It’s been hot and dry throughout our area this summer – in fact it’s been excessively hot throughout the US. When it gets this hot drought stress can be a problem. As living things, trees need water for survival and even a few days of dry, hot weather can damage a weak or newly planted tree. Water is essential for tree and shrub growth and development. Water maintains all of the physiological processes within plants and is the fluid that moves nutrients from the roots to the leaves. A large tree may absorb hundreds of gallons of water during a hot summer day, but it retains only a few gallons; most of the water is lost into the air by a process called transpiration. When enough water isn’t available, movement of nutrients throughout the tree will be reduced, the process of photosynthesis will slow down causing the tree to lose vigor. Too little water is not the only problem a tree can face. Too much water in soil can have a negative impact on tree health as well. Many homeowners try and compensate for drought and end up adding too much water. Over can damage roots. Damaged roots cannot absorb oxygen as they normally would. Diseases prosper in wet soil conditions and add further stress. Effective water management provides a balance between excess soil moisture and drought conditions.
The timing of watering and amount of water to apply to trees and shrubs depends on the health of the plant as well as the site conditions. Other factors for an Arborist to consider are soil type, soil drainage and prevailing weather patterns. With normal rainfall and good soil conditions, trees will naturally adjust to water availability by opening or closing small holes in the leaf. If natural weather patterns do not supply enough water for your tree, an irrigation system should be considered. As a general rule, most established trees or shrubs require a combined total from rain and irrigation of one to one and a half inches of water per week. This equates to about 1/2 gallon per square foot, 6 gallons per square yard or 620 gallons per 1000 square feet. These rates may need to be doubled for sandy soils. There is a simple way to calibrate your system. Place a bucket in the area to be watered and run the irrigation system for 10 or 15 minutes to measure the depth of the water. This should give you an idea of the total output of water. A typically built-in sprinkler system will deliver about ½ inch of water in 15 minutes. In this case, the timer should be set to run for 15 minutes two times per week.
It is best to run any sprinkler system during the early morning, between 4 a.m. and 8 a.m. This reduces evaporation and reduces foliage-wetting periods. Wet foliage, whether the water is from irrigation or dew, is more susceptible to disease infection. The second best time is in the late afternoon when temperatures are lower and the foliage can dry before sunset.
On new transplants or trees with root damage, a sprinkler system may not direct the water where it is needed the most. In these cases, hand watering directly into the rootball is preferred. This can be done with a hose or with a water container such as a “Tree Gator”. Avoid application of water directly to the trunk for all application methods as this may increase canker diseases.
You may want to consider using mulch as part of your overall tree health plan. Mulch greatly reduces water loss from the soil surface and keeps soil temperature much lower. Two to four inches of wood chips or other coarse organic matter should be used. Mulch should not be in contact with the trunk – but, if you are a regular reader, you already know this!