Watermelons (Citrullus lanatus) are a classic addition to the annual home vegetable garden, but are somewhat difficult to grow, especially for the novice gardener. If you are growing watermelons for the first time or you're having trouble with them setting fruit, keep in mind the several reasons they might fail to do so.
Male and Female Blossoms
You may notice the first blooms to appear on the watermelons never produce fruit. But this doesn’t mean there is anything wrong with the melon plant. Because watermelons do not have perfect flowers -- meaning the male and female sex organs are contained within separate blooms -- the male flowers must open first to be ready with pollen when female blossoms open. Bees then carry the male pollen to the female blossoms. Because watermelons grow only from the female flowers, the first few flowers to open won't produce fruits.
Watermelons are not pollinated by wind blowing pollen from male blossoms to female blossoms, but rather by insects carrying it there. These insects are usually bees, and a lack of bees in the area may explain misshapen fruit or failure to set fruit at all. Make an effort to protect bees and draw them to your garden. You can do this by planting other flowering plants that they love, such as English thyme (Thymus vulgaris), which grows in U.S. Department of Agriculture plant hardiness zones 5 through 9.
Growing Only Seedless Watermelons
If you plant only seedless watermelons, you will get no fruit. While seedless varieties lack true seeds and may be sweeter because they do not expend energy producing them, they are sterile hybrids and so do not produce viable pollen. To grow seedless watermelons, you must also plant seeded types that produce viable pollen nearby. If possible, use pollinator plants of a visibly different cultivar so you can tell which is which at harvest. Some seed companies sell their seedless watermelon seeds already mixed with pollinator seeds to make this easier for you.
It is possible for you to successfully grow vines that may then wilt or die before setting fruit. Several diseases may be responsible for this, including powdery mildew, verticillium wilt and fusarium wilt. All of these may cause the plant to die before it sets fruit. Buy resistant varieties, if possible. When choosing starts at the nursery, choose only the healthiest-looking ones, free of spots, wilting or other signs of ill health.