The black watermelon is one of the most striking fruits you can grow in your garden. With its deep, dark green rind that appears almost black in the sunlight and its brilliant red, incredibly sweet flesh inside, the Black Diamond watermelon is both a visual showstopper and a delicious summertime treat. Despite its dramatic appearance, growing black watermelons is surprisingly accessible for home gardeners with the right conditions and a little patience.
- Table of Contents
- What Is a Black Watermelon?
- Popular Black Watermelon Varieties
- Growing Conditions & Requirements
- Step-by-Step Planting Guide
- Care & Maintenance Tips
- Common Problems & Solutions
- How to Know When to Harvest
- Nutrition & Health Benefits
- The Famous Japanese Densuke Watermelon
- Frequently Asked Questions
- What does a black watermelon taste like?
- How big do black watermelons get?
- Are black watermelons natural or GMO?
- Why are Japanese black watermelons so expensive?
- Can I grow black watermelons in a small garden?
- How do I store a harvested black watermelon?
- When is the best time to plant black watermelon seeds?
In this complete guide, we cover everything you need to know about black watermelon varieties, how to grow them from seed, care tips for maximum sweetness, and how to tell when they are ready to harvest.
Table of Contents
What Is a Black Watermelon?
A black watermelon is not actually black – it is a watermelon variety with an extremely dark green rind that appears black from a distance. The dark coloring comes from high concentrations of chlorophyll in the rind. Several watermelon cultivars produce this striking dark appearance, with the Black Diamond being the most well-known and widely grown variety in North America.
Black watermelons are not genetically modified or artificially colored. They are naturally occurring varieties that have been selectively bred over many decades for their dark rind, large size, and exceptional sweetness. The contrast between the near-black exterior and the vibrant red flesh makes them particularly impressive when sliced open.
Black Watermelon vs. Regular Watermelon
| Feature | Black Watermelon (Black Diamond) | Regular Watermelon (Crimson Sweet) |
|---|---|---|
| Rind color | Very dark green (almost black) | Light green with dark stripes |
| Flesh color | Deep red | Red to pink |
| Average weight | 30 – 50 lbs (up to 75 lbs) | 15 – 25 lbs |
| Days to maturity | 85 – 95 days | 80 – 85 days |
| Seeds | Large, dark brown to black | Dark brown, smaller |
| Sugar content (Brix) | 10 – 12 | 10 – 11 |
| Rind thickness | Thick, durable | Medium |
| Vine spread | 15 – 20 feet | 10 – 15 feet |
Popular Black Watermelon Varieties
1. Black Diamond
The classic American black watermelon. Developed in the 1940s, Black Diamond produces massive fruits averaging 30 to 50 pounds, with specimens occasionally exceeding 75 pounds. The rind is uniformly dark green, tough, and excellent for shipping and storage. The flesh is deep red, crisp, and very sweet. This is the variety most commonly found in seed catalogs and garden centers.
2. Sugar Baby
A compact “icebox” variety with a dark green to nearly black rind. Sugar Baby melons are much smaller (8 to 12 pounds) and mature faster (75 to 80 days), making them ideal for small gardens and shorter growing seasons. The flesh is deep red and exceptionally sweet despite the small size.
3. Moon & Stars
A heritage variety with a dark green rind speckled with tiny yellow “stars” and one or two larger yellow “moon” spots. Fruits average 25 to 40 pounds. Nearly extinct by the 1980s, this variety was saved by seed preservation efforts and has become a favorite among heirloom gardeners.
4. Densuke (Japan)
The most famous and expensive black watermelon in the world. Grown exclusively on the Japanese island of Hokkaido, Densuke watermelons have a perfectly smooth, jet-black rind with no stripes at all. They sell for $250 to $6,000 each at auction in Japan. More on this variety below.
5. Black Tail Mountain
An early-maturing variety (70 to 75 days) with a dark green rind, developed specifically for northern climates with short growing seasons. Fruits are smaller (6 to 15 pounds) but reliable producers even in zones with cool summers.
| Variety | Weight | Days to Maturity | Best Growing Zones | Seed Availability |
|---|---|---|---|---|
| Black Diamond | 30 – 50 lbs | 85 – 95 | 5 – 10 | Widely available |
| Sugar Baby | 8 – 12 lbs | 75 – 80 | 3 – 10 | Widely available |
| Moon & Stars | 25 – 40 lbs | 90 – 100 | 5 – 10 | Specialty seed catalogs |
| Densuke | 15 – 25 lbs | 80 – 90 | N/A (Hokkaido only) | Not commercially available |
| Black Tail Mountain | 6 – 15 lbs | 70 – 75 | 3 – 8 | Specialty seed catalogs |
Growing Conditions & Requirements
Black watermelons thrive in the same conditions as standard watermelons, but their larger size means they need more space, nutrients, and growing time.
