Vegetable · Cucurbitaceae
Watermelon
Citrullus lanatus
Give it full sun, warm soil, and room to sprawl, and watermelon hands back 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 big, sprawling annual from the gourd family, so it's a cousin of cucumbers and squash. It doesn't climb; it just runs flat along the ground, and a single plant can stretch 5 to 10 feet (1.5 to 3 m) wide while staying only ankle high. Don't let that easygoing look fool you, though, because underneath it there's a crop that really wants heat and a long warm stretch to swell its fruit and pack it with sugar.
Here's the thing about the flowers: each plant carries separate male and female blooms, so bees have to carry pollen from one to the other before any fruit shows up. That's a big deal with the seedless kinds, which can't fruit at all unless a seeded variety grows close enough to pollinate them. Sort out the pollination and the melons pretty much take care of themselves.
This one's a warm-season crop through and through, and it's got 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. Then, as the melons get close to ripe, you ease back on the water so the flavor concentrates into something the supermarket rarely matches.
Care
The short version: watermelon wants full sun, warm well-drained soil, and 1 to 2 inches of water a week while the vines grow and set fruit, then a bit less as the melons ripen. Feed it lightly with a low-nitrogen mix, give it room to sprawl, and you're good.
Light
Watermelon wants full sun, eight or more hours of it, and no real shade. The vines turn all that sunlight into sugar, so the brighter the spot, the sweeter and heavier the melons you'll get.
Water
Give it about 1 to 2 inches (2.5 to 5 cm) of water a week as deep soaks every 7 to 14 days, and keep the soil evenly moist while the vines grow and set fruit. Then ease off as the melons ripen so the sugars concentrate, and water at the base, since dry leaves catch a lot less disease.
Soil
Plant it in deep, well-drained sandy loam with plenty of organic matter mixed in. A pH of 6.0 to 6.8 suits it best, and that loose, warm ground lets the long roots run and drains quickly after rain.
Temperature
It loves real heat, somewhere around 68°F to 95°F (20°C to 35°C), and won't take frost at all. Cold soil stalls the seed and chilly spells stall the fruit, so it pays to wait for warmth before you plant.
Humidity
Average outdoor humidity of about 40% to 70% is fine, and there's no number to fuss over here. What matters more is airflow, because a damp, crowded canopy is exactly how mildew and blight get going.
Feeding
Work a low-nitrogen balanced fertilizer like 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 end up with leafy vines and not much fruit.
Pruning
Watermelon usually needs no pruning at all, so just let the vines run wherever they like. Cut off any dead or diseased leaves, and if you want bigger melons, hold each vine to a few fruit so the plant puts everything into those.
Repotting
This is a ground crop, not really a container plant, so repotting hardly ever comes up. If you do grow one in a pot, only move it up when it's badly 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, 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 down.
When to plant
Watermelon really does live and die by warmth. Start seed indoors a few weeks before the last frost, or just wait and sow into warm soil once the cold's gone, and you'll 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 of the trouble shows up on the leaves or the growing fruit, and nine times out of ten it comes down to water, weather, or pollination. Match what you're seeing below and step in early, while there's still time to turn things around.
Vines going yellow and wilting one runner at a time
This is Fusarium wilt, a soil fungus that clogs the plant's plumbing from the inside, and once it's in there's no cure. Split a lower stem and you'll spot brown streaking, and it's worse in warm soil. Pull and destroy the infected plants, don't ever 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 showing up at the bottom of the fruit
That sunken, leathery patch is blossom end rot, and here's the twist: it comes from soil bouncing between wet and dry, not from a shortage of calcium in the ground. When the soil dries out and then floods, the plant just can't move calcium into the fruit. So water deeply on a steady schedule, mulch to keep the moisture even, and pull off any marked melons so the vine can spend 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 up the leaves and flowers
Those are cucumber beetles, and besides the holes they chew, they can pass along bacterial wilt, though watermelon handles it better than cucumber or muskmelon do. Handpick them, or cover young plants with a floating row cover until they flower. And if you cut a wilted stem, press it, and see a stringy thread ooze out? That vine's got 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 creeping across the leaves
That talc-like dust is powdery mildew, a fungus that moves in when warm days meet a crowded, humid canopy. It won't just rub away, and it hits the oldest leaves first. Open the planting up for airflow, keep water off the foliage, and pull the worst leaves before it spreads down the vine.
