How to Grow Watermelon in the UK

Much to many people’s surprise it is quite possible to grow watermelon right here in the UK. 

While growing watermelons can provide a number of challenges to those of us in cooler climates I’m here to discuss how you can succeed. Read on to discover how even in cold old Blighty you can be harvesting your own home-grown watermelons before the end of this summer…

Best Watermelon Varieties for the UK

Hunt around and you’ll find a surprising lack of watermelon varieties available to buy as seed. In truth, the standard watermelons grown in balmy southern Europe simply don’t fare well in our cooler, damper climate. 

Furthermore, these varieties of watermelon are normally grown like pumpkins and other squashes, allowed to run riot across the surface of the ground. Within the confines of a greenhouse or polytunnel however this really isn’t ideal. 

We need a watermelon that can grow up a frame, taking up the minimum of ground space, letting us pack as many different fruit and vegetable plants as possible into our limited growing space.

For the purposes of growing watermelon in the UK really the most suitable variety is one known as “Rosario”. This watermelon is quite easy to grow from seed, and produces a decent crop of modestly-sized fruits. Rosario fruits typically measure some 4-5” in diameter, which is more than enough for a tasty watermelon treat for two people. 

A handful of other varieties are available from better seed merchants, and some suppliers like Suttons and Thompson & Morgan will sometimes sell established seedlings in the spring. This can help to kick-start your watermelon growing experiments if you’ve missed the start of the seed-planting season. 

What is the Best Month to Plant Watermelon?

Like peppers and tomatoes, watermelon need as long a growing season as you can possibly offer them. I recommend planting watermelon seeds in February or March. The seeds will take a few weeks to germinate, and can then be grown on a sunny windowsill till the frosts are definitely past.

Once you get to April or May, and you’re confident that the cold temperatures have truly gone away, your watermelon plants can then be relocated from your windowsill to your greenhouse. 

Watermelons are unlikely to thrive outdoors in the UK, so you’ll really need some kind of protection to get the best crop. That could be an unheated greenhouse or a polytunnel. 

Planted into their final growing position, your carefully-tended watermelon plants will then romp away with all the sunshine and warmth, visibly growing almost by the day. Within a matter of weeks you’ll likely see the attractive yellow flowers appearing, and soon afterwards watermelon fruits will begin to swell. 

How Many Years Does it Take to Grow a Watermelon?

In the UK watermelon plants should be considered annuals. The seeds are planted in the spring and the fruits are harvested in the late summer and the autumn. Seeds can then be saved from the fruits, ready to replant the following spring. 

In truth, as the cooler weather of autumn starts to hit you’ll find that your watermelon plants start to wilt, wither and die off. Normally by around October your previously vigorous watermelon plants will be but a distant memory. 

How Many Watermelons Do You Get Per Plant?

Watermelon plants can produce a decent number of fruits per plant, though the exact number can be affected by a number of factors. Based on my experience I would suggest that 3-6 fruits per plant is typical. 

Some of the factors that can affect how many watermelons you get per plant can include:

Plant size & age – Bigger, more established plants are capable of producing more fruits than younger or smaller watermelon plants. This is a good reason to start your watermelon plants nice and early in the season. That way, by the time the real summer heat arrives your plants will be big and vigorous. 

Nutrients provided – Feed your plants generously and they’ll have the nutrients necessary to produce a number of fruits. 

Fertilization rates – Most watermelons require fertilisation if the fruits are to ripen. There are a number of issues that can prevent this. For example flowers may be knocked off before being fertilised, or pollen may not be transported from male flowers to females. For best results consider manual fertilisation of your watermelon flowers.

Grower action – In truth, more watermelons isn’t necessarily better. In the temperature British climate it is often best to limit each plant to just 2 or 3 fruits, nipping off any others that you spot. In this way your watermelon plant can put all of its energy into ripening just a handful of fruits guaranteeing a delicious crop in summer. 

Growing Watermelons in a Greenhouse

First, it’s important to understand that watermelons are almost-exclusively greenhouse plants in the UK. I’ve tried growing a variety of different watermelon cultivars outside and our climate simply isn’t conducive to their success. The one exception may be growing watermelons in a greenhouse for the first part of the season, then potentially moving them outside to a warm, sunny and protected area for the best of the summer season.

