Hybrid seedless (triploid) watermelons have been grown for over 40 years in the United States. However, it was not until recently that improved varieties, aggressive marketing, and increased consumer demand created a rapidly expanding market for seedless watermelons. The seedless condition is actually sterility resulting from a cross between two plants of incompatible chromosome complements. The normal chromosome number in most living organisms is referred to as 2N. Seedless watermelons are produced on highly sterile triploid (3N) plants which result from crossing a normal diploid (2N) plant with a tetraploid (4N). The tetraploid is used as the female or seed parent and the diploid is the male or pollen parent ( Figure 1 ). As shown by the schematic drawing within figure 1, several steps are necessary in triploid watermelon seed production: a diploid (2N) female parent plant is treated with colchicine to produce the solid-colored female tetraploid (4N) parent; this is corssed with a striped male parent (2N) which results in triploid (seedless) watermelon seed(3N). To produce a crop of seedless watermelons, the triploid seed is interplanted with a pollenizer variety (2N). Since the tetraploid seed parent produces only 5 to 10% as many seeds as a normal diploid plant, seed cost is 10 to 100 times more than that of standard, open-pollinated varieties and 5 to 10 times that of hybrid diploid watermelon varieties. Tetraploid lines are usually developed by treating diploid plants with a chemical called colchicine.

Figure 2.
Tetraploid parental lines normally have a light, medium, or dark-green rind without stripes. By contrast, the diploid pollen parent almost always has a fruit with a striped rind. The resulting hybrid triploid seedless melon will inherit a striped pattern ( Plate 2 ). Growers may occasionally find a non-striped fruit in fields of striped seedless watermelons. These are the result of accidental self pollinations of the tetraploid seed parent during triploid seed production. Tetraploid fruit are of high quality but will have seeds and must not be sold as seedless. The amount of tetraploid contamination is dependent upon methods and care employed in triploid seed production.

Plate 2.
Sterile triploid plants normally do not produce viable seed. However, small, white rudimentary seeds or seedcoats, which are eaten along with the fruit as in cucumber, develop within the fruit. The number and size of these rudimentary seeds vary with variety. An occasional dark, hard, viable seed is found in triploid melons. Seedless watermelons can be grown successfully in areas where conventional seeded varieties are produced. However, they require some very unique cultural practices for successful production.