Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Delayed fruit set in pumpkin can be caused by many factors including late planting, heat and water stress, poor pollination and excess fertility (too much N). When set is delayed until August, the question is will the pumpkin develop and color in time for sales.
Under favorable summer growing conditions pumpkins will start to color about 4 weeks after fruit set and will be completely colored by 7 weeks after set. If fruit set is delayed until August, reduced day lengths and cooler temperatures may increase the time for full color development. Varietal differences in days to maturity also come into play.
In research at Purdue University, pumpkins that set in August were tagged and then evaluated for maturity in October. They found that for flowers that opened between August 10 and August 21, at least 70% produced pumpkins that were either turning or fully orange by October 2 and 10, respectively. The remaining 20 to 30% either never set a fruit, or the fruit was still immature at the time of harvest.
This indicates that pumpkins set in mid-August will be ready for October sales. In fields with delayed set, it will be critical to keep vines healthy through September. This will mean additional fungicide sprays through the month with special attention being paid to powdery mildew and downy mildew.