Managing Heat Stress in Young Trees

The current forecast for central Washington calls for a period of temperatures in the upper 90's and possibly in the low 100's coming up this week. Most apple growers by now should be well-prepared to protect their fruit from sunburn with shade cloth, overhead evaporative cooling, and/or sprayable protectants. 

But what about the millions and millions of young trees that have been planted the past couple of years in central Washington? I've been getting a lot of questions about those recently, and how to best protect them from the upcoming heat wave so they don't shut down and stop growing.

A common misconception is that applying an antitransiprant will protect against heat stress. Most of these products work by forming a film over the leaf that physically covers or plugs stomata, thereby reducing water loss out of the leaves through the normal transpiration process. Other antitranspirants, such as abscisic acid, are plant growth regulators and physiologically close the stomata.

Antitranspirants may be beneficial in situations of drought, high winds, and recent transplants, but can actually be harmful if used during heat. Apple and cherry trees have an inherent ability to cool themselves when it's hot. They do so through an evapotranspiration process where water transpires through the leaf stomata, and then evaporates from the leaf surface. It's nature's own evaporative cooling system, and it works quite well as long as we don't interfere with it. 

If an antitranspirant covers, plugs, or closes the stomata, it stops the transpiration of water to the leaf surface, preventing the natural evaporative cooling from taking place. This results in increased leaf temperature, which in turn increases rather than decreases the stress load on the plant. Blocking the stomata also interferes with gas exchange and photosynthesis, further increasing plant stress.

So what, then, is the best strategy for reducing heat stress on young non-bearing apple and cherry trees? Simply put - water. For the whole evapotransiration process to work properly, there must be an adequate supply of water for the roots to take up and move through the xylem stream to transpire through the leaf stomata.

Young trees, especially those just planted this spring, have very shallow root systems. And, if they are on a dwarfing rootstock like M9-337, Bud 9, or Mark for apple; or Gisela 5 or 6 for cherry, those root systems are by design small and shallow. 

Supplying adequate water to maintain the evapotranspiration process during periods of heat to those small, shallow root systems requires frequent, short irrigation sets. When it's hot, this could mean as many as 2 - 4 irrigation sets per day, of maybe only an hour or two per set depending on the irrigation system (micros vs. drip). This is even more critical if the trees are in sandy or gravelly soil with a low water-holding capacity.

The key is to keep the root-zone well-watered without over-doing it and water-logging the soil. The concept is similar to evaporative cooling to reduce sunburn and heat-related disorders on fruit, but in this case the idea is to maintain an adequate soil moisture level in the root zone so that the young trees can cool themselves.

Once the hot spell passes, normal irrigation regimes should resume.