black diamond watermelons

How To Grow Black Diamond Watermelons

I love a good watermelon. Nothing spells summer to me like splitting open a watermelon, enjoying its sweet, red guts, and having a seed spitting contest! I’ve also been known to unload a bottle of vodka into a fresh watermelon for a special adult treat. When I heard about black diamond watermelons, I knew I had to give them a try.

All about black diamond watermelons

Similar to Black Densuke watermelons, Black diamond watermelons are an heirloom variety of watermelon that grow similarly to others – approximately the same shape but very large. The main difference is its peel, which comes in dark black-green. These heirloom seeds are either grey or black and maintain the same size and structure as other watermelon seeds. The flesh of the black diamond watermelon is dark red, much like other varieties.

Read More: What Are Heirloom Seeds?

Growing black diamond watermelons

Growing from seed: Plant indoors 4 to 6 weeks before last frost of spring. You can also plant seeds outdoors 3 to 4 weeks after final frost. If planting indoors, keep soil at 70 degrees Fahreinheit. Plant 4-6 seeds one inch deep. Eventually thin the seeds.

Transplanting: When it comes time to transplant seedlings, dig a hole one foot deep and two feet wide. Fill with compost and manure. Use the soil removed for the hole to create a mound over the hole. Plant your seedling at the top of the mound.

Fertilizing: Ensure plenty of nitrogen at the time of planting. As the plants grow, fertilize with phosphorus and potassium fertilizer. Liquid seaweed is a good option.

Spacing: Black diamond watermelons, like most other varieties, grow very large. They’ll need to be spaced out about 60 inches apart.

Soil: Your soil will want to be well drained and nutrient rich. It helps to add nutrient-rich compost and organic matter. Soil should be 70 degrees Fahrenheit before planting seedlings.

Watering: Watermelon are thirsty plants and like moist soil. Keep watermelon watered consistently throughout the growing season. Once the fruits have reached the size of a softball, ease off the watering and let the soil dry between waterings.

Companion plants: Corn, nasturtiums, marigolds, oregano. Nasturtiums are known to attract aphids, so plant carefully.

Harvest time

Black diamond watermelons will be ready to harvest approximately 85 days after sowing. It could be as early as 70 days or as late as 90 days. Your watermelons are ripe when you hear a dull thump when knocking on it. The underside should also be yellow.

Read More: How To Pick The Perfect Watermelon

Cody Medina
Small Scale Farmer
Hi there! I'm Cody, a staff writer here at The Garden Magazine and a small-scale farmer living in Oregon. I've been gardening most of my life and now live on a quarter-acre farmstead with chickens, ducks, and a big garden.