Watermelons are possible to grow in Britain, but they are not an easy outdoor crop in an average summer. They need sustained warmth, sunlight and a long growing season to develop sweet, properly ripened fruit. For most UK gardeners, a greenhouse or polytunnel gives the best chance of success.
Watermelon is Citrullus lanatus, a different species from the cantaloupes and honeydews commonly grouped under the word 'melon'. Growing advice overlaps, but the fruit size, varieties and ripeness tests can differ, so don't follow instructions for a honeydew without checking whether they also apply to watermelon.
Pick a small, early-maturing variety
Choose a watermelon variety recommended for cooler climates or greenhouse growing. Compact, small-fruited varieties are generally more realistic than trying to grow a giant exhibition watermelon in a short northern summer.
Read the seed packet for expected spread, fruit size and maturity period. The Royal Horticultural Society mentions compact varieties such as 'Little Darling' as candidates for UK growing under cover.
A packet of seeds is inexpensive, but one vigorous watermelon plant can occupy a lot of growing space. If your greenhouse is small, start with one or two plants rather than crowding the entire floor.
When to sow watermelon seeds
Sow indoors in spring, normally around April or May if you can supply suitable warmth. Melon seeds germinate best in warm compost; the RHS gives about 18–21°C for its melon sowing guidance, although some watermelon varieties prefer warmer conditions.
Fill individual small pots with peat-free seed compost and sow at the packet's recommended depth. Keep moist but not saturated. Once seedlings emerge, give them plenty of light and avoid sudden cold nights.
If two seedlings germinate in one pot, select the stronger plant and remove the other carefully. Pot on when necessary, handling the roots as gently as possible. Watermelons dislike rough transplanting and long periods in crowded cells.
Where to plant watermelon
Choose the warmest, sunniest position available. A greenhouse, polytunnel or large coldframe is generally more suitable than an exposed bed. Outdoor success is most likely in exceptionally warm, sheltered locations.
Before planting outside or into an unheated greenhouse, harden young plants off gradually and wait until nights are reliably warm. The RHS advises planting tender melons when temperatures remain around 12–15°C or higher, with watermelons benefiting from additional warmth.
Prepare rich, free-draining soil using well-rotted organic matter. Roots need both moisture and air, so don't place seedlings into persistently waterlogged ground.
Can you grow watermelons in containers?
Yes, provided the container is large enough and the variety suits it. A pot around 40cm wide and deep is a useful starting point for melon cultivation, but vigorous watermelon varieties may require still more root space and strong support.
Fill the pot with fresh peat-free multipurpose compost and position it somewhere warm. Choose a stable container with proper drainage holes. A large plant carrying several developing melons can become top-heavy if stems are trained vertically.
A growing bag is another option if it provides sufficient compost volume. Check moisture regularly because bags can dry quickly under glass in summer.
Training sprawling vines
Watermelons make long trailing stems with tendrils. You can let them spread across a warm greenhouse border or guide them up a sturdy trellis to save floor space.
If you train plants vertically, provide secure support for the fruit itself. A heavy watermelon hanging from a slender stem can break off; gardeners sometimes use strong fabric or net hammocks anchored to the main support.
Don't repeatedly twist established vines or move large fruits around. Tender stems can split, and damaged connections may stop a developing fruit from growing.
Pollination and fruit formation
Watermelon plants produce separate male and female flowers. Female flowers have a small swelling behind the petals that resembles a miniature melon, while male flowers sit on slimmer stems.
Bees and other insects transfer pollen between flowers. In a greenhouse, pollination can be poor if insects cannot enter, so hand pollination may help. Transfer fresh pollen from a male flower into the centre of a fully open female flower using a small clean brush or the male flower itself.
If tiny fruits turn yellow and drop, poor pollination may be involved. Temperature extremes and water stress can also cause problems, so examine the growing conditions before adding fertiliser.
Watering and feeding watermelons
Watermelons have extensive foliage and need steady moisture while fruit is developing. Water the roots thoroughly during dry conditions, then allow the surface to dry appropriately without letting the whole rootball become parched.
Containers need more frequent checks than ground soil. If compost is already soaking wet, extra water can damage the roots. Aim for a consistent supply rather than allowing the plant to wilt repeatedly.
Feed according to the growing medium and a suitable vegetable fertiliser's label. Fruiting plants may benefit from potassium-rich feed once fruits begin developing. Avoid excessive nitrogen that encourages a mass of foliage without a balanced crop.
Keep greenhouse temperatures under control by ventilating during intense sunshine. Extremely hot enclosed air can interfere with pollination.
How many watermelons should you leave on a plant?
This depends on variety, plant size and growing conditions. In a short UK summer, leaving fewer fruits to mature may produce a better result than allowing many small fruits to compete.
Remove damaged or obviously poorly developing fruit if necessary. If you're experimenting with a compact variety, follow its specific guidance rather than assuming that the general figures given for honeydew melons will be identical.
Healthy leaves are essential for making the sugars that eventually sweeten the fruit. Don't strip foliage to expose every watermelon to direct sunlight.
How to tell when a watermelon is ripe
Watermelons do not use exactly the same ripeness clues as cantaloupes. Many varieties develop a creamy yellow patch where the fruit rests on the ground. A tendril nearest the fruit stalk may begin to dry, and the fruit's appearance and sound can change as it matures.
No single clue works perfectly for every variety. Compare the expected days to maturity, the fruit's colour and the seed supplier's guidance. Harvesting too early can produce bland flesh, and a fruit picked unripe won't develop all the sweetness it would have gained on the plant.
Cut the stalk with clean secateurs rather than pulling or twisting the fruit off. Store and handle carefully to avoid bruising.
Problems that prevent a good crop
Lots of leaves but no fruit: Look for female flowers and consider whether pollination is taking place.
Fruit starts growing then shrivels: Poor pollination, cold or water stress may be responsible.
Small or bland melons: Insufficient warmth, a short growing season or premature harvesting are common causes.
White powder on foliage: Powdery mildew can develop, particularly later in the season.
Leaves wilt in a wet pot: Check drainage and root health before watering again.
Planning a UK watermelon crop
Choose a compact variety, raise it indoors in warm conditions and plant one healthy seedling into a large greenhouse container. Provide good light, plenty of room, steady moisture and pollination support when needed.
Treat the first season as an experiment. Even a modest watermelon crop in the UK is an achievement, and your experience with local warmth and timing will make next year's attempt more successful.
Source
Royal Horticultural Society: How to Grow Melons, including its specific advice for watermelons.