How to Grow Black Diamond Watermelons

Citrullus lanatus 'Black Diamond' fruit

Grygorii Shvets / Getty Images 

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. These dark, blue-green-black melons typically lack the usual stripes of more familiar watermelon varieties and have rich red flesh. This watermelon variety can easily weigh up to 50 pounds. Because of their large size, these melons 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 require a long growing season and can take around 90 days to mature. Give them plenty of sunshine and water to keep them strong and healthy. These plants can either be allowed to trail along the ground or trained to climb a trellis, however this can prove much more challenging with such a large melon variety. When growing watermelons on the ground, it is best to place them atop a piece of wood or cardboard to help keep them dry. 

Before harvesting fruits, search 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.  

Light

As most melons do, the black diamond watermelon requires lots of sunshine. At least 6 to 8 hours every day are required 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.

Since they need moist conditions to thrive and produce juicy fruits, these plants respond well to a layer of mulch added to the top of the soil. This will help retain soil moisture. 

Water

Watermelons may be some of the juiciest fruits around, so it is no surprise that they need consistent water to create their famous fruits. Keep the soil moist, though not soggy. A consistent watering schedule will keep these plants thriving.

Try to avoid getting the leaves wet when watering, as this can lead to fungal problems. Once the melons grow to about the size of a tennis ball, only water once 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. 

Fertilizer

Black diamond watermelons love rich soil and do very well in organic material. Amending the soil with plenty of compost before planting will ensure these melons receive the nutrients they need. If needed, fertilizer can be applied when flowers appear to help encourage healthy fruit production. It is best to give 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. Lightly cover the seeds in a rich growing medium 1/4-inch deep. 
  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. 
  3. Once seedlings appear, reduce watering. 
  4. When seedlings grow their first true leaves, cut back on watering again. 
  5. 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. Check your seed packet for specific spacing instructions.
  3. Keep the soil moist.

Potting and Repotting Black Diamond Watermelon

These melons can be grown in pots, but their large size may make this a bit difficult. If planted in a container, be sure to choose a large pot with very good drainage. There should be plenty of room around the pot to allow the vines to travel. Alternatively, a sturdy trellis can be provided for the vines to travel up. It is best to choose a pot large enough to support this plant’s mature size, as transplanting will add stress to these annual plants.  

Common Pests & Diseases

When growing black diamond watermelons, be vigilant for the most common pests it faces: 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 watermelon plant. It is best to avoid planting melons in an area where plants of the cumber family were grown the year prior.