General Climatic Requirements. Seedless watermelons are a warm-season crop, preferring relatively high temperatures for optimum growth. Daytime temperatures of 80 to 95 degrees F. and night temperatures of 60 to 70 degrees F. are best. When temperatures are lower, plant growth is slowed considerably. With favorable weather, seeded fields can produce ripe fruit in 85 to 100 days.
Planting. Poor seed germination is the main problem with growing seedless watermelons. When direct seeding, the soil temperature should be minimum of 70 degrees F. at a depth of 4 inches. Soil temperatures below 70 degree F. will reduce germination and emergence. When growing transplants, use 12- to 2-inch seedling cells or peat pellets. Soak the planting medium thoroughly, and let drain 4 to 6 hours before sowing. Plant 1 or 2 seeds per cell or pellet. The greenhouse temperature should be 75 to 85 degrees F. during the germination period. Do not allow the growing medium to become dry, but do not over water during initial germination. Begin watering, as needed, after 10 to 15 percent of the seedlings have emerged. Plants should be ready for transplanting in 3 to 4 weeks. Transplants should have not more than 3 true leaves when set in the field. Use of older, larger transplants can cause slow, stunted growth and poor yields. In-row and between-row spacing is generally 48 X 80 inches.
But some times to guarantee a good germination for seedless watermelon. We need crack the seeds first then soak. Crack like the following you’re your mouse or with some tools. Crack the seeds just when you hear a sound of”ka”.
Pollination. The male and female flowers are born separately on the watermelon plant. Female flowers must be pollinated for fruit to set. Also, cross pollination must occur between a seedless and a regular type watermelon for seedless fruit to be produced. This is best accomplished by planting a standard watermelon variety in the garden. Approximately one-third of the plants in the garden should be of the standard or ‘pollinator’ variety. Honey bees are the principal insects that pollinate watermelons. Pollination is a must, and poor or partial pollination may result in misshapen fruit and no seedless melons.
Harvesting. The lower side or ‘ground spot’ of the fruit should be cream-colored or yellowish. Thump fruit to check for ripeness. The result will vary. Generally, a solid sound indicates ripeness, while a sharp echoing sound indicates a greener fruit. The tendril or ‘tail’ which occurs in the axils of leaves (where the leaf attaches to the vine) along the stem can be used as an indicator of ripeness. Experienced harvesters say that if the 2 tendrils nearest the fruit are dry, the seedless watermelon is ripe. It is important to note that the first few mature melons in the garden may frequently contain small seeds. This condition is most prevalent under stressed conditions, such as low soil moisture, insufficient fertilizer, temperature extremes, or disease pressure, which affect normal plant development.
Each planting of seedless watermelons actually produces 3 different types of watermelons — the regular seeded watermelons (from pollinator plants), the true seedless melons, and a melon that produces a very limited number of seeds.