It’s hot! It’s barely mid July and we’ve already had some 100-degree days in the Denver area. The hottest time of year here is the last week of July and first week of August, so we’ll be struggling with these temperatures for some time. Some parts of the world are, of course, even hotter. Heat stress forces plants against the upper limit of their adaptations, and they may require intervention.
Signs of heat stress in plants
When the weather is hot, water evaporates much more quickly from leaves and from the soil. This is especially true in low humidity, which is even harder on plants. Plant tissues that lose too much moisture will wilt, scorch or die back.
Symptoms of heat stress overlap with symptoms of drought stress, but there are some differences. Because of the way heat peaks in the middle of the day, plants suffer from repeated assaults rather than a gradual scarcity of water.
Wilting
The most immediate sign of heat stress in plants is wilting. When wilting is related to heat, it is notable that plants can wilt even when the soil is moist. The roots are still taking up water, but they can’t keep up with the rate of evaporation from foliage. The short-term solution is to wet the soil even more thoroughly to increase the absorption rate. You can further ease the strain on roots by mulching with wood chips, straw or leaves to cool the soil.
Leaf curl
When dehydration occurs repeatedly or lasts for days, it can change the way young leaves develop. Cells on the margins of a developing leaf can’t get enough water to expand, while leaf centers develop normally. The uneven expansion forces leaves to crinkle or curl as they mature, and sometimes the deformation is permanent. This type of drought stress is especially common on fast-growing plants like sunflowers, tomatoes and peppers. (Note: viruses and other diseases can also show up as leaf curl. But if the problem began in abnormally hot weather and leaves are not discolored or dying, the likely cause is heat stress). The typical solution to leaf curl is to water plants more regularly.
Scorched leaf margins
Severe or repeated heat stress can scorch mature leaves by dehydrating and killing the outer edges of the leaves. Scorched leaves will not improve and the plant won’t look better until it grows new shoots. Mulch, deeper watering, and shade protection on atypically hot days will reduce the incidence of scorch. Resist the temptation to cut off damaged leaves; they don’t look good, but they still photosynthesize and that helps the plant to grow new, undamaged leaves. Meanwhile, the dead, scorched tissue helps to shade and protect lower foliage from further damage.
Sunscald
Unlike wilting and scorch, which are primarily caused by dehydration, sunscald appears when plant tissues are killed by bright sunlight. In the same way that ultraviolet light burns human skin, high-energy photons from the sun damage the chlorophyll and DNA in plant cells beyond the plant’s ability to repair them. Scalded tissue is permanently bleached white or tan. It’s especially common on plants that have recently been moved or brought home from the nursery. To prevent sunscald, move plants to a brighter setting by acclimating them gradually with shade, part shade, light shade and finally full sun. That allows leaves to produce compounds that absorb excess light or free radicals and will prevent sunscald from occurring.
Thermal burn
Leaves can burn on exceptionally hot days, especially when air temperatures are above 100 degrees. Foliage in direct sunlight, or plant tissues in contact with rocks, gravel or metal reach lethal temperatures. Plants are literally cooked to death: the absolute maximum temperature plant cells of any species can tolerate is 140 degrees, while 100-degree temps can kill cells if they go on for hours and the plant is not acclimated to heat. Thermal burn on leaves resembles frost damage. Stems through solar-heated rocks or gravel can be killed from the base, cutting the plant off from the roots. On extremely hot days—the kind of heatwave that happens once every few years—only shading can protect foliage and it’s impossible to prevent some damage. Water and mulch soil with organic material to prevent thermal burns on stems. To protect plants from further damage, leave burnt leaves in place to provide shade until the heat wave is passed.
Yellowing leaves
It’s normal and healthy for plants to drop older leaves and grow new ones, and plants do so regularly. Older leaves don’t produce as much energy for the plant, and plants regularly recycle their canopies. When leaves turn yellow before they fall, they are dying in a controlled process that usually doesn’t harm the plant. However, during times of severe stress, yellowing can accelerate and many or most leaves may drop.
Spider mites
Spider mites are barely visible to the naked eye, clinging to the sides or undersides of leaves and sucking the fluids out of plant cells. Mite damage appears as small, yellow dots that can expand and fuse together as the mites proliferate. Plants sensing this damage can drop all their leaves in an attempt to starve the mites out or make them go away. Mite-damaged perennials usually survive, but the damage can set them back for a season or severely stunt their growth. Smaller plants or annuals may die. Mites increase their activity in hot weather, and can overwhelm and kill plants in a severely drought-stressed garden. Water and humidity lower spider might activity. Insecticides, which kill beneficial predatory insects, can exacerbate the problem in outdoor gardens. The long-term solution is to water, mulch, and make sure gardens are full of heat-tolerant species planted at a density that the water supply can support.
Shock or early dormancy
Extreme heat stress can force a plant into dormancy as a last-ditch effort to survive. Trees will occasionally do this if they are transplanted in summer. Many cool-season grasses and lawns can go into and out of dormancy easily. Other plants will be left weakened or traumatized when they finally begin to regrow. Some plants are unable to go dormant at all, and just die. The solution is to water, and adapt your garden to drought so that it doesn’t happen again. If plants go dormant easily, it may be better to withhold water and let them stay dormant until cooler weather comes.