Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Some pumpkin fields have seen a delay in fruit set due to high temperatures in late June and July in 2022. Excess nitrogen can also cause delays in fruit set. When set is delayed until August, the question is will the pumpkins develop and color in time for sales?
Vigorous pumpkins grown no-till in ladino clover cover crop from a late June planting that was partially killed by a glyphosate herbicide treatment. Note clover growing between rows. Pumpkin set was delayed due extra nitrogen from the clover and fertilizer N applications. Pumpkins are now setting well and will be in color by early October.
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.
Recently set pumpkin. Most fruit at this stage on the 18th of August will turn orange color by the first week in October.
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.
Well advanced pumpkin that will color by mid-September.