by Rob Barrett
Droughted dogwood. Photo courtesy Jerry Giordano, CCE Westchester County.
In Upstate New York we have learned to expect the unexpected when it comes to weather. Many times it’s the mildest of winters, other times it’s the summer with no sun. This time, we are talking about the drought of 2016. We had higher than average temperatures, but not unbearable. The real issue was the staggering lack of rainfall, it seemed as though it would never end.
Trees are complex plants, and through evolution, species have adapted to conditions present in their respective zones. This does not make them invincible. The urban landscape can be unforgiving, full of multiple stresses. It would be foolish to think we, as humans, could alleviate all stress on trees and shrubs. There are, however, quite a few things we can do to lessen their effects.
Let’s first discuss the importance of water and how too much or too little can matter. We need water, plants need water; it is a critical component of life. Trees and shrubs use water for transpiration, moving from the smallest roots, through the trunk or stem, and out through the leaves. Along the way, its used to build and protect plant tissues, and through photosynthesis in the leaves, it is converted into the air we breathe. The essential nutrients in the soil can only be accessed with adequate moisture. These nutrients are then shuttled up the plant along the water columns to areas where they are required. Water is even used to move the carbohydrates synthesized within the leaves. This may all seem redundant and obvious to plant lovers, but it deserves repeating every now and then.
There is such a thing as too much water. Yes, yes there is. In many cases, too much water causes quicker and more terminal injury to trees and shrubs. All species tolerate saturated soils in different ways. Read planting labels when choosing plants for wet or dry areas. Some trees can tolerate wetter conditions, such as some birches and willows. These same locations would mean certain death for most evergreens. Also, keep in mind that soil conditions can change; new development greatly changes drainage. This has led to many clumps of spruce in backyards to drown.
How can plants drown? Good soil structure requires adequate pore space for air as well as water. Supersaturated soils have their pore space filled with water rather than air; conditions become anaerobic, transpiration ceases and roots rot. Another factor is the soil itself, whether it is sand, loam, or clay. Soils are typically some combination of these. Sandy soils are well drained and those with clay often retain water.
Back to the matter at hand though: the drought. We have discussed how water is important. What happens to plants when water is scarce? Over time, trees have adapted to cyclic rain events. They know that typically in our region, soils will be moist in late winter from the melting snow and low temperatures. In spring, during and after bud break, we receive quite a bit of spring rain. They know summer can get hot and dry, they hope for a few thunderstorms now and again. They also expect cooler temps and more precipitation in the fall. Last year what they knew was thrown out the window. Our winter was mild, our spring was dry, the thunderstorms didn’t develop, and on top of that our summer pushed right into fall. This kind of pattern would stress the most established landscapes, not to mention newer plantings, or those under other stresses already.
The responses of plants to these conditions can vary; they can often attempt to protect themselves during these less than favorable situations. Sensing a lack of soil moisture they may slow transpiration through a variety of methods. Sometimes we see leaf curl, as in the dogwood family that reduces the leaf surface area and slows down transpiration. If water is short during leaf development, very often we see smaller leaves and shoot growth. Some species can regulate their stomata, openings in leaves used in gas exchange. Still others will prematurely drop interior, less important leaves. We saw this a lot in birch species last year. Another coping mechanism may be activation of anchor roots, which tap into moisture reservoirs in the subsoils. Some of these ideas are relatively new and not well understood, but these adaptations may very well be the basis for success in certain species.
What happens to a plant under drought stress? Very often we are amazed that trees “seem” unaffected, when the turf has gone into dormancy and been brown for two months. I will tell you that the tree has put on a brave front. They will attempt to mitigate the stress the best they can; they will use up critical energy stores and slow other metabolic processes. This can lead to tissue death within the root system, as well as the vascular system, and into the canopy. The lack of water has weakened their defenses. They are now susceptible to an array of secondary attacks. Insect pests can now penetrate the bark and attack. Boring insects are opportunistic and take full advantage of lone birch trees and pine forests alike. Most fungal pathogens flourish in wet weather. Unfortunately there are some that only attack drought-weakened tissue. Canker diseases are some of the more common drought induced diseases we encounter in plant health care. The fungus lies in wait until the plant becomes so weak it cannot stop the fungus from entering the weakened tissue. Once inside, the plant has little or no defense. This can either lead to dieback or a very slow death. Under dry conditions, the fibrous roots experience dieback as well. They are now candidates for fungal diseases as well. The scariest part of all this is, we may not see symptoms of the stress created by the drought for a few years. Trees especially can take a longtime to fail; we often use the analogy of construction stress. We often take great measures to save mature trees near construction sites, only to see them deteriorate in three to five years.
Black knot canker on cherry.
What can you do? If you are seeing signs of damage, or know that injury has occurred, do whatever you can to aid in recovery. This will most likely be making sure the planting has adequate water, treating for any insects and disease. Fertilizing a stressed plant may not be the best option, but certainly adding organic mulch would help. If you do fertilize, use a low nitrogen rate fertilizer with a low salt index. You are pretty much attempting to limit as many stresses as possible, and giving the landscape it’s best chance to recuperate.
Going forward, it would be best to plan ahead, as this will certainly not be the last dry season we have. Having your entire landscape evaluated is a crucial part to any plant health care program. Knowing which plants have special requirements, or are prone to insect or disease damage, will allow you to prevent problems before they arise. Pay attention to weather patterns and rainfall in your area. Periodically do a walkthrough checking for changes in leaf color and texture. Check soil moisture and prune dead branches as needed. Look for anything out of the ordinary. Maintain an organic mulch layer under your plants and trees, and water as necessary.
The drought of 2016 was disheartening for all of us plant lovers. We will never be able to prevent the stresses associated with drought, but if we pay attention and take action, we will make a difference. That being said, I’m sure the weather will be perfect this year!
Rob Barrett is the manager of Plant Health Care at Ted Collins Tree and Landscape in Victor, NY.
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