Vegetable · Cucurbitaceae
Watermelon
Citrullus lanatus
Give it full sun, warm soil, and room to ramble, and watermelon pays you back with sweet, heavy fruit by the end of summer.
Full sun
Every 7–14 days
68–95°F · 20–35°C
70–90 days
Overview
Watermelon may look like a project for seasoned growers, but it is friendlier than its reputation. It belongs to the gourd family alongside cucumbers and squash, and rather than climbing it settles into a low, sprawling vine that can reach 5 to 10 feet (1.5 to 3 m) across while staying barely ankle high. Give it warmth and a long, easy summer, and it will do most of the hard work of swelling and sweetening the fruit on its own.
Each plant carries separate male and female flowers, so bees do the matchmaking that turns a bloom into fruit. This matters most with the seedless kinds, which cannot fruit at all unless a seeded variety grows nearby to pollinate them. Do not let that worry you, since getting pollination going is simple once you plan for it, and the melons follow close behind.
This is a heat lover with no fondness for the cold, happiest in full sun with deep, well-drained soil and steady moisture while the vines run and the fruit fills. As the melons near ripeness, ease back on the water and let the flavor gather. The reward is a homegrown melon far sweeter than anything a store can offer, and any early stumbles are easy to recover from.
Care
The short version: watermelon wants full sun, warm and well-drained soil, and 1 to 2 inches of water each week while the vines grow and set fruit, with a little less once the melons ripen. Feed it gently with a low-nitrogen mix and hand it plenty of room to sprawl, and it will settle in happily.
Light
Watermelon loves full sun, eight or more hours of it, with no real shade to slow it down. The vines turn all that light into sugar, so the brighter the spot you can give it, the sweeter and heavier the melons will grow.
Water
Aim for about 1 to 2 inches (2.5 to 5 cm) of water each week as deep soaks every 7 to 14 days, keeping the soil evenly moist while the vines grow and the fruit sets. Ease off gently as the melons ripen so the sugars gather, and pour the water at the base to keep leaves dry and disease at bay.
Soil
Plant into deep, well-drained sandy loam enriched with plenty of organic matter. A pH between 6.0 and 6.8 suits it best, and that loose, warm ground lets the long roots stretch out and drains freely after a downpour.
Temperature
Watermelon is at ease in real warmth, roughly 68°F to 95°F (20°C to 35°C), and it has no tolerance for frost. Cold soil holds the seed back and chilly spells slow the fruit, so give it time and wait for settled warmth before planting out.
Humidity
Ordinary outdoor humidity of about 40% to 70% is perfectly fine, and there is no figure here to fuss over. What counts far more is airflow, since a damp, crowded canopy is where mildew and blight like to begin.
Feeding
Work a low-nitrogen balanced fertilizer such as 5-10-10 into the bed before you plant, then side-dress once before the vines run and again after bloom. Keep a light hand with the nitrogen, or you will grow lush leaves instead of the melons you are hoping for.
Pruning
Happily, watermelon asks for almost no pruning, so let the vines wander where they please. Snip away any dead or diseased leaves, and if you would like larger melons, gently limit each vine to a few fruit so the plant can pour its energy into those.
Repotting
This is a plant for open ground rather than a pot, so repotting rarely enters the picture. If you do keep one in a container, only move it up when it is truly crowded, with roots matting at the base and the vines wilting between waterings.
Size & spacing
The vines themselves stay low, only 4 to 8 inches (10 to 20 cm) tall, yet they spread 5 to 10 feet (1.5 to 3 m) wide. Give each plant that generous room so the runners can roam and air can move through to keep disease away.
When to plant
Watermelon takes all its cues from warmth. Start seed indoors a few weeks ahead of the last frost, or wait and sow into ground that has warmed through, and you will be picking 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 troubles show up on the leaves or the swelling fruit, and the root of it is usually water, weather, or pollination. Take heart, because if you match what you see below and step in early, there is almost always time to set things right.
Runners going yellow and limp one at a time
This is Fusarium wilt, a soil fungus that quietly clogs the plant's plumbing from within, and once it takes hold there is no cure. Split a lower stem and you will find brown streaking inside, with the damage worse in warm soil. Pull and destroy the affected plants rather than composting them, and next season lean on resistant or grafted varieties and rotate well clear of other cucurbits.
Fix:
- Remove and destroy infected plants
- Do not compost diseased vines
Sunken brown patches forming at the fruit's base
Those sunken, leathery patches are blossom end rot, and the surprise is that they come from soil swinging between wet and dry rather than a true lack of calcium in the ground. When the soil dries hard and then floods, the plant simply cannot carry calcium into the fruit. Water deeply on a steady rhythm, mulch to keep moisture even, and pick off the marked melons so the vine can spend its strength 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
Spotted or striped beetles feeding on foliage and blooms
The culprits here are cucumber beetles, and beyond the ragged holes they leave, they can carry bacterial wilt, though watermelon shrugs it off better than cucumber or muskmelon do. Pick the beetles off by hand, or drape young plants with a floating row cover until they flower. If you cut a wilted stem, press it, and see a stringy thread of ooze, that vine has the wilt and is kindest removed.
