Growing Watermelon: What can go wrong?

Maybe you’re like me and growing watermelon successfully is a goal for your garden. The penultimate summer fruit, there’s nothing like cracking open a cold watermelon in the heat of summer. (Salted, of course!)

For me, growing watermelon has never been a decision for my garden. It’s my favorite food, so of course, I’m going to make room for it. (Even if it does take over my garden space…more on that below.)

But is watermelon easy to grow? Well, it’s not hard, but there are common pitfalls that can occur with beginner gardeners and experienced ones alike.

In this post, we’ll look at what can go wrong in growing watermelon, so you can prevent and troubleshoot common issues that may occur.

Watermelon Problems in Planting

The first hurdle you’ll have to clear when growing your own watermelon is in planting — specifically if you sow your own seed.

Direct Sowing Watermelon

The easiest way to plant watermelon is to directly sow the seed in your garden. But if you plant the seed and it doesn’t germinate within a week or so, there might be a problem.

The biggest issue with direct sowing watermelon seeds is planting too early. Watermelon, as a warm season crop, wants to sprout in warm soil — no less than 70F soil temperature, though 85F is ideal. So, if your seed hasn’t sprouted, low soil temperature could be the cause. If you realize that was the problem, you can either keep waiting (and if the seed hasn’t rotted in cold, wet soil, it will sprout), or you can replant when the soil warms up.

Transplanting Watermelon

While direct sowing watermelon seeds is the simplest way to plant, I’ve grown to prefer transplanting watermelon. As a beginning gardener, I purchased my transplant from a garden center, and it worked great.

Now, though, I start my watermelon seeds indoors about three to four weeks before I intend to plant the transplant outside.

A common problem with indoor seed starting watermelon is starting too early. Watermelon grows rapidly, and while it will transplant easier than its cousins squash and cucumbers, it still doesn’t appreciate being root-bound in a container.

I prefer to start my watermelon seeds in Epic cell trays and then transplant them (pot them up) into 3″ or 4″ containers. Then, once the weather warms to the 60s at night, I’ll plant out in the garden.

(Epic Cell Trays are all I use for indoor seed starting these days. Follow my affiliate link here to learn more and you can use code JOURNEYWITHJILL for 5% off your purchase.)

Watermelon Growing Problems

Whether you direct sow or transplant your watermelon, it’s not uncommon to encounter issues between the planting and the harvesting. Let’s look at a few of those.

Slow growth

Watermelon plants grow rapidly in ideal conditions. But if yours isn’t, the problem is likely related to the temperature. Remember, watermelons are warm season crops, so they like temperatures in the 80s and even 90s during the day and 60s and 70s at night.

If you live in an area with a cool start to your summer, consider using a floating row cover to heat up the environment around your watermelon plant. But be sure to provide ventilation so it doesn’t “cook” on sunny days.

If your season is long and you have the time, simply wait to plant your watermelon. Sure, you’ll hear you can plant them out after your average last frost date, which is true. But watermelons will grow best when those nighttime temperatures are nowhere near freezing. I like to wait a few weeks after my average last frost date before planting.

Lack of vigor

Related to slow growth, you may notice that the growth of the watermelon plant lacks vigor. Maybe the leaves are a light green or they are more spindly than full. What could the problem be?

Sun. First, watermelon plants want full sun. That means 6-8+ hours of direct sun per day. I didn’t realize how much sun played a role in watermelon growth until I had two plantings: one in full sun and one with several hours of part-shade per day. The difference was significant.

Water. Watermelons — as their name implies — need water. Use drip irrigation if possible to keep water going to the root. This will also help prevent fungal diseases that thrive when leaves are wet. I prefer the Garden in Minutes Garden Grids when I’m growing watermelons in raised beds. These full-coverage grids provide ample water that watermelons need.

Nutrients. As a member of the cucurbit family along with squash and cucumbers, watermelons love compost-rich soil. Before planting, amend the planting area with high-quality compost. That’s all I usually add to my soil, but if you know your soil is lacking of nutrients, nitrogen is one you want to make sure to have. With nitrogen, the plant will produce more leaves, which promotes photosynthesis to develop the fruit.

Poor fruit set

What if your watermelon looks strong, but you don’t see any fruit developing? First, it may be a matter of time before those melon-producing flowers appear.

But more commonly, you’ll see the female flowers (with a baby melon at the base), but they don’t grow into melons. This is caused by a lack of pollination.

Watermelons need bees to take pollen from male to female flowers, and those flowers are usually only open for a few hours on any given morning. So, if the flowers open when it’s a rainy or cool morning — when bees aren’t active — they won’t get pollinated. You also may not have pollinators in your garden yet for a variety of reasons.

Most of the time, this problem corrects itself as the weather warms and the pollinator activity increases. The plant will continue to produce female flowers with potential to become melons. If you find you still have a problem in this area, consider hand-pollinating.

Watermelon rots at the blossom end

Tomatoes aren’t the only garden fruit that suffers from blossom end rot. If you notice that the non-stem end of the watermelon is showing a black, rotting place, you know your fruit has been affected.

Just like with tomatoes, the best way to prevent blossom end rot is to keep the plant irrigated. While the cause is a lack of calcium to the fruit, most often there’s plenty of calcium in the soil (especially if you amended your bed with compost). The problem is a lack of transportation of that calcium in the form of water.