Climate
- Temperature: Soil temperature must be at least 70°F (21°C) for seed germination. Air temperatures of 80 to 95°F (27 to 35°C) during the day are ideal.
- Growing season: Black Diamond requires 85 to 95 frost-free days minimum. Gardeners in zones 3 to 5 should start seeds indoors 3 to 4 weeks before the last frost date.
- Humidity: Moderate humidity is fine, but excessive moisture promotes fungal diseases.
Soil
- Type: Sandy loam is ideal. Well-draining soil is essential – watermelons are susceptible to root rot in waterlogged conditions.
- pH: 6.0 to 6.8 (slightly acidic to neutral).
- Preparation: Amend soil with 3 to 4 inches of compost before planting. Work in balanced fertilizer (10-10-10) at a rate of 3 pounds per 100 square feet.
Space
- Row spacing: 8 to 10 feet between rows.
- Plant spacing: 4 to 6 feet between plants within rows.
- Vine spread: Each Black Diamond plant can spread 15 to 20 feet, so plan accordingly.
- Minimum garden size: Dedicate at least 50 square feet per plant for best results.
Sunlight
Full sun is non-negotiable. Black watermelons need at least 8 hours of direct sunlight per day. More sun equals sweeter fruit. Position your melon patch in the sunniest part of your garden, away from shade-casting trees or structures.
Step-by-Step Planting Guide
Starting Seeds Indoors (Cold Climates)
- Start seeds indoors 3 to 4 weeks before your last expected frost date.
- Use peat pots or biodegradable containers (watermelons dislike root disturbance during transplanting).
- Plant seeds 1 inch deep in moist seed-starting mix.
- Keep soil temperature at 80 to 85°F using a seedling heat mat.
- Seeds germinate in 5 to 10 days.
- Harden off seedlings for 7 to 10 days before transplanting outdoors.
- Transplant after all frost danger has passed and soil temperature reaches 70°F.
Direct Sowing (Warm Climates)
- Wait until soil temperature reaches at least 70°F (typically late May to June in most areas).
- Create mounded hills of soil 12 inches high and 3 feet wide, spaced 6 to 8 feet apart.
- Plant 4 to 6 seeds per hill, 1 inch deep.
- Water thoroughly after planting.
- Once seedlings develop 2 to 3 true leaves, thin to the 2 strongest plants per hill.
Using Black Plastic Mulch
Many experienced watermelon growers lay black plastic mulch over the planting area before setting out transplants. The black plastic warms the soil, conserves moisture, suppresses weeds, and keeps developing fruit clean. Cut X-shaped holes in the plastic for transplanting.
Care & Maintenance Tips
Watering
Watermelons need 1 to 2 inches of water per week. Consistent, deep watering is critical during vine growth and fruit development. Use drip irrigation or soaker hoses to water at the base of plants, keeping foliage dry to prevent fungal diseases.
Important: Reduce watering significantly during the last 1 to 2 weeks before harvest. This concentrates sugars in the fruit, producing sweeter watermelons. Excess water during ripening dilutes flavor and can cause fruit splitting.
Fertilizing
- At planting: Apply balanced 10-10-10 fertilizer.
- When vines begin to run: Side-dress with nitrogen-rich fertilizer (such as blood meal or 21-0-0).
- When flowers appear: Switch to a phosphorus and potassium-heavy fertilizer (5-10-10) to promote fruit development and sweetness.
- Stop fertilizing once fruits reach full size and begin ripening.
Pollination
Watermelons produce separate male and female flowers on the same vine. Male flowers appear first, followed by female flowers (identifiable by the small fruit swelling at their base). Bees are the primary pollinators. If bee activity is low, hand-pollinate by transferring pollen from male to female flowers with a small brush.
Fruit Support
As Black Diamond melons grow to 30 to 50 pounds, place a piece of cardboard, straw, or a melon cradle beneath each fruit to prevent bottom rot from soil contact. This also makes harvesting easier and prevents the rind from developing flat, pale spots.
Common Problems & Solutions
| Problem | Cause | Solution |
|---|---|---|
| Blossom end rot | Calcium deficiency, inconsistent watering | Maintain even watering, add calcium (gypsum or lime) |
| Fruit splitting | Sudden heavy watering after dry spell | Water consistently, reduce watering near harvest |
| Powdery mildew | Fungal disease from humid conditions | Improve air circulation, apply neem oil or sulfur spray |
| Fusarium wilt | Soil-borne fungus | Rotate crops, plant resistant varieties, solarize soil |
| Aphids | Small sap-sucking insects | Spray with insecticidal soap, attract ladybugs |
| Cucumber beetles | Beetles that spread bacterial wilt | Use row covers early in season, hand-pick, use neem |
| Poor fruit set | Insufficient pollination | Attract bees, hand-pollinate, avoid pesticides during bloom |
| Bland or flavorless fruit | Overwatering, low sunlight, early harvest | Reduce water before harvest, ensure full sun, wait for ripeness signs |
How to Know When to Harvest
Knowing when to pick a black watermelon is one of the trickiest parts of growing them, since you cannot see the flesh through the dark rind. Look for these five signs:
- The tendril test: The small curly tendril nearest the fruit stem turns brown and dry when the melon is ripe. This is the most reliable indicator.
