Black Jewel Watermelon with sliced watermelon surround on a white background.
Watermelon Seeds, Black Jewel or Kurokodama
Black Jewel Watermelon with wedges on light background.
Black Jewel Watermelon with steam and leaves on white background.
Black Jewel Watermelon, whole and halved with wedges and Oriental fan.
Black Jewel Watermelon with sliced watermelon surround on a white background.
Watermelon Seeds, Black Jewel or Kurokodama
Black Jewel Watermelon with wedges on light background.
Black Jewel Watermelon with steam and leaves on white background.
Black Jewel Watermelon, whole and halved with wedges and Oriental fan.

Watermelon Seeds,
Black Jewel or Kurokodama

43 ReviewsSKU: #WM223
Price
Regular price $4.00
Regular price $0.00 Sale price $4.00
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Min. 15 seeds

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Description

(Citrullus lanatus). A sugary sweet, deep crimson watermelon from Kumamoto, Japan. The skin is thin and dark green, nearly black. The flesh is super firm, sparsely seeded, and intensely sweet. Watermelon growing is serious business in Kumamoto; the fruit from this region is famously delicious and rigorously tested for high brix content and firm flesh! With its distinctive dark skin and sweet flesh, this Kurokodama cultivar is related to the famous Densuke Black Diamond watermelons grown only in small quantities on the island of Hokkaido, but it is much smaller and doesn’t typically command the same high prices at market.

Growing Tips: Black Jewel watermelon prefers warm, sunny locations and light, sandy soil rich in compost. Provide ample space for vine development and mulch around the base to conserve moisture. Because the fruits are smaller and more compact, they are well suited to smaller gardens or intensive growing systems like vertical trellising.

Plant Details

Botanical Name
Citrullus lanatus
Life Cycle
Annual
Growth Habit
Vining
Sun Requirement
Full sun
Days to Germinate
5–10
Days to Maturity
80–85
Planting Depth
1 in.
Plant Spacing
36–48 in.
Ideal Temperature
75–95°F
Frost Hardy
No

Growing Guide

When to Grow

Start seeds indoors 3–4 weeks before the last frost date or sow directly outdoors once soil temperatures consistently reach 70°F or higher. Transplant only after danger of frost has passed. If you’re not sure how to find your first and last frost dates, consult our blog post 'Our Experts Answer Questions, Share Resources.'

How to Sow and Plant

Plant seeds in raised mounds or rows to encourage warmth and drainage. Sow 2–3 seeds per mound, 1 inch deep, and thin to one plant per mound after germination. For row planting, space plants 36-48 inches apart and leave 5–6 feet between rows for vine growth.

How to Grow

Water deeply once or twice weekly depending on rainfall, allowing the soil to dry slightly between waterings. Avoid overwatering near harvest to improve sugar content. Fertilize with a phosphorus-rich, organic formula early on, and side-dress with compost as vines develop. Use row covers to protect young plants from pests and remove once flowers appear.

How to Save Seeds

  • Choose fully ripe or overripe fruits from healthy, true-to-type plants.
  • Scoop seeds from the flesh and rinse in a strainer to remove pulp.
  • Spread seeds on a screen or paper towel in a warm, dry place out of direct sunlight for at least 7-10 days.
  • Stir daily to ensure even drying.
  • Once completely dry, store in an labeled, airtight container in a cool, dark place.

FAQ

Where does Black Jewel watermelon come from?

Black Jewel, also called Kurokodama, comes from Kumamoto, Japan, where watermelon growing is serious business and the fruit is rigorously tested for high brix content and firm flesh. It is related to the famous Densuke Black Diamond watermelons of Hokkaido, but it is much smaller.

Can I grow Black Jewel watermelon in containers?

Although compact compared to other varieties, Black Jewel still requires space for its vining habit. Large containers or grow bags may work if provided with a strong trellis and full sun.

Why is my Black Jewel watermelon still small at maturity?

This variety produces naturally smaller fruits, typically 5–8 pounds. Ensure it gets full sun, consistent moisture, and warm temperatures to reach peak size and flavor.

Browse our full range of watermelon seeds.

Customer reviews

4 / 5 3 Reviews

What gardeners are saying

  • Easy to grow: Took off from direct sowing with little soil prep.

  • Good eating: Flavor and melon quality rated great.

  • Size varies: Melons ranged from 4 to 8 lb up to 20 lb across gardens.

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