Besides consistent, deep irrigation (especially during the watermelon’s growing stage, which is usually hot and dry), also mulch the soil well. Mulch prevents evaporation and keeps moisture in the soil and near the plant’s roots.

Most likely, these measures will prevent blossom end rot, but if you still run into issues, test the pH of the soil. A too acidic soil can also keep calcium from being accessed by the plant.

Fruit bottlenecking

When you notice your watermelon plant misshapen in the form a bottle, usually that’s the result of incomplete pollination. One side of the melon is more narrow than the other, indicating that part of the fruit received pollination while the other didn’t. (That’s an oversimplification.)

Personally, I cull the fruit when I see this happening. Seldom does this go on to product the quality of fruit I want. By removing this fruit, the plant focuses its energy on creating and developing a properly-forming watermelon from another part of the vine.

Bottlenecking can also be caused by fluctuations in watering, so as mentioned already, keep the watering consistent.

Problems with Watermelon Harvest

The most discouraging problem when growing watermelon is when your fruit doesn’t meet your long-awaited expectations. Here are the most common sources of that disappointment.

Watermelon not sweet

It’s realistic to expect your homegrown watermelon to taste like no other store-bought watermelon does. You expect juicy, sweet, exquisite flavor. And often, you get it. But what happened when you take that first bite of your garden watermelon and it’s flavor is…meh?

This could be caused by a couple of things. First, remember how watermelon is a warm season crop? It will have the best flavor when it ripens in warm conditions.

Another — more common I’d say — issue is when your garden receives a heavy downpour of rain close to harvest (or your irrigation is set a bit too high when your melon is almost ripe). With extra water, the plant takes up that moisture and funnels it to the fruit, diluting its natural sugars.

Truly, my most flavorful watermelons have been harvested during times of heat and drought — when everything else suffers. Those sugars concentrate to create a flavor explosion like none other.

Of course, you can’t control rain, but if you’re irrigating, you can control how much water your melons receive. Hold off watering during the final week before you expect to harvest. When is that? Look for the tendril closest to the fruit. When it begins to dry at the tip, it’s time to withhold water.

Watermelon split

Heavy rains not only can dilute the sugars of your watermelon, but it can also cause the fruit to split. When the fruit can’t contain all the extra water it has been given, it can burst.

To prevent watermelon split, first, make sure you harvest at the right time. Too often I’ve left my watermelons in my garden beyond their ripeness (more on that below), and all it took was a rainfall or regular irrigation for the melon to split.

If you see a heavy rain in the forecast during the summer, check all your melons for ripeness, and harvest the ones that are ready.

You can also turn off your irrigation temporarily until harvest, as mentioned above.

Watermelon not ripe

It is not easy to know — especially as a beginner — when a watermelon is ready to harvest. More common than harvesting too late, though, is harvesting too early. When you harvest too early, the watermelon isn’t good to eat. It’s tough and lacks flavor. Watermelon does not keep ripening off the plant, so harvesting at the right time is critical.

How do you know a watermelon is ripe? There are four signs I use in my garden that have been the most dependable:

1. The tendril nearest the fruit dries up completely
2. The “field spot” where the melon rests on the ground turns from a white/pale yellow to a deep golden color
3. The sound of the watermelon when you “thump” it is a deep tone
4. The sheen of the rind of the melon goes from a slick, shiny appearance to dull

In this video you can see and hear each of these signs:

Pests & Disease

In addition to basic growing issues, you may find yourself troubleshooting pests and disease with watermelons, but most of the time, these issues are minor. I’ve seldom had issues with pests and disease with my watermelons, but here are some that can occur:

• Skeletonized leaves can be caused by cucumber beetles. If this is an issue for you, cover the plant with insect netting until flowering.
• Powdery coating on the leaves indicates powdery mildew. You can apply some of these remedies for powdery mildew, or seek out disease-resistant varieties in the future.
• Sudden wilt can be caused by fusarium wilt, but this is a different strain than the one that attacks cucumbers and tomatoes. If you’ve had this issue in the past, rotate watermelons to a different place, and/or plant a disease-tolerant variety like Charleston Grey or Jubilee next time.
• If you notice necrotic lesions on the leaves and around the crown of the plant, and then leaves start to die this could be anthracnose, alternaria, or even downy mildew. Remove diseased leaves and mulch to try to prevent the spread. For your next planting, rotate your crop and space out the melons further.
• Leaves can become distorted and twisting at stem end when affected by watermelon mosaic virus. Fruit may be absent or distorted. Sadly, the plant will need to be pulled and disposed of (not composted).

If you notice issues not on this list, check out these more comprehensive lists from Texas A&M and Oklahoma State University.

Final thoughts

After years of growing watermelon, with some some years giving me more than I could eat and others ending the season with disappointment, I encourage you not to give up.

Try new varieties if the ones you’ve tried didn’t work for you. Know that some years are better than others, and rainfall definitely affects the final product. My best watermelon year occurred in a hot drought; my worst was in a year of plentiful summer rains.

Regardless of if you find growing watermelon easy or challenging, once you harvest and cut into your own homegrown watermelon, it’s all worth it!