once a watermelon leaves the vine, it stops getting any sweeter. What you pick is what you get, so it pays to harvest at peak ripeness rather than hoping the kitchen counter will finish the job.
Thumping for a hollow sound is the only reliable way to tell when a watermelon is ripe.
The thump is a rough guide at best. The dried, brown tendril nearest the fruit and a ground spot that has turned from white to creamy yellow tell you far more, and a dull rather than glossy rind backs them up.
Watermelon fruit is toxic to dogs and cats.
The flesh is perfectly safe for pets, and only a big helping of rind or seeds might cause a brief stomach upset. The ASPCA even lists the closely related honeydew melon as non-toxic to dogs, cats, and horses.