How to Grow and Care for Black Diamond Watermelons

Black diamond watermelons are known for their enormous size and juicy, sweet flesh. This late-season heirloom variety is popular for home gardens, and large, commercial farms. The dark, blue-green-black melons have rich red flesh. and typically lack the usual stripes of more familiar watermelon varieties. The melons, which can easily weigh up to 50 pounds, require a long growing season to produce mature fruit. 

How to Plant Black Diamond Watermelons

When to Plant

Whether you plant seedlings from a nursery or garden center or start your own from seed, plant watermelons in the spring after the danger of frost has passed and temperatures remain consistently between 70°F and 80°F. If you direct seed watermelons in your garden, wait until the soil temperature is at least 70°F, or they won’t germinate. Covering the soil with black plastic before seeding helps warm up the soil. In areas where the growing season is short, the seeds can be started indoors about four weeks before your area's last average frost date.

Selecting a Planting Site

Watermelons need a location in full sun with well-draining, rich soil. And they need lots of space—the vines can grow up to 20 feet long and work best when allowed to trail along the ground.

Spacing, Depth, and Support

Plant the seeds 1 inch deep and space plants 8 feet apart. When direct seeding, plant some extra seeds in between. When plants are 3 inches tall, thin them out.

Trellising is not recommended due to the heavy weight of the fruit.

Black Diamond Watermelon Care

Here are the crucial care requirements for black diamond melons:

• Plant black diamond watermelons in full sun.
• Keep black diamond melons happy in rich, well-draining, and slightly acidic soil.
• Water this melon consistently to keep the soil moist but avoid getting the leaves wet.
• Grow black diamond watermelons in dry, hot climates; they do not thrive in the cold.

Light

Like all melons, the black diamond watermelon requires at least six to eight hours of sun daily to keep these plants growing strong. 

Soil

Black diamond watermelons enjoy soil rich in organic material. They prefer neutral to slightly acidic, warm, moist, well-draining soil mediums.

A layer of organic mulch such as straw underneath the vines and fruits will help retain soil moisture. 

Water

Watermelons are some of the juiciest fruits, so it is no surprise that they need consistent watering to thrive. Keep the soil moist, but not soggy.

Water the vines where they are rooted into the soil and try to avoid getting the leaves wet which can lead to fungal problems. Once the melons grow to about the size of a tennis ball, only water the plants when the soil becomes dry. Around two weeks before the fruit is ripe, stop watering to increase the sugar content in the fruits.

Temperature and Humidity

These melons prefer hot temperatures and do not handle cold well. They prefer moist conditions, though very high humidity can cause fungal problems. 

Fertilizer

Black diamond watermelons do well in rich soil with organic material. Amending the soil with plenty of compost before planting will ensure these melons receive the necessary nutrients. Fertilizer can be applied when flowers appear to help encourage healthy fruit production. It is best to use nitrogen-based fertilizer in the early growing stages, then switch to phosphorus and potassium-based fertilizers when the plant produces flowers. For amounts to use and application instructions, consult the fertilizer product label.

Pollination

Watermelons have male and female flowers on each vine which need pollination to produce fruit. Make sure you have planted flowering plants nearby that attract nectar-collecting bees, which are the primary pollinators of watermelons. Don't use broad-spectrum insecticides, as they indiscriminately kill pests and pollinating insects.

Harvesting Black Diamond Watermelons

Solely keeping track of growing time does not tell you when a black diamond watermelon is ripe, as ripening time depends on the growing conditions and weather. Before harvesting fruits and their seeds, look for signs that the melons are ripe.

Check the tendrils where the melons connect to the plant. The tendrils near a ripe melon will be dry and brown while tendrils near an unripe melon will be green. Check the rind of the melon. The rind of a ripe melon should be hard and the part that rested on the ground should have a creamy coloring. Harvest seeds from one of these ripe melons by cutting it open and scooping out only the plump black seeds.

Saving Black Diamond Watermelon Seeds

Put the seeds from the melon in water for about two days to clean them. Keep only the seeds that sink to the bottom of the bowl and toss out the ones that float. Rinse and dry the good seeds on a paper towel placed in a sunny spot for about a week until they are so dry they snap when bent. Seeds kept in an airtight container and stored in a cool, dry, dark place are good for a few years.

Pruning

Only watermelons grown on a trellis require pruning to control excessive vine growth. The heavy black diamond watermelons are unsuitable for trellising so there is no need to prune the vines. 

How to Grow Black Diamond Watermelon From Seed

Watermelons can be direct seeded but take up to three months to mature so to get a head start on the growing season, start seeds indoors about one month before the last frost. Biodegradable pots work best because watermelons don't like their roots disturbed.

1. Plant the seeds 1/4 inch deep and lightly cover with seed starting mix.
2. Keep the soil moist and warm. Seeds germinate better in warm temperatures, so keep the soil at 70°F or higher. A humidity dome will help, however, it should be removed as soon as seedlings emerge.
   
3. Once seedlings appear, reduce watering. 
4. When seedlings grow their first true leaves, cut back on watering again. 
5. Once temperatures warm, begin the hardening-off process until the seedlings can be planted in the garden. 

For areas with long, hot growing seasons, black diamond watermelons can be planted directly into the garden. Follow these steps:

1. Around two weeks after the last frost, sow the seeds in rich, moist, well-draining soil. Cover the seeds with soil to a depth of 1 inch.
2. Melons also grow well using the mounding method. Create a mound of soil about 1 foot in diameter and 3 to 4 inches high. Press three to five seeds about an inch into the soil of the mound. Thin to two or three plants once seedlings emerge, leaving about 8 feet of space between plants.
3. Keep the soil moist at all times.

Common Pests and Plant Diseases

When growing black diamond watermelons, be vigilant for the most common pests including melon aphids, cucumber beetles, squash bugs, and squash vine borers.

Fungal diseases, such as powdery mildew, Alternaria leaf spot, stem blight, and downy mildew also attack this plant. Avoid planting melons in an area where plants of the cucurbit or squash family were grown the previous year.

Watermelons growing on the ground are prone to rot and soft spots, which you can discourage by placing the fruit on a piece of wood or cardboard to help keep them dry as they ripen.