Squash season is just ramping up, and some home gardeners will be wondering why they have plenty of blooms, but no fruit. Or why they have tiny fruit that just rot instead of maturing.
All members of the cucurbit family (Cucurbitaceae), including summer and winter squash and zucchini, cucumbers, melons and gourds have separate male and female flowers. It is very common for these plants to produce only female flowers when they first begin blooming.
Female flowers have what looks like a tiny fruit attached to them below the bloom, called an inferior ovary. Without male flowers nearby to pollinate them, these female flowers will drop off or their little ovaries will rot on the vine. Inside the bloom, female squash flowers only have stigma.
Male flowers do not have this ovary, but inside the bloom they have pollen-bearing anthers.
In order to get fruit, you must have both male and female blossoms and a pollinator. The American native squash bee, Peponapis pruinosa, is the best pollinator of cucurbits. Because it flies before sunrise, it is rarely seen by gardeners. Unfortunately, our native squash bees are also in decline, suffering from pesticide use.
Bumblebees also visit and pollinate squash blossoms very effectively. Honeybees are effective pollinators, but less so than either of the other two bees.
If you have both male and female blossoms of the same species, but no fruit set, you probably do not have enough pollinators, although high temperatures and rain can both cause pollination failure. If the female flower falls off but the ovary stays and grows quickly, you have successful pollination. However, poor levels of pollination produce misshapen fruit, or fruit that only partially develops and then dies. Poorly pollinated fruit are edible, but grow slower and are less attractive. They should definitely not be used for seed saving.
If you do lack pollinators, squash can easily be pollinated by hand: pick off a male blossom, remove the flower petals to reveal just the pollen-bearing anthers, then gently brush the stamens inside the female flowers with the anther. Each male blossom can be used to pollinate several female blossoms. Once you know how to hand pollinate squash, this opens up new options for you. You can cover plants to protect them from flying pests, create your own hybrids, or use hand pollination to ensure purity when seed saving.
In the long run, cultivating a garden and landscape which is friendly to bees and other pollinators will save you much labor.