How to Grow Black Diamond Watermelons

Citrullus lanatus 'Black Diamond' fruit

Grygorii Shvets / Getty Images 

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Black diamond watermelons are known for their enormous size and their juicy, sweet flesh. This heirloom variety is popular for home gardens as well as 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. 

Botanical Name Citrullus lanatus ‘Black Diamond’
Common Name Black Diamond Watermelon
Plant Type Annual
Mature Size vines can be 20 ft. long
Sun Exposure Full
Soil Type Loamy, Sandy, Moist but Well-drained
Soil pH Acidic, Neutral
Bloom Time Summer
Flower Color Yellow
Hardiness Zones 3-9, USA
Native Area North America

Black Diamond Watermelon Care

These large, sweet melons can take around 90 days to mature and need plenty of sunshine and water to keep them strong and healthy. These vines can grow to 20 feet in length and work best when allowed to trail along the ground. Trellising is not recommended due to the heavy weight of the fruit. When growing watermelons on the ground you can discourage rot and soft spots by placing the fruit on a piece of wood or cardboard to help keep them dry as they ripen.

Before harvesting fruits, look for signs that they 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 still be green. Also, check the rind of the melon. The rind should be very hard and the part that was resting on the ground should have a creamy coloring.  


Like all melons, the black diamond watermelon requires lots of sunshine. At least 6 to 8 hours every day are needed to keep these plants growing strong. 


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. 


Watermelons may be some of the juiciest fruits around, so it is no surprise that they need consistent water. Keep the soil moist, but not soggy. Regular watering will keep these plants thriving.

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 when the soil begins to dry out. 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. 


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

How to Grow Black Diamond Watermelon From Seed

These juicy melons take some time to mature, so it is best to start seeds indoors about one month before the last frost. Here’s how: 

  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 try to keep the soil at 70 degrees Fahrenheit 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 into the garden. 

For areas with long, hot growing seasons, these melons 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 3 to 5 seeds about an inch into the soil of the mound. Thin to 2 or 3 plants once seedlings emerge.
  3. Check your seed packet for specific spacing instructions.
  4. Keep the soil moist.

Common Pests & Diseases

When growing black diamond watermelons, be vigilant for the most common pests which include melon aphids, cucumber beetles, squash bugs, and squash vine borers. Fungal diseases, such as powdery mildew, alternaria leaf spot, stem blight, and downy mildew, can also attack this plant. It is best to avoid planting melons in an area where plants of the cucurbit or squash family were grown the previous year.