As living things, trees need water for survival and even a few days of dry, hot weather can be damaging to 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 moves nutrients from the soil to the leaves. A large tree may absorb hundreds of gallons of water during a sunny, summer day, but it retains only a few gallons. Most 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 and the tree will begin to decline. Too much water in soil can have a similar impact on the plant. In this case, water is not available in the leaves because nutrient movement in the plant is reduced by root rot and “water-logged” roots. Damaged roots cannot absorb oxygen. Root diseases prosper in wet soil conditions adding further stress. Effective water management provides a balance between excess soil moisture and drought conditions.
With normal rainfall and good soil conditions, trees will adjust to water availability by opening or closing small holes in the leaf. These openings are called stomates. When trees can not regulate or adapt to the moisture levels irrigation may be necessary. Proper Irrigation is beneficial for all new transplants and for trees with root damage. Watering should start when soil moisture levels drop to a critical level and end when the soil is recharged. Of course, this can be hard to determine. 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 or 6 gallons per square yard on a soil surface area basis. There is a simple way to calibrate your system. Run the irrigation system for 10 or 15 minutes and measure the depth of the 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. During severe droughts this can be increased to 20 minutes.
Timing of irrigation is important as well. It is best to run any sprinkler system during the early morning, between 4 a.m. and 8 a.m. This reduces evaporation, increases efficiency and reduces foliage-wetting periods. Wet foliage, whether the water is from irrigation or rain, is more susceptible to disease infection. Try not to water in the evening – this can leave the foliage wet for over 9 hours and lead to leaf disease
On new transplants and trees with root damage, a sprinkler system may not be the best method because it may not get water where it is needed the most – into the rootball! 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.” Transplants less than one inch in diameter grow well with 10 gallons of water applied once per week. For one to two inch diameter trees, 20 gallons should be applied. Avoid application of water directly to the trunk for all application methods as this may increase canker diseases.
Finally, Mulch. Mulch greatly reduces water loss from the soil surface and keeps soil temperature much lower. During drought years, mulch is an excellent means of keeping plants healthy with less irrigation. 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 Arborist‘s View reader, you already know this!!
As always, I hope you learned something!