So, how are seedless watermelons grown? Chromosomes are the building blocks that give characteristics, or traits, to living things including plants and watermelons. Watermelon breeders discovered that crossing a diploid plant (bearing the standard two sets of chromosomes) with a tetraploid plant (having four sets of chromosomes) results in a fruit that produces a triploid seed. (Yes, it has three sets of chromosomes). This triploid seed is the seed that produces seedless watermelons!
In other words, a seedless watermelon is a sterile hybrid which is created by crossing male pollen for a watermelon, containing 22 chromosomes per cell, with a female watermelon flower with 44 chromosomes per cell. When this seeded fruit matures, the small, white seed coats inside contain 33 chromosomes, rendering it sterile and incapable of producing seeds. This is similar to the mule, produced by crossing a horse with a donkey. This process does not involve genetic modification.
How They're Planted
The seedless watermelons need seeded watermelons in order to be pollinated and set fruit. In a watermelon field where they're growing seedless, roughly 25% of the plants are seeded varieties and 75% are seedless varieties. Hives of bees are brought into the fields to cross-pollinate from the seeded plants to the seedless plants. Without this cross-pollination, the seedless watermelon plants would not produce fruit.
Seedless watermelon varieties are more difficult to grow than their seedier cousins but it can be done. Your best bet is to germinate these pricier seeds indoors at an optimal 85º F, and plant them outdoors after soil temperatures have warmed to at least 70º F. It's easier to buy melon transplants and set them out when temperatures are in the 80s. Because seedless melons are sterile and produce little pollen, you'll need to also plant a normal pollinator variety of watermelon in the patch so the bees can pollinate the flowers, contributing to the development of flavorful, well-formed fruit.