How to Grow Watermelon

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Nothing brings back memories of warm summer picnics like watermelon. It’s the perfect balance of sweetness and nutrition.

Most of us know the taste of store-bought fruit, but there’s nothing better than a homegrown, freshly-harvested watermelon.

Whether you want to grow one unique variety to taste-test or a whole bunch of picnic watermelons to enjoy with your family and friends, we have you covered with this growing guide.

Image of a growing guide overview that shows the amount of sun, water, fertilizer, and when to harvest.

When to Plant

To grow a watermelon, you will need a long, warm growing season, sandy, well-draining soil, and a full-sun location with plenty of space.

Generally you’ll want to plant your seeds about 2-3 weeks after your last frost date.

How Far Apart Space the Plants

Unfortunately, melons aren’t container plants that you can grow plentifully on an apartment balcony. They need to be spaced at least 5 feet apart from one another, and one vine can grow up to twenty feet in length.

Preparing Your Garden

Preparing your garden before sowing crops is a crucial step to success.

But before you dig your hands into the soil, learn about your area’s climate. You need to know how long the growing season lasts in your climate.

This will help you to choose a variety that suits you best. Picnic watermelons take much longer to grow than the smaller, icebox varieties, so select a species based on your region.

If you live in a colder climate, you’ll likely need to start your seeds indoors and transfer them outside a few weeks after your last frost.

You can also cover your garden area with black plastic to retain heat in the soil. This will allow you to plant your seeds sooner and with more success.

A couple of sliced watermelon

Sunlight

Watermelons are full-sun plants, which means a bright, unfiltered location is best. They need at least 6-10 hours of sun per day. Avoid shady areas, such as under a tree, or places where southern-facing light is blocked off, such as at the side of a building.

If you start your seeds indoors, place them on a southern-facing windowsill for best results. If they begin to grow leggy, as they may if an overhang blocks your window or the weather outside is cloudy, purchase a grow light for plants to avoid future problems.

Although there are many of these on the market, I’ve found reusing an old lamp and buying a light bulb suited for plant growth is cheapest. You’ll get much better results for your money this way.

Soil

Once you’ve chosen a bright location with plenty of space, take a look at your soil. Watermelon like well-draining, sandy soil with a pH of 6-6.5. Although they can tolerate slight variations from these numbers, a pH that drops too low will damage the plant.

If you have a clay-based soil, the easiest thing to do is create a raised garden bed for your melons. This way, you can create new soil from scratch.

You can also amend clay soil with compost and other drainage additives, but this will take more work on your part.

Watermelons are hungry plants, so when you’re preparing your soil, be sure to add in plenty of nutrients through a balanced fertilizer or compost. You can check out our recommendations for the watermelon fertilizers

You can also consider to plant your watermelons with some beneficial and synergistic companion plants.

Watering System

These melons thrive in soil that is consistently moist, but not waterlogged. They also do best when water doesn’t fall on their stems and leaves.

For this reason, many gardeners use irrigation systems, such as soaker hoses, to wet the ground and keep watering consistent. Short of this, you can keep a watering schedule yourself and aim your hose or watering can straight at the plant’s base.

Knowing your options and deciding on a watering method before planting will help you be more prepared for the season ahead.

A plump watermelon sitting in a garden almost ready to be harvested

Pest Deterrent

Once your watermelons begin to grow, you’ll need to keep the fruit from touching the soil by placing a surface between the two.

This could be straw, cardboard, or another substance. This prevents the underside of the fruit from rotting and also protects against many pests.

Covering your fruit will also stop pests and wildlife from munching on your crops. Floating row covers are great for this purpose.

Row covers allow most sunlight through while blocking insects, wildlife, and other pests from your plants. They also help to keep crops warm, which is a great bonus for such a heat-loving plant.

Remove your row covers once your vine has grown both male and female flowers. You’ll be able to tell the difference as the female flowers are plumper.

During flowering, remove row covers so that bees can access your crop to pollinate it.

Starting Seeds Indoors

Seedling with 2 true leaves sprouting from a pot

If your growing season isn’t long enough for the variety you’re growing, you’ll need to begin the seedlings indoors.

Don’t begin too early, though, as large seedlings have more trouble being transplanted. Instead, plant your seeds 2-3 weeks before the growing season begins.

Biodegradable pots are best to start your seeds in, as you won’t have to remove your seedlings from the pot or touch their fragile root systems when planting them outdoors. However, any four-inch pot with a drainage hole will do.

Fill your pot with sandy, freely-draining potting soil and push 2-4 seeds into the soil with your fingertip. Keep the soil consistently moist, but not muddy or waterlogged.

Once seedlings begin to sprout, avoid watering the plant’s foliage by pouring water onto the soil directly, or bottom watering through a drainage hole in the pot.

