Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
Would you like a watermelon that is sweeter, easier to eat, has a longer shelf life, and no messy seeds to clean up? This year, try "seedless" watermelons.
What makes a watermelon seedless? Plant breeders utilize genetic manipulation. Where a normal watermelon is diploid, meaning it has two sets of chromosomes, a "seedless" watermelon is triploid, having three sets of chromosomes. This is the result of crossing a diploid (2N; having two chromosomes) parent with a tetraploid (4N; having four chromosomes) parent produced by Colchicine treatment. The hybrid receives one set of chromosomes from one parent and two sets of chromosomes from the other. Thus, the new seed and resulting plant have three sets of chromosomes (3N). When the 3N hybrid attempts to reproduce, it cannot divide into an even or stable number of chromosomes, so it is sterile. However, pollination stimulates the development of the fruit, and the nonviable eggs develop into white, translucent, empty hulls that aren't even noticed when eaten.
Seeds to produce "seedless" watermelon are more difficult to germinate and establish in the garden than are normal watermelon seeds. Warm temperatures are a must. Watermelon plants usually do best when germinated indoors with temperatures of 75 to 85F. Use a sterile mix and plant the seeds one per peat pot to avoid damaging the roots when moving to the garden. Set the plants outside when the runners are 6 to 8 inches long (approximately three weeks) and temperatures are well into the 70s. If you must plant directly outside, wait until soil temperatures are above 70F.
It is necessary to grow "seedless" (3N) watermelons adjacent to normal (2N) watermelons. This is to ensure pollination for fruit development. Planting in a ratio of one normal to two "seedless" plants is a good guideline to follow. Also make sure that the fruit of your normal variety can be easily distinguished from your "seedless" variety plants, so you won't be disappointed by finding unexpected seeds if the wrong fruit is harvested.
The seed is only viable for 10 to 12 months, so it should be purchased new each year. Also, there are fewer sources from which to obtain the seed, and it will cost more than the normal cultivar. A few of the more popular cultivars are 'Jack', 'Queen', or 'King of Hearts' from Japan; 'Sun World' from California; and 'Mickylee' and 'Minilee' from Florida.
(Originally published as "Seedless Watermelons," by Leigh Anne Starling, Extension Technician, in The Virginia Gardener Newsletter, Volume 9, Number 3.)
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