Near record breaking heat will have an impact on our garden’s vegetables. In general, daytime temperatures that exceed 92°F and nighttime temperatures above 72F will cause problems. The longer the heat lasts, the bigger the impact. Problems include:
Sunscald – Usually a problem with late season heat waves when loss of foliage is more prevalent, but any fruit exposed to direct sunlight can experience bleaching and browning of the exposed parts.
Pollination issues in cucurbits – Heat favors male flower production over female flowers, resulting in fewer fruits, at least temporarily. It may also lead to drop of already set fruit. Bee flights/activity is reduced in hot weather which causes poor pollination and flower drop. In some cases, fruit will be distorted and crooked due to incomplete pollination. Cucumbers are especially sensitive to this disorder.
Tomato/Pepper flower drop – Flowers on these plants have a 48-hour window to pollinate. High temperatures can cause pollen sterility and flower drop. You may not notice the impact until 5 to 6 weeks later when yields are suddenly reduced for a period that aligns with the length of the heat wave.
Sweet corn yields – Although a heat loving crop, daytime temps over 95F can result in poor tip fill, missing kernels, smaller ears, and reduced yield.
Snap bean flower drop – Beans respond by dropping flowers. As soon as temperatures cool down, flowers will develop normally. Unfortunately, this may result in a split set, where some flowers set before the heat and others after. This is a big problem for one-time mechanical harvest but doesn’t have much impact on garden grown beans.
Potato crinkle and heat sprouts – Distorted, yellow leaves may look like herbicide damage as chlorophyll is reduced. High soil temps can pause tuber development. Once growth resumes, rather than continued tuber expansion, tubers may sprout, producing above ground stems or chain tubers.
Poor color development – Although a little early for ripe tomatoes, fruit may look less red and more orange. Some varieties may show more yellow shoulders. Carrots and beets will also have duller colors.
Reduced sweetness – High temperatures increase respiration in plants, burning off accumulated sugars. This can result in bland tasting melons, sweet corn, and other crops. Cucumbers will become more bitter. Radishes will develop a spicy flavor.
Premature flowering in greens – Spinach and lettuce will quickly bolt and develop bitter flavors prior to seeing the flower stalk.
The most important thing to do during a heat wave is maintain an effective irrigation schedule. Water stressed plants will close their leaf pores (stomata) and go in to survival mode, resulting in reduced yield and quality. Raised beds will dry our more quickly and require more water. A raised bed 4 feet x 10 feet in size (40 feet2) needs a minimum of 25 gallons water per week to equal an inch of rain. Mulches like straw or paper can reduce watering needs by 50%. If using overhead watering, try to irrigate first thing in the morning as evening or night irrigation can increase plant disease. Soaker hoses and trickle irrigation can be used anytime.
For more information on irrigation systems and beating the heat in your garden, read the resources below:
University of California: Drought and Water-Wise Gardening
University of Georgia: Make Every Drop Count: Managing a Water-Wise Landscape
Steve Reiners is a Professor of Agriculture at Cornell Agritech. His short-term projects are designed to provide growers with the best cultural methods to grow crops, i.e., fertility management; irrigation; optimizing plant populations; and variety selection for the processing vegetable industry. Long-term projects include fine-tuning cover crop use to maximize nutrient recycling and reduce disease incidence; improving fertilizer recommendations; and determining how variety selection and fertility practices affect disease tolerance.