- The ground spot: The spot where the melon rests on the ground turns from white to creamy yellow.
- The thump test: Tap the melon firmly. A ripe watermelon produces a deep, hollow sound. An unripe melon sounds higher-pitched and metallic.
- Rind resistance: Press your fingernail into the rind. A ripe melon resists puncture; an overripe melon yields easily.
- Day count: Count from the date of pollination (when you first noticed the tiny fruit). Black Diamond typically ripens 35 to 45 days after pollination.
When at least three of these indicators align, the melon is ready. Cut the stem with a sharp knife, leaving 2 inches of stem attached to the fruit.
Nutrition & Health Benefits
Black watermelons share the same impressive nutritional profile as standard watermelons, with some studies suggesting that darker-rinded varieties may have slightly higher concentrations of certain antioxidants.
| Nutrient | Amount per 1 Cup (152g) | % Daily Value |
|---|---|---|
| Calories | 46 | – |
| Water content | 91% | – |
| Vitamin C | 12.3 mg | 14% |
| Vitamin A | 865 IU | 17% |
| Lycopene | 6,890 mcg | – |
| Potassium | 170 mg | 4% |
| Fiber | 0.6 g | 2% |
| Sugar | 9.4 g | – |
Watermelon is one of the richest natural sources of lycopene, a powerful antioxidant linked to reduced risk of heart disease and certain cancers. The high water content (91%) makes it an excellent hydrating food during hot summer months.
The Famous Japanese Densuke Watermelon
No discussion of black watermelons is complete without mentioning the legendary Densuke watermelon from Hokkaido, Japan. These melons are considered the most premium watermelons in the world.
What Makes Densuke Special
- Appearance: Perfectly round with a uniformly glossy, jet-black rind – no stripes, no spots, no variations.
- Flesh: Exceptionally crisp with an unusually high sugar content and complex flavor described as having subtle notes of honey.
- Scarcity: Only about 100 Densuke watermelons are produced each year, grown on a small number of farms in the town of Toma, Hokkaido.
- Price: Regular Densuke melons sell for $250 to $500. Premium specimens sell at auction for $1,000 to $6,000. In 2008, a single Densuke watermelon sold at auction for a record $6,100.
Densuke seeds are not commercially available outside Japan, and the unique growing conditions of Hokkaido (volcanic soil, cool nights, and specific humidity levels) contribute to the flavor that cannot be replicated elsewhere.
Frequently Asked Questions
What does a black watermelon taste like?
Black watermelon varieties like Black Diamond taste similar to regular watermelons but are often sweeter and crisper. The flesh is deep red with a refreshing, classic watermelon flavor. Sugar content (measured in Brix) typically ranges from 10 to 12, compared to 10 to 11 for standard varieties.
How big do black watermelons get?
Black Diamond watermelons typically grow to 30 to 50 pounds, making them one of the largest commonly grown varieties. Under optimal conditions with plenty of water, sunlight, and fertilizer, individual fruits can exceed 75 pounds. Competition growers have produced Black Diamonds over 100 pounds.
Are black watermelons natural or GMO?
Black watermelons are completely natural, non-GMO varieties developed through traditional selective breeding over many generations. The dark rind color is a naturally occurring trait caused by high chlorophyll concentrations. No genetic modification is involved.
Why are Japanese black watermelons so expensive?
Densuke watermelons from Hokkaido are expensive due to extreme scarcity (about 100 produced annually), exclusive growing region, labor-intensive cultivation, cultural significance as luxury gifts in Japan, and exceptional flavor quality. They are status symbols in Japanese gift-giving culture.
Can I grow black watermelons in a small garden?
Standard Black Diamond vines need 50+ square feet per plant, which is challenging for small gardens. Instead, try compact varieties like Sugar Baby (needs about 16 square feet) or Black Tail Mountain. You can also grow them vertically on strong trellises with sling support for the heavy fruit.
How do I store a harvested black watermelon?
Whole black watermelons store well at room temperature for 7 to 10 days. For longer storage (up to 2 to 3 weeks), keep them at 50 to 60°F. Once cut, wrap tightly in plastic and refrigerate. Cut watermelon lasts 3 to 5 days in the refrigerator.
When is the best time to plant black watermelon seeds?
Plant seeds outdoors after all frost danger has passed and soil temperature reaches at least 70°F (21°C). For most of the United States, this is late May to mid-June. In zones 3 to 5, start seeds indoors 3 to 4 weeks before the last frost date for a head start.
Last updated: March 2026
