No doubt, you’ve noticed that it’s been hot and dry. The National Oceanic and Atmospheric Administration (NOAA) says July 2016 was the hottest month on record for the globe, and as of the middle of August, the U.S. Drought monitor recorded that roughly two-thirds of Tennessee is experiencing abnormally dry to severe drought conditions. Parts of six counties in the southeastern part of the state remain under an extreme drought.
University of Tennessee Extension specialists explain how heat, humidity and soil moisture might affect your late summer gardens and landscapes. As you might have guessed, most of the news is not promising.
David Lockwood, UT Extension Fruit Crops Specialist, says the recent weather will definitely not benefit your fruits trees. “High temperatures along with high humidity during the day causes a mid-day decline in photosynthesis. High temperatures at night keeps respiration rates high, resulting in utilization of stored reserves in the plant. Both color development and sugar accumulation in fruits are negatively affected,” he said.
Lockwood has even more bad news about fruit crops and heat and drought. He says that high temperatures and dry conditions will have a negative impact on fruit size – even when irrigation is being used.
Finally, Lockwood notes, “Sunscald on fruits is worse than under cooler conditions. Damaged fruits are more prone to rots as a result. Also, the lack of alternate food sources and water results in greater damage to fruits from birds and other wildlife. Fruit damage from wildlife will increase the severity of insect damage and rots.”
Annette Wszelaki, UT Extension Vegetable Specialist, adds that vegetables will scald as well, especially tomatoes and peppers, if there isn’t sufficient plant cover. “We also see a lot of bloom drop on several crops if nighttime temperature does not drop below 70 degrees and if daytime temps are above 85. This affects tomato, pepper, green beans and other fruiting vegetables,” she said.
Tom Samples, UT Extension Turfgrass Specialist offers a little hope for some lawns, however. “Warm-season turfgrasses such as bermudagrass and Zoysia actually grow best at air temperatures from 80 to 95 F, if the soil is moist,” he said. “Tall fescue and Kentucky bluegrass on the other hand, suffer severe high temperature stress when air temperatures remain in the 90’s for several days in a row.”
Samples says the combination of heat stress and low soil moisture can kill cool-season turfgrass plants if the weather remains hot and dry. “The lawn will first appear grayish-blue as leaves roll in response to drought before eventually turning yellow, then brown.” Some form of irrigation is recommended.
Carol Reese, UT Extension Horticulture Specialist in West Tennessee, points out that ornamental plants will exhibit heat scorch, especially on plants that are also experiencing a water deficit. “Even though there may be adequate soil moisture, some plants cannot move it from the root zone as fast as it is being lost from the foliage, especially on breezy days with low humidity. This is especially true with hydrangeas for example, which may wilt midday even when they are well watered.”
Reese says that some plants that normally flower all summer will slow down or even stop flowering during the hottest parts of summer. This is true of some of our garden vegetables, too.
Reese adds that high night temperatures are also very stressful and probably more important, since high night temperatures increase plant respiration, the reverse of the daytime photosynthesis. “This results in a depletion in the carbohydrates produced to satisfy the plant’s needs, and the plant will decline and be more susceptible to other problems. These heat related issues are most prevalent in the warmer, low altitude western regions of Tennessee. Humidity also increases disease pressure, so it is another factor that causes plants to fail,” she said.
Scott Stewart, an entomologist at the West Tennessee AgResearch and Education Center in Jackson, added that hot dry summers can be a mixed bag for field crops. “Many field crops tend to thrive and grow best when temperatures are in the 85- to 90-degree range, but it’s also generally true that hot temperatures are associated with less rainfall, which for most crops poses a problem.” Stewart says high heat during flowering can result in pollination problems for corn, and this is especially bad if associated with droughty conditions. “We’ve seen some of this in Tennessee this year,” he said.
Stewart adds that while corn and soybean don’t exactly “enjoy” 90+ degree days, higher temperatures can help cotton mature faster. “This can be especially beneficial in Tennessee where accumulating enough heat units to mature the crop can sometimes be a problem,” he said.
The moral of this story? Every weather condition can pose a problem or present an advantage to the various plants that we like to cultivate. Consult your local county UT Extension Office if you have production questions, or visit the UT Extension publications website and type in key words to search for relevant, science-based information regarding the crop or plants you wish to grow.