Pumpkins (Cucurbita spp.) prefer a sunny spot with rich, well-draining soil and a pH between 5.8 and 6.8. These annual vining plants may grow to heights of 3 feet, with long sprawling vines that grow up to 15 feet, depending on the cultivar. The flowers typically appear in mid to late summer. But not all blossom will produce fruit because pumpkins produce both male and female blooms.
The first eight blooms to appear on a pumpkin vine are usually male. These flowers open approximately one week before the female flowers appear, to attract bees to the garden. Male blooms are held on a slender stem. The male flowers produce both pollen and nectar. The bees are attracted to the nectar, but in the process, pollen from the male bloom sticks to the bees' legs and hairy bodies.
The female flower opens after the male flowers have attracted bees to the garden. It has a tiny, swollen ovary at the base of the bloom that looks like a miniature pumpkin. The swollen ovary may be green or yellow, depending on the type of pumpkin. The female bloom does not produce pollen, but does produce nectar to attract bees.
When a bee visits male blooms, it collects pollen on its legs and body, which it carries with it to the female bloom. When the bee enters the female bloom to get nectar, the pollen rubs off and pollinates the female bloom. For complete pollination to occur, bees must visit the female bloom up to 15 times. Female blooms open in the morning and close by afternoon. After 24 hours, the bloom drops from the plant, whether it is pollinated or not. If the bloom is pollinated, the tiny fruit begins to grow. Otherwise, the miniature pumpkin shrivels and rots and drops from the vine.
Poor pollination may occur when bee activity is low, the weather is too hot or your plants are under stress from disease or insect pests. Too much nitrogen fertilizer can also delay blooming and fruit set. Other factors that affect pollination and fruit set include prolonged rainy periods that may limit bee activity and either over- or under-watering your pumpkins.