by Lisa Johnson, horticulture educator for Dane County UW-Extension
Often, this time of year, I get calls from gardeners asking why some vegetables are not fruiting. Unpredictable spring and summer weather temperatures can adversely affect crops, especially tomato, pepper, and eggplant in the Solanaceae family, and winter squash, cucumbers, pumpkins, melons, and summer squash in the Cucurbitaceae family. These plants rely on a certain range of temperatures to initiate flowers, bring them to maturity, and produce fruit. Fruit production can be affected any time during this process.
Temperatures at night have the greatest impact. If temperatures are too hot or too cold for even a few days during flowering, plants may abort flowers or fruits. For solanaceous plants, daytime temperatures above 85 F for several days, nighttime temperatures above 70 F, or nighttime temperatures below 55 F cause fruit to abort. If you think back to 2012, which had high daytime and nighttime temperatures, low fruit production is understandable. In fact, temperatures over 104 F for four hours can cause tomato flowers to abort.
Temperatures and Pollination
Tomatoes, peppers, and eggplants are self-pollinating, but cucurbit flowers must receive multiple pollinator visits for complete pollination to occur. Squash, for example, requires an average of 12 visits by a pollinator to set fruit. And it has to happen fast. Pumpkin and squash flowers open at temperatures above 50 F; cucumber and watermelon, above 60 F; and muskmelon above 65 F. But these flowers only stay open and viable for a day in the case of watermelons, muskmelons, and cucumbers, and half a day or less for other cucurbits.
Also, all cucurbits are not alike. Some plants have separate male and female flowers, or female flowers only, or have perfect flowers, with male and female parts in the same flower. If they have separate male and female flowers, usually male flowers open first. Early in the season, more male flowers are open than female flowers, but you need both to produce fruit! As the plant ages, the proportion of female flowers increases. Cucurbits are affected by other factors that influence whether a flower will be male or female. Cool temperatures promote female flowers in cucumber, squash, and pumpkin. Conversely, high temperatures promote male flowers and delay female flower development. In pumpkins, temperatures of 90 F during the day and 70 F at night lead to abortion of female flower buds.
Light levels also affect flower development in cucurbits. High light levels promote female flowers; shade can reduce those numbers. The bottom line is that a lot can happen between flowering and fruiting!