Heat effects on vegetables
Gardeners might be wondering at the reaction of some of the common vegetable plants to the heat this summer. In many cases, as evidenced in the information below, too much sunshine and heat can really be too much of a good thing.
Here is helpful information from Rhoda Burrows, PhD, Horticulture Extension Specialist at the West River Agricultural Center.
Tomatoes: Ideal temperatures for growing tomatoes are 75 to 80 degrees. Temperatures over 100 degrees F can prevent fruit set, and temperatures in the 90’s can also prevent fruit set if nights are warm (over 70 degrees) or the humidity is high. On ripening fruit exposed to strong sun, sunburn can occur, and temperatures over 85 degrees decrease development of red color in the fruit. Heat also tends to increase blossom end rot because the fruit expands too rapidly for the plant to take up calcium quickly enough to distribute it to the expanding fruit. Uneven watering will also result in the same problem, as the plant needs moisture in order to take up and move calcium to the fruit.
Squash: High temperatures (over 86 degrees) accelerate flower closing (mid to late morning), so pollination must be accomplished by bees early in the morning. Squash and pumpkin flowers must be pollinated within a few hours of opening, or will fall off the plant.
Peppers: Drought stress early in the season decreases leaf area and fruit yield, especially during blossoming. The optimal temperature for growing bell peppers is 72 degrees; hot peppers can withstand somewhat higher temperatures. Temperatures above 90 can stop fruit set altogether on bell peppers, especially under dry conditions, and even temperatures in the 80s can decrease yield by 50%.
Potatoes: Drought can cause tubers to crack, resulting in misshapen tubers at harvest.
Cucumbers: Heat and drought increase bitterness.
Green beans: Fruit set of beans will be reduced or stopped altogether at temperatures over 85 degrees, with some variation in cultivars. Bush-type (as opposed to pole) beans have fairly shallow root systems, so gardeners need to be careful to keep their soil moist. Smaller-seeded cultivars germinate better in warm (over 80 degrees) soils; larger-seeded cultivars in cooler (54 degree) soils.
Sweet corn: Corn is one of the most heat-tolerant vegetables, but is still sensitive during silking. The primary concern with hot temperatures is to maintain water supply to the roots to ensure good “tip-fill” of the ears.
Lettuce: Many types of lettuce will not germinate when soil temperatures are over 80 to 85 degrees, so late summer plantings for a fall crop must be grown from transplants germinated in a cooler place.
Broccoli & Cauliflower: Temperatures over 80 degrees disrupt head development, leading to small scattered bunches of florets. Water stress can cause the heads to develop too quickly, with similar results.
What can a gardener do to ameliorate the effects of high temperatures? Some tomato growers in other areas of the country are resorting to shadecloth or even mist systems to cool the plants. Although we generally encourage drip systems to avoid plant diseases and to conserve water, short periods of overhead watering may be beneficial to cool the plants during the hottest hours of the day, especially when humidity levels are low. However, avoid having water on the foliage for more than a few hours at a time, as longer periods of leaf wetness allow diseases to invade.
Gardeners might also be interested in reading Heat Zone Gardening by Dr. Marc Cathey (1998). Ask your library to get it for you. He explains the cellular damage done in plants by the heat. It is informative and helpful.