A Basic Guide to Growing the Not-So-Basic Black Diamond Watermelon

By Charisse Miller
slices of watermelons

The black diamond watermelon is sometimes described as the “king” of the garden. Weighing more than 50 pounds (or more), this classic oblong watermelon is known for its enormous size, as well as its bright red, crisp and delicious flesh. Unlike other watermelons, this black watermelon lacks the typical stripes, which makes them more unique.

The black diamond watermelon is also the quintessential watermelon of the summer season. It is equally popular with both commercial and home growers. If you’re interested in growing this variety of watermelon and want to grow your garden, here’s everything you need to know.

What is a Black Diamond Watermelon?

Black diamond watermelons (scientific name: Citrullus lanatus) are considered heirloom watermelons that fall under the open-pollinated category. This black watermelon can produce vigorous vines and yield fruits that weigh more than 23 kilograms.

Apart from its heavyweight, this watermelon variety sets itself apart from other melons with its black-green peel and seeds (which are either black or gray; they also maintain the same size and structure).

Because of its large size, expect this watermelon variety to have a long growing season before you can harvest fully ripe watermelons. It takes 80 to 90 days for your black diamond watermelon to grow to full size. It seems like a long wait, but once the melons have matured, you’ll be rewarded with hard rinds and delicious, pink-red flesh.

Black Diamond Watermelon Care

Despite its drastic difference from other watermelon varieties, growing black diamond watermelons is similar to other types.

To grow your own black diamond watermelon patch, here are the basics every gardener should know.

How Much Light Do Black Diamond Watermelons Need?

Like other watermelons, the black diamond variety needs plenty of sunshine. They require at least six to eight hours of sunlight every day to keep the melons growing strong and healthy.

What Should the Temperature and Humidity Levels Be?

watermelon cut in half
Photo by aung nyi on Unsplash


These black watermelons are big fans of hot temperatures and do not thrive in cold temperatures. They prefer moist conditions. However, growing your watermelons in a very humid place might trigger fungal infections.

How Much Water Does a Black Diamond Watermelon Need?

Watermelons are known for being juicy, so it makes sense that they need plenty of water. A rule of thumb when it comes to watering your melons: keep the soil moist, but avoid getting it soggy. Regular watering ensures the healthy growth of your black watermelon.

When it comes to watering your melons, water the vines where they are rooted and avoid getting the leaves wet to avoid fungal problems. Once your watermelons have reached the size of a tennis ball, water them only when the soil starts drying out. Two weeks before the melon reaches maturity, stop watering them to increase the fruit’s sugar content.

What Type of Fertilizer Do I Use for Black Diamond Watermelons?

Black diamond watermelons thrive in rich soil and any organic material. Give them the nutrients they need by amending the soil with plenty of compost. You can also apply fertilizer when the flowers appear. This encourages the healthy production of fruits.

During the early growing stages, use a nitrogen-based fertilizer for your black diamond watermelon. Switch to potassium and phosphorus-based fertilizers once the melons produce flowers.

What are the Common Pests and Diseases to Watch Out For?

When growing your black diamond watermelons, look out for common pests, which include squash bags, cucumber beetles, squash vine borers and melon aphids. Alternaria leaf spot, downy mildew, stem blight and other fungal diseases can attack your plants.

Growing Black Diamond Watermelons: Step by Step

These watermelons take their sweet time to ripen, so it’s best to start your seeds indoors for a month.

Here’s how:

  1. Plant your seeds 1/4 inch deep into the soil and lightly cover them with starting mix.
  2. Always keep the soil warm and moist since the seeds germinate better in these conditions. Maintain a temperature of 70 degrees Fahrenheit (or higher).
  3. Once the seedlings appear, cut back on watering. When the seedlings grow leaves, reduce the watering again.
  4. Once the temperatures start to rise, begin the hardening-off process until you can plant the seedlings into the garden.

If you live in a place with high temperatures or warm growing seasons, you can directly plant the melons into your garden.

Just follow these steps:

  1. After the last frost, plant your seeds in moist, rich and well-draining soil. Cover the seeds with soil.
  2. Apply the mounding method. Create a mound of soil and press three to five seeds about an inch into it. Once seedlings emerge, thin to two or three plants.
  3. Check the seed packet for spacing instructions.
  4. Keep the soil moist.

Harvesting Your Black Diamond Watermelon

Similar to other watermelon varieties, determining when the melons are at with their ripeness can be challenging.

When it comes to picking a ripe black diamond watermelon, pay attention to the tendril that connects the melon to its plant stem. If the tendril is green, your black diamond watermelon isn’t ripe yet. On the other hand, if the tendril is brown and dried, your watermelon is ripe.

Before you harvest your watermelons, look for other signs that the fruit is ready. Rolling or lifting the watermelon always works. Also, check the place where the melon rests on the ground. If the melon is ripe, the area of the rind will be cream in color.

Once they are ripe, the rinds of a black diamond watermelon will harden. Check this by scratching the melon’s rind with your fingernail. If you can easily scratch the surface, the watermelon is ripe.

How to Save Black Watermelon Seeds

If you enjoyed growing black watermelons or wish to grow another batch of them, save the seeds!

To do so, just scoop out seeds from a ripe black watermelon and place them into a wire mesh sieve. Run the seeds over water and rub them gently against the mesh to remove the stringy fibers. Next, place the clean seeds in a bowl of water and stir. You’ll notice that some of the seeds float to the top of the water. This means they are sterile or immature black watermelon seeds. Discard of these.

Stir the water a few more times until there are no more immature seeds. Drain the water from the remaining seeds and place them on a baking pan or heavy plate lined with waxed paper. Let them air dry in a sunny spot.

Stir the seeds on the pan for the next few hours to ensure all of them are exposed to fresh air. After a day, bring the seeds back inside and let them dry for another week or two. Make sure they are dry before you package them for storage. 

If you love watermelon but want something different, grow a black diamond watermelon! It takes a while, but the fruits are worth it.

Scroll to Top