Vegetable · Cucurbitaceae
Watermelon
Citrullus lanatus
Give it full sun, warm soil, and room to sprawl, and the low green vines swell into sweet, heavy fruit by late summer.
Full sun
Every 7–14 days
68–95°F · 20–35°C
70–90 days
Overview
Watermelon grows as a low, ground-hugging vine, a member of the gourd family beside cucumbers and squash. The runners never climb; they lie flat and creep outward, one plant threading 5 to 10 feet (1.5 to 3 m) across the soil while the whole tangle rises only to your ankle. Along those trailing stems the fruit swells heavy and rounded, given enough heat and a long warm season to fill.
Set among the foliage are separate male and female flowers, and only when a bee carries pollen from one to the other does a small green fruit begin to bulge behind a female bloom. The seedless kinds hold sterile flowers that swell nothing on their own, so a seeded variety has to grow near enough to lend the pollen. Watch closely and the successful blooms give way to fruit almost overnight.
Each plant carries a rounded melon that grows dense and heavy, its rind hard and smooth over crisp, deep red flesh. Under full sun, deep well-drained soil, and even moisture the fruit fills and colors, and as it nears ripe the watering eases so the flesh turns sweeter and richer. The cut face of a ripe melon runs a vivid red, studded with dark seeds, far deeper in color than most store fruit.
Care
The short version: watermelon wants full sun, warm well-drained soil, and 1 to 2 inches of water a week while the low vines run and the fruit sets, then less as the melons ripen. Feed lightly with a low-nitrogen mix and give the sprawling stems plenty of room.
Light
Watermelon wants full sun, eight or more hours of it falling on the broad, ground-level canopy. Those sunlit leaves feed sugar straight into the fruit, so the brightest spot builds the sweetest, heaviest melons.
Water
Give it about 1 to 2 inches (2.5 to 5 cm) of water a week in deep soaks every 7 to 14 days, keeping the soil evenly damp while the vines run and the round fruit sets. Ease off as the rind hardens and colors so the flesh sweetens, and lay the water at the base to keep the trailing leaves dry.
Soil
Root it in deep, well-drained sandy loam dark with organic matter. A pH of 6.0 to 6.8 suits it, and the loose, warm, crumbling ground lets the long roots snake down and sheds water fast after rain.
Temperature
Watermelon leans into real heat, roughly 68°F to 95°F (20°C to 35°C), and the tender vines collapse at frost. Cold soil leaves the seed dormant and cool air stalls the swelling fruit, so warmth has to come first.
Humidity
Ordinary outdoor humidity of 40% to 70% asks nothing of the plant. Moving air matters more, since a still, damp, crowded canopy is where the pale mildew and dark blight first settle on the leaves.
Feeding
Work a low-nitrogen balanced fertilizer such as 5-10-10 into the ground before planting, then side-dress before the vines run and again after the flowers open. Too much nitrogen throws up lush green leaf and stem at the cost of the fruit.
Pruning
Watermelon needs little cutting, and the vines are left to trail and knit across the ground. Strip off any leaf gone dead or diseased, and holding each vine to a few fruit lets the remaining melons swell larger and rounder.
Repotting
This is a plant of open ground, its long runners built to spread, not to circle a pot, so repotting rarely comes up. In a container it needs moving up only under real crowding, with roots matting at the base and the vines going limp between waterings.
Size & spacing
The vines lie low, only 4 to 8 inches (10 to 20 cm) off the ground, yet the runners reach 5 to 10 feet (1.5 to 3 m) outward. Give each plant that spread of bare soil so the stems can trail flat and open air can pass between the leaves.
When to plant
Watermelon moves entirely with the warmth. Seed goes in indoors a few weeks before the last frost or straight into warmed soil once the cold lifts, and the heavy, rounded fruit colors up 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
Watermelon shows its troubles right on the surface, in the color of the leaves and the skin of the swelling fruit, and water, weather, or pollination usually lies behind them. Look closely at what has changed and step in early, while the vine can still recover.
Runners yellowing and going limp one at a time
When one runner yellows and wilts while its neighbors stand, the cause is Fusarium wilt, a soil fungus that plugs the stem's inner channels, and nothing cures it once inside. Slice a lower stem lengthwise and a brown streak runs through the pale core, sharpest in warm soil. The wilted plants are pulled and burned rather than composted, and resistant or grafted stock with a wide rotation keeps the next crop clear.
Fix:
- Remove and destroy infected plants
- Do not compost diseased vines
Sunken, leathery brown rot at the base of the fruit
That dark, leathery, sunken patch on the blossom end is blossom end rot, and it grows from soil swinging between wet and dry rather than any calcium missing in the ground. When the earth bakes dry then floods, the plant cannot draw calcium out to the far end of the fruit. Deep, even watering, a blanket of mulch, and lifting the marked melons let the vine pour into the sound ones.
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
Yellow-and-black striped or spotted beetles on the leaves
Those small yellow-and-black beetles, some striped and some spotted, chewing ragged holes in the leaves and flowers are cucumber beetles, and they can carry bacterial wilt, though watermelon shrugs it off better than cucumber or muskmelon. Pick them off by hand or tent young plants under a floating row cover until they flower. Cut a wilted stem, press it, and a fine stringy thread drawing out confirms the wilt, and that vine has to go.
