The philosopher William James once remarked, “When you’re through with learning, you’re really through”. Indeed, life is a learning experience and, since gardening is a part of life, it (too) should be a learning experience. Each and every year new problems arise or situations occur that cause us to pause and ask: “why” or “how” did that happen? Frankly, the challenge of the unexpected is one of the aspects that make working with plants so interesting. As the calendar year draws to a close it is appropriate that we pause to consider the past growing season and contemplate the lessons we have learned.
The summer of 2012 will live in infamy for decades to come for those involved with growing plants, be the latter for commercial or for recreational purposes. The combination of prolonged high temperatures and lack of nearly any precipitation is sure to make this past summer one that will be used as a benchmark for weather austerity in years to come. The result of this deadly combination was the loss of innumerable plants (including many woody species) that might have been able to withstand either of the two separately, but not both together. Let’s explore the reason for this.
The concept of a lack of one environmental factor exacerbating the effect of another is termed “synergism”. Looking at this concept in mathematical terms, it is an established fact that two plus three equals five. If synergism were involved between the numbers two and three, the sum might be something like six, seven or more. In short, this past summer plants suffered from a synergistic affect between high temperatures and drought stress.
We all know that plants take up water via their root system in an attempt to keep their leaves turgid and stomata open. The latter are minute pores on the surface of leaves which, when open, allow for the entry of carbon dioxide-laden air into the interior of the leaf. Carbon dioxide is one of the basic, raw ingredients needed in the photosynthetic process and critical for plant growth and productivity. However, open stomata also allow water vapor to escape from the leaf in a process called transpiration.
Although transpiration can lead to a water deficit in plants, it does serve several useful purposes. For example, transpiration is necessary to translocate dissolved minerals from the soil to other parts of the plant, to move sugars and other plant products within the plant, to maintain cell turgidity, and (importantly) to help cool leaf and stem tissues. The rate of transpiration is dependent on several environmental factors, the most important being leaf temperature as influenced by the radiant energy of the sun. As the temperature of the leaf increases, so does the rate of transpiration (or water loss from the plant).
Plants are in a constant state of water flux. When the rate of transpiration exceeds the rate of water uptake, wilting occurs. Recovery from wilting happens when the rate of water uptake exceeds that of transpiration. Often the latter occurs after the sun goes down and leaf temperatures cool. Permanent wilting (followed by death) occurs when there is insufficient soil moisture to reverse the wilting that occurred during the day.
On a hot day, transpirational cooling is sufficient to keep leaf temperatures near or slightly above the temperature of surrounding air, depending upon the amount of radiant energy received and wind speed. Research has demonstrated that temperatures of rapidly-transpiring leaves, on the average, were 2-4 oC cooler than those of non-transpiring leaves. When this cooling effect is reduced or stopped altogether because of a lack of moisture, leaf temperatures can rise to levels significantly above the ambient air temperature.
Heat stress in plants occurs when temperatures are of sufficient magnitude and duration to cause damage to plant function. Perhaps the most common form of heat stress is starvation injury. The latter occurs when the rate of respiration exceeds the rate of photosynthesis. At higher temperatures heat stress results in more dire, irreversible consequences. For example, for many woody species a temperature death threshold is reached when plant tissue temperature reaches approximately 115°F, according to one authority. Temperature death threshold depends on factors such as the duration of hot temperatures, the maximum temperature reached, the age and hydration of plant tissue, and the ability of the plant to adjust to temperature changes.
Suffice to say, then, there is a synergistic affect between temperature and water. The hotter the temperature, the greater is the rate of water loss via transpiration. Under periods of drought stress, when adequate soil moisture is not available, the matter is aggravated by a lack of transpirational cooling which, in turn, leads to higher leaf temperatures. This tends to lead to a downward spiral of plant function and (ultimately) the death of the plant.
Mark Twain once remarked, “Everyone talks about the weather but no one seems to do anything about it”. Indeed, there is not a lot we can do about hot temperatures. Shading is effecting in reducing leaf temperatures and might be practical for smaller plants but not for larger ones. Misting helps to cool leaf tissue but the beneficial effect is very short-lived.
If the summer of 2012 represented the “new normal” when it comes to temperatures in our region, then species selection probably is a gardener’s best defense against oppressively high temperatures. Gardeners long have had a tendency of trying to coax plants out of their “comfort zone” when it comes to temperature preference. Those species more content in cooler climates are the first to display heat stress here in the Midwest. In the future, if non-native species are planted, the attempt probably should be made to identify species more adapted to warm summers.
Water is an environmental parameter serious gardeners should be able to control. Many people pay adequate attention to the water needs of their herbaceous plants by supplying supplemental water during periods of high temperatures and drought stress. Most herbaceous plants require between one and two inches of water per week. Drip irrigation systems are excellent in that they tend to distribute the water where it is needed while keeping the foliage of the plants dry.
Woody species, however, often tend to suffer from neglect during periods of hot, dry weather since we consider them to be more self-sufficient. Often, supplemental irrigation is delayed until after symptoms of severe water stress such as leaf scorch appears. “Rescue irrigation” is better than nothing but frequently results in too little, too late from the standpoint of plant health. Prevention always is the best cure and this holds true for water (heat) stress as well. Again, applying between one and two inches of water per week is a “best management practice” for landscape plant maintenance. Some species might require more; others may to able to get by with less.
Gardeners tend to be optimists. Who else would plant a tulip bulb in October with the full expectation of seeing a beautiful flower in April? As discouraging as this past summer might have been to our gardening efforts, giving up is not an option. Hopefully, the summer of 2013 will be more conducive to plant production. If not, at least we now can say “we have been through this before” and (consequently) will be better able to deal with it.
REVISED: December 31, 2012