While there’s a chance that a squash-loving garden pest has found your yellow squash vines to explain why they turn brown and shrivel up, there’s a better chance that what’s happening can be explained with some sex education, summer squash-style. It’s likely that your squash needs more love, though not from garden pests. You often hear about gardeners desperately trying to give their summer squash yields away, but if the insects that pollinate plants aren't plentiful in your garden, squash vines that appear healthy are unable to set fruit that grows to maturity, and so the flowers turn brown and die.
Summer Squash Basics
Yellow squash is a cultivated variety of the species Cucurbita pepo, which includes all squash commonly referred to as summer squash. Squash are monoecious, meaning that a single plant bears both male and female flowers. Some monoecious plants, as is the case with squash, bear only male flowers at the start of the growing season, developing female flowers later. The early male flowers eventually wither and fall off without producing anything – only the female flowers, once pollinated, produce fruit.
As the season progresses, squash plants start bearing both male and female flowers. They can be distinguished from one another by the vine behind the blossom. A flower that’s attached with just a narrow, straight stem is male. If a small, round fruit – actually an ovary – is present directly behind the flower, it’s female. For this ovary to continue to grow and develop into a fruit that can be harvested, pollination needs to occur between the two flowers. It’s not uncommon for the first few female flowers to turn brown and fall off without producing fruit; the plant simply produces another rash of flowers to attract pollinators like bees. If your plant keeps producing flowers that wither and die, however, you may need to play matchmaker to help things along.
For whatever reason -- it might be that your garden's pollinators are busy elsewhere or that your plant is heavy on blossoms of only one sex or the other -- self-pollination is not occurring. Helping to pollinate your plants is a fairly straightforward process that entails transferring pollen from a male flower to the reproductive parts of a female flower. Working in the morning is best, when the pollen from a male flower is most viable. Snip off a male flower and peel back the petals to reveal the pollen-covered anther. Leaving the female flowers attached to the plant, gently open the petals to reveal the stigma at the base of the flower. Swab the anther of the male flower over the stigma of the female flower, making sure that pollen is transferred to the stigma. Within just a few days, you should see baby yellow squash developing.
There are other possibilities to consider if your yellow squash continues to produce flowers that wither and die. Extreme temperatures – either too low, such as below 55 degrees Fahrenheit or too high, above 85 degrees, can cause flowers to die back, even if they have been pollinated. Other conditions, such as too much shade or sun or too much fertilizer, may be contributing to plants that turn brown and shrivel up before producing fruit. Common squash pests, such as the squash vine borer and squash bug, also cause squash plants to suddenly wilt and turn brown.