Fortunately Rosario doesn’t take up too much space in the greenhouse, and it is entirely possible for even new growers to produce healthy, tasty fruits in an unheated greenhouse. 

Here are the most important factors to consider:

When to Plant

While we’ve discussed planting watermelon seeds indoors, ideally in March, you should wait to transfer your watermelon plants to your greenhouse until the last frosts are over. 

Of course, those few lucky readers with a *heated* greenhouse can transplant as early as they see fit. For the rest of us (like me) with an unheated greenhouse or polytunnel it’s best to plant out when the weather is reliably warm. Think of your watermelons plants as only half-hardy and you’ll be along the right lines. 

How Much Space to Give

Assuming you’re growing Rosario then your watermelon plants don’t necessarily need to take up too much space in your greenhouse. It is entirely possible to plant 2-3 watermelon plants in a decent-sized grow bag for example. Furthermore, by training them up a supporting trellis they will grow upwards, leaving plenty more growing space for other crops. 

When grown up a trellis or support, your watermelon plants can easily reach 3-6 feet in total height, though they can also gently wrap around smaller supports, gently weaving the stems into the structure. This is what I choose to do, with my total support frame being only roughly 4 feet (120cm) in total height. 

What to Plant In

Watermelon plants can be planted in a variety of containers. They may be grown in large plastic pots, for example, where I would recommend a container with a diameter of at least 8 inches. 

Alternatively, watermelon plants can be easily grown in grow bags. These are freely available from almost any decent garden centre or hardware store in the UK in season. Depending on the size of the growbag add 2 or 3 plants to each one. 

Lastly, if you use beds in your greenhouse these can also be used, though be sure to mix in plenty of decent-quality compost to add nutrients to the soil. 

Ideal Growing Conditions

Watermelon plants are one of the more “demanding” crops to grow in the UK so you’ll need to be prepared to put in the modest amount of time and effort required.

The ideal growing conditions for watermelons in the UK are:

High Temperatures

Warmer temperatures lead to healthier watermelon plants and more fruits. The perfect greenhouse would therefore receive sun for most of the day, warming up pleasantly right from the very start of the day. Watermelons may struggle to thrive in cooler conditions, hence the reason they don’t tend to do too well outdoors. 

Generous Watering

Watermelons are thirsty plants, as you’ll find when you cut into your first ripe watermelon fruit. In hot, dry conditions they can quickly become dehydrated, and you’ll see the beautiful large leaves start to sag. Be prepared to water your watermelons several times a day in the hottest summer days. 

Sunny Position

Watermelon plants appreciate the sunniest position you can possibly give them. Don’t hide them at the back of your greenhouse in the shade of other plants – instead give them the best possible positioning so they can soak up the sun’s rays all day long. 

Supporting Your Watermelon Plants

I recommend building a support structure for your watermelon plants. Growing your watermelons up some kind of frame helps to minimise the amount of ground-space they take up, but also has a number of other benefits. 

Adding a support helps to keep fruits off the ground. In damp old Britain, fruits left in contact with the earth can be at risk of rotting or going moldy. Growing your watermelons up a frame also helps to ensure suitable air movement around the plants, cutting down on the risk of pests or diseases taking hold. 

Lastly, it means your watermelon fruits themselves can ripen comfortably in the sun, ensuring the best possible harvest.

When building a support for your watermelon, remember that the fruits themselves can get quite heavy. As a result, the frame needs to be quite sturdy. I create a “cage” out of garden canes attached to each other with cable ties. 

The plants are then gently woven around this structure, being tied into place with garden twine as necessary. 

Growing Watermelons in Pots 

Watermelons can be grown in pots in the UK. On all but the warmest of weather these pots are still best positioned within a greenhouse or polytunnel. When the really hot days of summer roll around in July and August, however, the pots can potentially be moved outside.

If you want to try your luck at growing watermelons in pots *without* a greenhouse here in the UK then you need to accept your odds of success are far slimmer. For best results aim to position the pots in the very best location possible – this means warm, sunny and sheltered. 

A good example of where to position your potted watermelon would be against a south-facing wall or fence, which will warm up during the day while offering some protection from strong winds. 

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