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 film dusting the leaf surfaces
That pale, talc-like film coating the leaves is powdery mildew, a fungus that settles in when warm days sit over a crowded, humid canopy. It clings rather than rubbing away and creeps first across the oldest, lowest leaves. Opening the tangle for air, keeping water off the foliage, and pulling the whitest leaves slows its march along the vine.
Fix:
- Improve air flow and avoid overhead watering
- Remove badly infected leaves
- Apply a labeled fungicide if needed
Fruit that fails to set or swells small and lopsided
Plenty of open flowers but few sound melons, some small and one-sided, points to poor pollination. The male and female blooms sit apart on the vine, and a bee has to bridge them, so too few visits leaves the young fruit unfilled or twisted. Seedless plants especially need a seeded variety within about 10 feet (3 m) to lend pollen, and holding off insecticide during bloom while adding flowers nearby keeps the bees at work.
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 pitting leaves, stems, and fruit
Dark, sunken, water-soaked spots pitting the leaves, stems, and fruit are anthracnose or gummy stem blight, two fungi that ride splashing water in warm, wet spells. The signs are tan to dark leaf spots ringed in yellow, sunken craters on the rind, and a sticky gummy bead weeping from the stem where gummy stem blight has taken hold. Drip watering, dry hands on the vines, cleared debris, and a labeled fungicide through the damp keep them in check.
Fix:
- Remove infected debris
- Use drip irrigation and avoid handling plants when wet
- Apply a labeled fungicide on a preventive schedule
Toxicity
Behind its heavy look, watermelon is safe all round. The crisp red flesh is a non-toxic treat for people and animals alike, and only the tough green rind or a mouthful of the dark seeds might bring a pet a brief, mild stomach upset. Serve the ripe, deseeded flesh in moderation and there is nothing here to fear.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats Safe
The red flesh is safe, and most cats do little but sniff at it. Only a good helping of the tough rind or the seeds might bring a passing bout of loose stool, which clears on its own.
Dogs Safe
Seedless or deseeded flesh makes a fine treat in moderation. A dog that bolts the hard rind or a mouthful of dark seeds, though, may carry a day of mild stomach upset.
Horses Safe
Horses take small pieces of the flesh with no trouble. Modest portions keep the sudden sugar from unsettling the gut.
Rabbits Safe
A thin sliver of the watery flesh now and then suits a rabbit. The rind is left off and the amount kept light, since the sugary fruit can loosen the droppings.
Livestock Safe
Cattle, goats, and sheep crunch through surplus melons, rind and all, without harm, and growers often pass them the culls. A gradual introduction keeps the change of feed easy on digestion.
Birds Safe
Backyard hens and pet birds peck at the wet red flesh and even the seeds, and it doubles as a cooling summer snack. Nothing in it harms them.
People Safe
Watermelon is eaten the world over, the red flesh and dark seeds alike. It is safe for everyone, children included.
Offer only ripe, rind-free flesh in moderation and remove seeds and rind before sharing with pets.
Varieties
Watermelons range from small round icebox melons to long, heavy giants, and their rinds carry different stripes and shades. These few are the ones worth knowing by sight.
Crimson Sweet
Crimson Sweet wraps sweet, crisp, deep red flesh inside a striped rind, and gardeners come back to it again and again.
Sugar Baby
Sugar Baby is a compact icebox melon, its fruit small, round, and dark green, ripening early enough for a short season.
Charleston Gray
Charleston Gray carries large, oblong fruit sheathed in a pale gray green rind, an old and steady standard.
Jubilee
Jubilee swells into a big, oblong striped melon grown for heavy yields, with open-pollinated forms like Jubilee II favored in Texas.
Seedless (triploid)
Seedless triploids are sterile hybrids whose flowers swell fruit only when a seeded diploid variety grows nearby to pollinate them.
How to propagate
Watermelon starts from seed, the flat, dark seed you find through the ripe flesh, and that is the only practical way in. The seed can be raised into transplants indoors or dropped straight into the warm bed.
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)
The flat seed resents any tug on its roots, so it starts in biodegradable peat or paper pots about 2 to 4 weeks before the last frost, later sunk pot and all into the bed. Held at 70°F to 90°F (21°C to 32°C), the pale seedlings break the surface in about 7 to 10 days. They harden off before going out, once frost has passed 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
The seed drops straight into the warm bed just as well, once frost has passed and the seed zone reaches 65°F to 70°F (18°C to 21°C). In that heat the seedlings push up in about 7 to 10 days, then thin to the strongest. Sowing in place spares the roots any shock, though the crop may color up to two weeks later than transplants.
Myths
A watermelon keeps ripening on the counter after you pick it.
Once cut from the vine, the flesh sweetens no further, however long the melon sits. What you pick is what you get, so the deep-red ripeness has to be there at harvest rather than hoped for on the counter.
A hollow thump is the only reliable sign of a ripe melon.
The hollow thump only hints. The dried, brown tendril curling at the stem nearest the fruit and the ground spot gone from white to creamy yellow show far more, and a rind faded from glossy to dull backs them up.
Watermelon fruit is toxic to dogs and cats.
The red flesh is perfectly safe for pets, and only a good deal of the tough rind or the dark seeds might stir a brief stomach upset. The ASPCA even lists the closely related honeydew melon as non-toxic to dogs, cats, and horses.