During the late summer and fall, many homeowners see their trees begin to defoliate and wonder if the tree is dead or dying – the answer can be yes, or no! Sorry for the confusion, but sometimes dealing with living things can be tricky; It depends on when defoliation happens and how often. Defoliation during the early part of the growing season (just after leaf expansion) is extremely injurious to a tree. Defoliation and leaf disease become less severe as the growing season draws to a close. If you are seeing defoliation and leaf disease around now, it may not be as bad as you think.
In plants, the leaf functions primarily in the manufacture of sugars and carbohydrates; a process known as photosynthesis. These substances are the basic food or energy sources for all life processes in the plant including growth, root development, flower and seed production, disease resistance, etc. Leaves also provide many indirect benefits such as releasing oxygen, screening out particulates and other air pollutants, intercepting precipitation to minimize erosion and shading the ground to modify surface temperatures.
The timing of the defoliation is critical. Defoliation early in the growing season when leaves just reach full expansion is most detrimental. At this time, considerable energy has been expended in budbreak and leaf development, but food reserves are not yet replenished by photosynthesis. Refoliation usually occurs following heavy defoliation early in the season, which will further weaken the tree. On the other hand, late season defoliation is seldom injurious because leaves have already manufactured and stored most of the needed food reserves.
Effects of defoliation on trees can range from a slight reduction in vigor to total death. Defoliation harms plants by destroying their food producing capability. The refoliation process, which frequently occurs immediately following defoliation, also requires energy, which causes further depletion of stored food reserves. The inability of the tree to manufacture food (energy) together with the depletion of stored food weakens the tree and results in reduced growth, stunted, pale-green new leaves and possibly twig and branch dieback. Generally the greater the amount of foliage removed, the greater are the adverse effects. However, healthy trees with a full crown can tolerate up to 50% loss in foliage without a significant reduction in vigor. Refoliation usually occurs when more than 50% to 75% of the foliage is removed unless defoliation occurs late in the growing season when the tree is entering dormancy.
Most healthy trees can tolerate a single heavy defoliation with only a slight reduction in vigor. Exceptions are evergreens, which are usually killed by one complete defoliation. Two to three consecutive years of early season defoliation can kill even the healthiest trees.
The big problem results from the loss of general health and vigor. Just as humans are more susceptible to disease when they are “run down” so are trees. Secondary organisms can invade a tree after it has been weakened by a stress-inducing factor such as defoliation, drought, etc. Common secondary organisms invading defoliated trees include insect borers, bark beetles, root decay and canker (stem disease) fungi. These are usually responsible for the ultimate death of the plant; it’s not the defoliation, it is the secondary pathogens that attack the weakened plant and eventually kill it.
So, what do you do? First, remember Bob’s first rule: Right tree, right place. If the tree is well suited to its environment it is more likely to survive. But, there are some measures to take if your tree is defoliated. First, watering during dry periods is recommended to aid the refoliation process and fertilization with a quick release high nitrogen fertilizer will also help encourage rapid refoliation as well as help replenish nutrients lost due to defoliation. With defoliation in mid to late season, the effects are not as devastating and fall fertilization with a low salt, well balanced fertilizer is recommended.