How Do Seedless Watermelons Propagate If They Have No Seeds?

Answer 1

You can actually buy the seed of seedless varieties from most major seed companies, & then you plant them. For pollination purposes, you need to plant a standard variety of watermellon along with the seedless variety. About one-third of the plants in the garden should be of the standard or 'pollinator' variety.

Where do the seeds come from? "Simply stated, the number of chromosomes (the threadlike bodies within cells that contain the inheritance units called genes) in a normal watermelon plant is doubled by the use of the chemical colchicine. Doubling a normal (diploid) watermelon results in a tetraploid plant (one having four sets of chromosomes). When the tetraploid plant is bred back, or pollinated, by a diploid or normal plant, the resulting seed produces a triploid plant that is basically a "mule" of the plant kingdom, and it produces seedless watermelons."
It's explaind in more detail here:
http://aggie-horticulture.tamu.edu/extension/newsletters/hortupdate/may00/h5may00.html
Good luck! Hope this is helpful.

Answer 2

cross pollination