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
A pale, powdery film creeping over the leaves
That talc-like dusting is powdery mildew, a fungus that moves in when warm days meet a crowded, humid canopy. It will not simply rub away, and it works through the oldest leaves first. Open the planting up so air can move, keep water off the foliage, and strip away the worst leaves before it travels along the vine.
Fix:
- Improve air flow and avoid overhead watering
- Remove badly infected leaves
- Apply a labeled fungicide if needed
Fruit that will not set or comes out small and lopsided
This usually points to poor pollination. Watermelon carries its male and female flowers separately on the same plant, and bees have to shuttle pollen between them, so plenty of blossoms 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. Hold off on insecticide sprays while the plants are in bloom, and set a few 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 appearing on leaves, stems, and fruit
These are the marks of anthracnose or gummy stem blight, two fungi that splash from leaf to leaf whenever the weather turns warm and wet. Watch for tan to dark leaf spots ringed in yellow, sunken patches on the fruit, and a gummy ooze on the stems where gummy stem blight has settled in. Switch to drip watering, keep your hands off the vines while they are wet, clear away infected debris, and stay a step ahead with a labeled fungicide if the damp lingers.
Fix:
- Remove infected debris
- Use drip irrigation and avoid handling plants when wet
- Apply a labeled fungicide on a preventive schedule
Toxicity
Here is a reassurance for pet owners: watermelon is safe all around. The sweet flesh makes a gentle treat for people and animals alike, and the only real caution is that a big helping of tough rind or a mouthful of seeds might leave a pet with a passing, mild stomach upset. Serve ripe, deseeded flesh in modest amounts and there is nothing to fear.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats Safe
The flesh is safe, and most cats do little more than sniff at it. Only a large helping of rind or seeds might bring on a passing bout of loose stool, and even that settles on its own.
Dogs Safe
Seedless or deseeded flesh makes a fine treat in moderation. Let a dog bolt down the tough rind or a mouthful of seeds, though, and a day of mild stomach upset can follow.
Horses Safe
Horses can enjoy small pieces of flesh with no concern. As with any sweet treat, keep the helpings modest so a sudden rush of sugar does not unsettle the gut.
Rabbits Safe
A thin sliver of flesh now and then sits well with a rabbit. Skip the rind and keep it light, since the watery, sugary fruit can loosen their droppings if they overdo it.
Livestock Safe
Cattle, goats, and sheep eat surplus melons without harm, and growers often feed them the culls. Bring it in gradually, though, so a sudden change of feed does not upset digestion.
Birds Safe
Backyard hens and pet birds peck happily at the flesh and even the seeds, and it doubles as a cooling summer snack. There is nothing here that will do them harm.
People Safe
Watermelon is a refreshing fruit enjoyed the world over, seeds and all. The flesh is safe for everyone, children included.
Offer only ripe, rind-free flesh in moderation and remove seeds and rind before sharing with pets.
Varieties
There are hundreds of watermelons out there, from little icebox types to true giants. These few are the ones worth knowing by name as you begin.
Crimson Sweet
Crimson Sweet is a well-loved open-pollinated melon wrapped in a striped rind, prized for the sweet, crisp red flesh that keeps gardeners coming back.
Sugar Baby
Sugar Baby is a compact icebox type whose small, round, dark green melons ripen early enough to reward a short season.
Charleston Gray
Charleston Gray is an old standard that ripens large, oblong fruit beneath a soft pale gray green rind.
Jubilee
Jubilee is a big, oblong striped melon grown for heavy yields, with open-pollinated forms like Jubilee II favored in Texas.
Seedless (triploid)
Seedless triploid melons are sterile hybrids that will only set fruit when a seeded diploid variety grows close by to pollinate them.
How to propagate
Watermelon grows from seed, and seed really is the only practical way to begin. You can raise young transplants indoors or sow straight into the bed once the weather turns kind, and either way is well within reach.
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)
Because watermelon dislikes having its roots disturbed, start seed in biodegradable peat or paper pots about 2 to 4 weeks before your last frost and later plant the whole pot straight into the ground. Kept warm at 70°F to 90°F (21°C to 32°C), the seeds break the surface in about 7 to 10 days. Harden the seedlings off gently, then set them 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
You can just as easily 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 coaxes the seedlings up in about 7 to 10 days, and you then thin to the strongest plants. Sowing in place spares the seedlings any transplant shock, though the crop may ripen up to two weeks later than transplants would.
Myths
Watermelon carries on ripening on the kitchen counter after picking.
Once a watermelon leaves the vine, its sweetness is set for good and it will not gain another degree. What you pick is what you get, so it truly pays to harvest at peak ripeness rather than trusting the counter to finish the job.
Thumping for a hollow sound is the one dependable ripeness test.
A 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 a great deal more, and a rind that has gone dull rather than glossy quietly confirms it.
Watermelon fruit is dangerous for dogs and cats.
The flesh is perfectly safe for pets, and only a large helping of rind or seeds might bring on a brief stomach upset. The ASPCA even lists the closely related honeydew melon as non-toxic to dogs, cats, and horses.