Jennifer Schultz Nelson
Extension Educator, Horticulture
While you enjoy the last breaths of summer at cookouts this Labor Day weekend, you may ponder this eternal question while enjoying a delicious slice of watermelon -- if seedless watermelons have no seed, where do you get the seed to plant it?
Japanese plant breeders developed the first seedless watermelons over fifty years ago. Understanding the process requires basic knowledge of genetics.
Plant and animal cells have chromosomes, threadlike structures containing genes, the instructions cells need to function. A complete chromosome set is called the "n" number of a species. The number of sets in a cell is the "ploidy" level.
In humans, our ploidy level is diploid, or 2n, meaning two chromosome sets—one set from our mother, another from our father. Seeded watermelons are also 2n, with one set of chromosomes from the "mother"—the plant whose flower produces the melon, another set from the "father"—the plant that pollinated the melon-producing flower.
To make seedless watermelon, seeds from seeded watermelon are treated with the chemical colchicine which doubles the ploidy level of the plants to 4n, which is called tetraploid.
The tetraploid plants are pollinated with pollen from seeded diploid watermelon. When crossed, each parent donates half its number of chromosome sets-- the tetraploid plant gives two sets, and the diploid plant gives one. The seed from this cross is 3n, or triploid. This triploid seed produces seedless watermelons.
The triploid seed develops into a sterile plant that cannot produce seed, much like a mule in the animal world cannot reproduce. The triploid chromosomes cannot pair up and divide as with diploid or tetraploid plants. The pollinated flower behaves normally, setting fruit and beginning to develop seed. True seeds never completely develop, but there may be small white rudimentary seeds that are completely edible and usually go unnoticed.
In order for seedless fruit to form, the triploid plants must be pollinated by a normal diploid watermelon plant. So a field of seedless watermelon will always include a seeded variety planted purely as a pollen source.
If you plant seedless watermelon at home, the packet you purchase will have a mix of the seedless cultivar and the seeded cultivar that is the pollen source. It is easy to tell the difference between the two cultivars, as seeds of the triploid seedless cultivar will be noticeably larger than the diploid seeded cultivar. Both types of seed need to be planted in your garden if you wish to enjoy harvesting a home-grown seedless watermelon.
This means you will need to dedicate a lot of space to watermelon if you want to grow seedless watermelon at home. Each hill of three seedlings needs from seven to ten feet of space to sprawl.
All types of watermelon need a fairly long growing season, anywhere from 70 to 85 days depending on the cultivar. Seed is planted after the danger of frost has passed and the soil has warmed.
Suggested cultivars of seedless watermelon suitable for home gardens include: 'Cotton Candy', 'Crimson Trio', 'Honey Heart' (yellow fleshed), 'Jack of Hearts', 'Nova', 'Ruby', 'Queen of Hearts', and 'Tiffany'.
Seedless watermelon usually commands a premium price at market, but considering the additional labor involved in producing both the seed and the fruit, it isn't a huge price to pay for a seedless slice of summer sweetness.