Much has been made in the news about the heat this summer. No wonder. Nobody needs much reminding, with large swaths of the country sweltering. There’s been drought too. This weather is tough on our infrastructure and most living things—plants are no exception. How does heat affect them, and how do they adapt to it?
Different types of plants differ in their sensitivity to high temperatures. Cool-season species are more sensitive to hot weather than are warm-season species. Some plants are most affected by high daytime temperatures, others by high nighttime temperatures. High temperature affects plants throughout their development and life cycles. Depending on the type of plant and the intensity of the heat, seed germination may be slowed or inhibited entirely. And the effects of heat vary with developmental stage. At reproduction, high temperatures can suppress flower development; or if flowers are produced, they may set no seed or fruit. The rate of reproductive growth can be increased and the period for photosynthesis so shortened that the amount of sugars contributed to fruit or seed production is severely reduced, a disappointment for the gardener to be sure but a crop failure for the farmer with a few hundred acres of soybean.
Plants that live in such places as the Mojave Desert, one of the hottest and driest places on Earth, have evolved ways to help them survive extreme heat (and drought). Features such as small, waxy and/or hairy leaves help reflect high light levels and retain water, and the ability to convert carbon dioxide to sugar at night allows these plants to live and grow in such environments.
But these are exceptions. Most plants grown in North America suffer irreversible damage when high temperatures continue long enough and absolute temperatures are high enough. Plants cool themselves, but a plant’s temperature is generally only slightly above the air temperature. The principal way they do this is by transpiration (water loss mostly from leaves). In addition to transpirational cooling, plants shed heat through air circulation (wind) and direct heat radiation, neither is much help at high air temperatures.
Transpirational cooling is similar to our sweating. Water absorbed by the roots is translocated to the plant vascular system (xylem vessels) and transported to specialized pores (stomata) on plant surfaces; there it evaporates, cooling the plant as well as the surrounding air. These stomata are a remarkable evolutionary adaptation; their function is dependent on one of the many remarkable properties of water—its high surface tension, the attribute of molecularly clinging to itself. You can see water’s surface tension at work in water drops that form on nonabsorbing surfaces or in the drinking glass filled with more water than its total volume.
Stomata are at one end of a continuous column of water that reaches down to the plant roots. Water’s surface tension makes possible this water column that can be as much as 360 feet long in the tallest of the coast redwood (Sequoia sempervirens) trees. Water evaporating at the stomata, creates a negative pressure differential that “pulls” more water from the soil and moves it to the top of the plant. This is the “transpirational stream”, and it ultimately drives and controls all water movement in vascular plants.
Stomata are flanked by a pair of “guard cells” that respond to water pressure inside plants and control whether stomata are open or closed. This allows plants to breathe—exchange the carbon dioxide and oxygen used in photosynthesis and respiration—as well as cool themselves.
Plants grow best at temperatures that we find comfortable—roughly 70 to 85° F. Within this temperature range (and with adequate soil moisture), plants will thrive and efficiently shed the heat that they absorb from their surroundings. But the warmer the air, the more water evaporates; as air temperature rises, plants lose more water from stomata. On warm, sunny days, leaves can easily lose water faster than roots can absorb it. Guard cells then lose turgor pressure, the plant begins to wilt, and stomata close, effectively limiting water loss—but also limiting transpirational cooling and photosynthesis. With adequate soil-water content, these daytime water shortages are minimized by water uptake at night, driven by the negative pressure within the transpirational stream.
But under high, persistent heat and drought conditions, things get bad for most plants. If you’ve got hundreds of acres of unirrigated crops, say in west Texas this year, you’re probably in trouble. Most gardens can be watered, though, and absolute temperatures are rarely high enough to kill plants outright.
A couple of simple tips when you water: do it slowly, deeply, and regularly. Recently planted (two growing seasons or less) trees and shrubs need lots of water. Deep irrigation will ensure that water gets to the root zone, and it will promote deep root growth and development. The amount of water needed will vary with soil type, environmental conditions (air temperature, wind, sun or clouds), and how big your plants are. If you water a plant and it still appears dry or is wilted the next morning, it needs more water. Trees and shrubs can be misted during the day. This will increase ambient moisture and clean the foliage in a dusty area.
Lawns of course suffer too. I’ve always thought of hot, dry summer conditions as a good reason not to mow them. Lawns in temperate regions are often composed of bluegrasses and fescues and ryegrasses; these stop growing during hot, dry periods. But they will green up and begin growing again when temperatures cool and rain returns. If a brown, dead-looking lawn bothers you and you want a green lawn, water it. But, again, water it slowly, allowing the dry and often water-repelling soil to be wetted and the water to soak into the root zone. Don’t water sporadically but keep watering regularly throughout the hot, dry period. And don’t cut your lawn as often or as short as you might normally.
Let’s hope that the extreme heat is pretty much over for this summer. But it will be back, maybe next year, maybe the year after. One last simple suggestion: nearly everyone uses mulch in winter to protect plants from freeze–thaw cycles, frost heaving, and the like. If you’re not already using mulch in summer, it’s just as useful as in winter. Many municipalities convert their green waste to mulch and deliver the finished product for a nominal charge. Garden stores have it too, as do independent suppliers who can be found in phone books. A few inches of organic mulch in your garden and around perennials and shrubs during hot weather will inhibit soil water loss and insulate the soil so that its temperature is reduced and does not vary much.