Often, I get the question: “My pumpkins bloom, but then the female fruit just shrivel up and die. Why is this happening?”
There is no one answer to this but here are some of the contributing factors to why the plant will abort fruit. Good news and bad news - it is nothing you did wrong but also there is often nothing you can do about.
- Heat Stress. Pumpkins in general do not like to set fruit when we have air temperatures above 95 degrees. One of the ways the plant can reduce its stress is carrying less fruit.
- Lack of Pollination. While you may have pollinators, if we have a stretch of rainy weather many pollinators don’t work as effectively which can lead to poor pollination causing the fruit to abort. On average it takes 10-15 bee visits to get a pumpkin flower enough pollen to pollinate. There is also work that has shown that bumble bees are better at pollinating under rainy conditions than honeybees.
- Variety. While we do not have a good index of this there are definite variety differences in fruit set, and some will abort fruit faster than others.
In the end, there is a lot of variation in why pumpkins will abort fruit. Fortunately, pumpkins are indeterminate in their fruit set so if you miss a few fruit, the plant will put out some more female flowers soon to try again. For most varieties, the plant will always put out more female flowers than it can support, and it will naturally balance the load of fruit it can handle. Since each pumpkin flower is only open one day, this is a mechanism the plant uses to help ensure fruit set despite environmental conditions at bloom!