Growing watermelons requires lots of space, lots of sun, lots of water and lots of nutrients.
They are greedy, rambling vines, like all plants in the cucurbiteae family (e.g. zucchini, squash, pumpkin, cucumbers…)
Watermelons are not particularly difficult to grow, but because they are so demanding I don’t consider watermelons a good plant for beginner gardeners. (You can get lucky if you live in optimum conditions). Like their cantaloupe cousins, watermelons demand 2 to 3 months of heat to produce ripe fruit, which makes growing watermelons in northern regions challenging, but not impossible. By using plastic mulch to warm soil and floating row covers to trap warm air near plants, gardeners in any part of the country can experience the homegrown goodness of watermelons.
Where and when can you grow watermelons?
In the true tropics the dry season (winter) is the best watermelon growing season.
Watermelons do not cope well with extreme heat or the humid, soggy conditions of our wet season/summer. Fungal diseases and bugs will wipe them out in no time.
If you live in a cooler climate, then summer is the time to grow watermelons.
You do need at least three months of reliably hot, sunny weather to grow and ripen a watermelon. During that time your average daily maximum temperature should be at least about 20-25°C or 70-80F. Warmer is even better.
(There are different watermelon varieties, so if you are at the low end of that, look for a faster maturing variety.)
Grow watermelons in full sun. You also need an abundant supply of water and nutrients (good soil).
And you need space. As I said, a rambling vine. They like to go wandering and smother everything around them.
Watermelons are grown from seed. You may be tempted to use seed out of a melon you bought, but don’t waste your time. It is almost guaranteed to be a hybrid.
Hybrid varieties are very special crosses that don’t grow true to type. (You would end up growing what we call pig melons. A melon variety that’s only good for feeding to the pigs…)
Buy your seed, and if possible buy an open pollinated heirloom variety. Because then you CAN use your own seed next year. The open pollinated varieties are also hardier.
You will find a lot more interesting varieties amongst the heirlooms then you cn find in the standard collection of you local gardening centre.
Start your watermelon seeds in the ground, right where they are supposed to grow. The soil should be at least 18°C for them to germinate.
Unless you have an extremely short growing season, do NOT start your watermelon seed in a pot or punnet. Do NOT buy watermelon seedlings from a nursery.
Watermelon seed germinates easily and quickly, within a few days. Watermelon plants outgrow the seedling stage very quickly, and they don’t like transplanting. You don’t save much time and you end up with a weaker plant.
Save yourself this totally needless extra work and stick your seeds in the ground, about two cm or an inch deep.
(If you have a long growing season, you may want to do several plantings, a few weeks apart.)
Watermelons need deep, rich, friable soils. To grow watermelons it helps to raise the soil (make mounds or ridges). Raising the soil has several advantages:
A mound or ridge is free draining (melons don’t like wet feet). If you have heavy clay soil, definitely raise the bed.
Mounds are also good if the soil is as poor as mine. I just make a mound of good soil with lots of compost in it to grow watermelons. Sometimes I plant them in what’s left over from a compost pile after I used most of the compost.
If you like growing things in neat rows, or if you want to plant a large area, grow watermelons on ridges, like the commercial growers do.
Rows should be about 2 m (6 ft) apart and the plants spaced at 30 cm/a foot apart. (Sow twice as many as you want, and keep the stronger ones.)
I prefer growing watermelons in clumps on a mound, in several different locations in the garden. (Mixing things up helps keeping pests and diseases at bay.) If you want several hills together, keep them about 2m apart.
The mound should be about one meter square and a foot high. Then I plant about ten seeds in it, in three groups of three to four seeds each. The groups are spaced about a foot apart (30 cm).
After a few weeks I can see which watermelon plants grow the strongest, and I snip off the weaker ones, leaving only one seedling in each group. (Don’t pull them up, cut them off. Or you disturb the roots of the others.)
If you have a very small garden but absolutely have to have watermelons, you can try growing them on a trellis. You need a very strong trellis, you need to train them up the trellis as they aren’t climbers, and you need to support the developing fruit so the trellis holds the weight, not the plant.
It is a lot of work but it can be done…
Harvest and Storage
Watermelons typically ripen over two weeks. As soon as one melon is ripe, the others won’t be far behind. About a week before a melon is ripe, water only as necessary to keep vines from wilting. Withholding water causes sugars to concentrate in the fruit. Too much water reduces sweetness.You can judge a watermelon’s ripeness by its skin color. The rind changes from a bright to a dull green, and the part that touches the soil shifts from greenish white or straw yellow to rich, creamy yellow. Gardeners also judge a watermelon’s ripeness by rapping on the skin and listening for a low-pitched thud. Tune your ear to the incorrect sound by rapping on a few fruits that aren’t ripe. Underripe fruits resonate with a high-pitched, tinny sound.
Watermelons will keep 2 to 3 weeks unrefrigerated. Place them in a cool basement to increase their holding time. After cutting, refrigerate unused portions. If you have extra melon on hand, dice or cut the flesh into balls and freeze for slushies.