I am in fall gardening mode which means I’m still thinking about some of the state-wide complaints from gardeners about lower than expected bean, pepper, and tomato yields during the hottest weeks of the past summer (mid-June through mid-August). Although many factors can reduce flowering and fruiting I believe that high night temperature is a growing, and overlooked, cause of this problem.
In general, warm-season crops can tolerate high day temperatures but can be negatively affected by higher than normal night temperatures, which hovered around 80°F in Maryland’s urban and suburban areas for most of July. Here’s an overview of this aspect of climate change from the USDA:
Some crops are particularly sensitive to high nighttime temperatures, which have been rising evenfaster than daytime temperatures. Nighttime temperatures are expected to continue to rise in the future. These changes in temperature are especially critical to the reproductive phase of growth because warm nights increase the respiration rate and reduce the amount of carbon that is captured during the day by photosynthesis to be retained in the fruit or grain…. Common snap beans show substantial yield reduction when nighttime temperatures exceed 80°F. http://downloads.climatescience.gov/usimpacts/pdfs/agriculture.pdf
Many of us observed healthy mid-summer snap bean crops with few flowers or pods. Yields increased once the weather cooled in mid-August. Experienced gardeners have lamented poor lima bean crops in recent years. Stink bug feeding has been a problem in beans but increasing night heat could also be at play. Research studies have shown that high night temperature causes sterile pollen and damages flower buds. Cowpea (a heat-loving crop!) yields consistently decline when night temperatures increase from 60°F to 75°F. http://www.plantstress.com/Articles/heat_i/heat_i.htm
On another global warming note… I attended the annual meeting of the American Society for Horticultural Science a few weeks ago and heard an interesting talk on how Shading Levels Affect Bell Pepper Fruit Yield (Juan Carlos Diaz-Perez; University of Georgia). The researchers found that erecting shade cloth over bell pepper plants increased yields. The lowest yields were from completely unshaded plants and the highest yields were from plants with 30% shading. Shading lowered leaf and root temperatures and reduced the incidence of blossom-end rot and sunscald.
So what’s the upshot? Climate change is real and night temperatures are on the rise.
Some garden-level responses? Try providing some shade to pepper plants in full-sun gardens. Plant snap beans multiple times and focus more on spring and late summer plantings. Plant limas late so they bear pods from late August through September. Experiment with planting bean, tomato, and pepper in spots where they will receive some late afternoon shade.