Fix:
- Improve air flow and avoid overhead watering
- Remove badly infected leaves
- Apply a labeled fungicide if needed
Melons that won't set or come out small and lopsided
This one's usually poor pollination. Watermelon keeps its male and female flowers on the same plant, and bees have to move pollen between them, so lots of blooms but little fruit means too few visits. Seedless types especially need a seeded variety within about 10 feet (3 m) to pollinate them. Skip the insecticide sprays while the plants are flowering, and plant a few flowers nearby to pull 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 turning up on leaves, stems, and fruit
This is anthracnose or gummy stem blight, a pair of fungi that splash from leaf to leaf in warm, wet weather. Look for tan to dark leaf spots ringed with yellow, sunken spots on the fruit, and a gummy ooze on the stems where gummy stem blight's at work. Switch to drip watering, keep your hands off the vines while they're wet, clear away the infected bits, 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 if you've got pets: watermelon is safe all round. The sweet flesh is a non-toxic treat for people and animals alike, and about the only catch is that a big helping of tough rind or a mouthful of seeds might give a pet a brief, mild tummy upset. Serve ripe, deseeded flesh in moderation and there's really nothing to worry about.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats Safe
The flesh is safe, and most cats will just give it a sniff and walk off. Only a big helping of rind or seeds might bring on a quick bout of loose stool, and even that clears up on its own.
Dogs Safe
Seedless or deseeded flesh makes a great treat in moderation. Let a dog scarf down the tough rind or a mouthful of seeds, though, and you might see a day of mild stomach upset.
Horses Safe
Horses can have small pieces of flesh with no trouble. Just keep the portions modest, like any sweet treat, so a sudden hit of sugar doesn't unsettle the gut.
Rabbits Safe
A thin sliver of flesh now and then is fine for a rabbit. Skip the rind and keep it light, since the watery, sugary fruit can loosen their droppings if they eat too much.
Livestock Safe
Cattle, goats, and sheep eat surplus melons without any harm, and growers often toss them the culls. Just bring it in gradually so a sudden change of feed doesn't upset digestion.
Birds Safe
Backyard hens and pet birds happily peck at the flesh and even the seeds, and it doubles as a cooling summer snack. There's nothing in it that'll hurt them.
People Safe
Watermelon is a refreshing fruit people enjoy all over the world, seeds and all. The flesh is safe for everyone, kids 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, everything from tiny icebox melons to real giants. These are the few that are worth knowing by name.
Crimson Sweet
Crimson Sweet is a favorite open-pollinated melon with a striped rind and sweet, crisp red flesh that gardeners keep coming back to.
Sugar Baby
Sugar Baby is a compact icebox type, and its small, round, dark green melons ripen early enough to work in a short season.
Charleston Gray
Charleston Gray is an old standby that ripens large, oblong fruit behind a pale gray green rind.
Jubilee
Jubilee is a big, oblong striped melon grown for heavy yields, and open-pollinated forms like Jubilee II are popular in Texas.
Seedless (triploid)
Seedless triploids are sterile hybrids, so they'll only set fruit when there's a seeded diploid variety growing nearby to pollinate them.
How to propagate
Watermelon grows from seed, and honestly that's the only practical way to start it. You can raise transplants indoors or just sow straight into the bed once things warm up, whichever suits you.
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 really doesn't like having its roots messed with, so start seed in biodegradable peat or paper pots about 2 to 4 weeks before your last frost and plant the whole pot later. Keep them warm at 70°F to 90°F (21°C to 32°C) and they'll break ground in about 7 to 10 days. Harden the seedlings off, then set them out once frost's 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 just sow straight into the bed once frost has passed and the seed zone hits 65°F to 70°F (18°C to 21°C). Warm soil brings the seedlings up in about 7 to 10 days, and then you thin to the strongest ones. Sowing in place skips the transplant shock, but keep in mind the crop can ripen up to two weeks later than transplants.
Myths
A watermelon keeps ripening on the counter once you've picked it.
Nope. Once a watermelon's off the vine, it stops getting any sweeter. What you pick is what you get, so it really pays to harvest at peak ripeness instead of hoping the counter will finish the job.
Thumping for a hollow sound is the only reliable way to check for ripeness.
The thump's a rough guide at best. The dried, brown tendril nearest the fruit and a ground spot that's turned from white to creamy yellow tell you way 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 tummy upset. The ASPCA even lists the closely related honeydew melon as non-toxic to dogs, cats, and horses.