Never leave your seedlings in standing water, as this will cause your plants’ roots to rot.

Place your seedlings next to a southern-facing windowsill or beneath a grow light so that they have a bright light source.

A small box of seedlings

Keep in mind that watermelon plants need warmth, especially when they’re young. If your windows are drafty, grow your seedlings underneath an artificial light source in a warm location, or use a heat mat to keep them warm.

I also recommend having a fan directed at your seedlings for at least a few hours daily. This strengthens them for outdoor life, where gusts of wind can damage unhardened plants.

2-3 weeks after your last frost date, or 4-6 weeks after planting your seedlings indoors, you can begin to transfer them outside. Don’t plant them directly into the ground in your chosen location, but instead allow them to adapt to full sun gradually.

Planting seedlings immediately in a full-sun location will burn them.

Once your plants have adapted to full sun, carefully remove them from their starter pot. Don’t touch the root system, but instead place the whole root clump, soil included, into the ground.

If you’ve started your seedlings in a degradable pot, bury everything in your chosen location.

Water seedlings immediately after transplanting.

Small foliage coming out of mulched soil without any watermelon growing from it yet.

Sowing

When sowing seeds straight into the ground outside, wait 2-3 weeks past your areas last frost date. Plant the seeds around an inch into the ground when soil temperatures are between 70-90 degrees Fahrenheit for the best results.

The warmer your soil is within this spectrum, the faster your seeds will sprout.

If growing in rows, space your seeds at least 5 inches apart. Some growers create small hills of dirt and bury the seeds within those piles. This allows water to run off from the base of the plant and gives your watermelon vines a bit more room to grow.

Mulch the top of your soil to help prevent weed spread and regulate soil temperature and moisture levels.

Here are some quick tips for seedling care:

  • As seedlings are growing, keep the soil consistently moist. Watermelon plants need 1-2 inches of water every week, but small seedlings will need a slightly more moist environment.
  • Once your seedling begins to sprout, avoid watering the foliage as this can lead to disease and rot. Instead, direct the water to the soil at the base of your plant.
  • Don’t prune your vines, as this can make your crop less sweet. If the vines are growing into other plants or covering walkways, train the vine into a more desirable position. (Some gardeners even grow them on trellises!)
  • Don’t forget to remove row covers from flowering plants so that bees can pollinate them. If you miss this step, your plants will not fruit.
  • When fruit forms, place material such as cardboard, aluminum foil, or hay between the fruit and soil to prevent rot.
  • Watermelons are hungry plants—fertilize them regularly with a balanced fertilizer, such as one with an NPK ratio of 4-4-4 or 10-10-10.
A plump watermalon that is almost ready for harvest

Harvesting

If you’ve followed the above steps, you likely have quite a few watermelons growing. But how do you know when they are ripe?

Tips such as knocking on the fruit and harvesting when you hear a hollow thud, or waiting for the nearest tendril to the fruit to dry out, only work for certain species. For others, they’re completely inaccurate and can lead to picking crops too early.

The best and most universal advice for picking watermelon crops is to wait until the melon turns from bright to dull green, and the underside turns from white to yellow.

If this is your first time growing your chosen variety, check into that species to see how long it takes to ripen on average. This timeframe can give you a good idea of when your fruit is ready.

Here are some more tips while you wait for your watermelons to ripen:

  • Pruning vines won’t make fruit ripen faster, and negatively impacts its taste.
  • Instead of the above, prune off any flowers that won’t have time to ripen. This ensures the plant doesn’t waste energy and can instead ripen existing fruit faster.
  • Aluminum foil or another light-reflecting surface placed underneath developing watermelons can make them ripen faster.
  • Water less frequently as the fruit ripens, but keep your schedule consistent and don’t allow the vines to wilt. Less water will improve the sweetness of your fruit and avoid an overly-watery consistency.

Storing

After you’ve harvested, you may be wondering how long it lasts and how to store the large spheres.

Outside of the fridge, a watermelon will hold up for about 2-3 weeks. If you want your fruit to last longer than this, it’ll need to be refrigerated.

After cutting into your fruit, it can last in the fridge for around a week if properly wrapped.

Store any watermelon you can’t use within this time frame in the freezer, or share it with some friends!

Half of a red watermelon sitting in a refridgerator

Pests and Diseases

Watermelons are prone to many problems with mildew, fungi, and pests. It’s essential to check them regularly and inspect the foliage and fruit for damage. One of the most common problems you’ll come upon are cucumber beetles.

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About the author: Carley Miller is a horticultural expert at TheGreenPinky. She previously owned a landscaping business for 25 years and worked at a local garden center for 10 years.

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