There can be fewer more powerful tastes of summer than the crisp, sweet, cool flesh of a watermelon. For many of us in cooler climates (like me in the UK) they conjure up images of hot, Mediterranean areas; Spain, perhaps, or Greece. Surely, nobody could grow watermelons in more temperate conditions?
As it turn out, you can, given a little time and effort. Here’s exactly how to do it…
Planting Watermelon Seeds
One of the most important lessons to learn when it comes to watermelons is that they like it hot. If you’re not lucky enough to be living in Florida or California then we need to “trick” our seeds into thinking that it’s suitable weather to germinate. This means providing a warm and inviting environment.
These days there are a huge number of different watermelons available on the market. However, not all of them will guarantee a top-quality crop. It is best, therefore, to select seeds that offer the very best chance of success. If you just pull some watermelon seeds out of a fruit you bought in your local supermarket then your odds are likely to be far less.
No, go to a specialist supplier (I use Chiltern Seeds or Thompson & Morgan here in the UK) and buy seeds fit for purpose; seeds for watermelon plants that will thrive in the less-than-ideal conditions they’re likely to experience.
The seeds should be planted roughly 2cm deep, in very rich potting compost. Watermelons are hungry plants, and grow rapidly, so placing the seeds into the most nutrient-rich compost available is a smart move. Additionally, choose a large pot (I would suggest a pot at least 6″ in diameter) to minimise how many times your seedlings will need to be repotted. Plant either one seed (if you feel confident) or two per pot, and if both germinate then simply remove the less vigorous seedling.
The pot should then be soaked in a tray of water until the compost has absorbed as much moisture as possible, before it is covered to keep in the moisture. Easy options are to use a piece of clean perspex, or even just wrap the whole pot in a clear plastic bag.
Then we need to provide the necessary warmth. Watermelons seeds rarely germinate at a temperature below 20’C, so our goal is to keep the pot warmer than this for some weeks. Here there are a number of solutions. Some people opt to keep their pots of seeds in a warm airing cupboard. Personally, I make use of artificial heaters. I place a heat mat on the windowsill, and place my pot on top. Within a few weeks, you should start to see the first signs of life appearing.
Note that watermelons need a long growing season, especially in cooler condition, so it can make sense to start the seeds early in the year.
I have found that watermelon seedlings tend not to cope very well with repotting. Whenever a seedling is moved to new conditions it seems to enter a period of “suspended animation” for some weeks as the roots re-establish themselves.
Your goal should therefore be to repot the seedlings as little as possible before placing them in their final destination. If and when repotting becomes necessary it makes sense to allow the pot to dry out gently, then tap out the whole thing and place it into the new pot. Try to disturb the roots as little as possible.
In warmer climates, watermelon are typically grown outdoors. In cooler areas, however, they’re likely going to need some protection in the form of glass or plastic. An ideal solution is to grow watermelon in a greenhouse, or in a large container in your conservatory.
The other alternative, if you lack the necessary glazing, is to grow them outside, but under the protection of a cloche. If this is the option you choose, be sure to put the cloche in place some weeks before you plant your watermelons, in order to gently warm the soil up in advance.
Whatever option you choose, watermelons should only be planted out after the last frosts have passed. Cold weather will quickly kill the plants, forcing you to start all over again.
Watermelons appreciate conditions that offer plenty of water, nutrients and sunshine. They should therefore be afforded “prime position” in your greenhouse, ensuring they are afforded as much direct sunlight as is possible.
I have found that digging out a good-sized hole in my greenhouse, and filling this with well-rotted compost and/or manure starts the plants off well. The side benefit is that manure heats up as it rots, further warming the roots of my plants and encouraging vigorous growth. Feed the plants weekly as the young melons start to appear. A liquid fertilizer, such as those sold for tomatoes, tend to work well.
Managing Adult Watermelon Plants
A healthy watermelon plant grows rapidly, and to impressive proportions. Plants should therefore be placed no closer than 1 metre apart. You should also be prepared to give them all the space that they need. While I leave my watermelons to ramble across the ground, some growers opt to train their watermelon vines up a trellis, allowing the plants to grow vertically rather than horizontally, thus saving space in the greenhouse.
There are two reasons why I personally leave my plants to ramble. Firstly, the fruits of most watermelon varieties are large and heavy. Supporting them on a vertical plant can be challenging, and risks damaging the sensitive fruits. Secondly, and more importantly, I use mulch under my plants. This not only protects the growing fruits from rot, but can actually speed up growth once again.
Many books, it seems, recommend the use of straw as a mulch under watermelon plants. Personally, my preference is for black weed-suppressing matting. Not only does this matting cut down on my ongoing maintenance (I often get grass growing when using straw) but the black colour helps to absorb and trap the sun’s warmth, further heating up my rapidly-growing fruits.
Watermelon Fruiting & Harvesting
The more watermelon fruits you let your plants produce, the longer it will take for them to ripen. In colder climates this is an issue, as we’re dealing with a very limiting growing window. We want your fruits to ripen as early as possible, as when the weather starts to turn in September you’ll be running on borrowed time. For this reason I tend to limit my plants to just 2-3 fruits each. The later in the season they start to appear, the fewer I will allow. It is not unusual for me to pluck off all the fruits except one if summer is at it’s peak, to guarantee at least one watermelon plant.
Assuming your plants were started early enough in the season, and have been fed with copious amoutns of warmth, sunshine, nutrients and water then all too soon the fruits should start to ripen. There are a number of ways to tell when a watermelon is ready for plucking. For one, you will notice that the rind starts to change colour, from a glossy green to a more matt-looking texture. Secondly, gentle twisting will quickly remove them from the plant. Thirdly, and perhaps most tellingly, tapping the fruits with your finger tips will produce a characteristic “hollow” sound. Try tapping your fruits regularly and you’ll hear when the change starts to occur.
As your watermelons start to ripen, consider reducing the volume of water you’re providing. This helps to intensify the flavours in the fruits, making them even more delicious than they otherwise would have been.
Upon ripening, the fruits can be safely harvested. Watermelon will keep for up to a week if refrigerated.
Prepare to enjoy your very own home-grown watermelons with pride!