Trees, like all living things, require water for life. The trees use water during the day when leaves are engaged in photosynthesis and at the end of the day when leaves are not doing photosynthesis, the stomata on the underside of the leaf opens to shed that excess moisture through the process called transpiration. We feel that moisture on the ground as dew in the morning. The entire transportation system in the tree depends on water. It is water pressure that gets the nutrients from the soil up to the highest leaves.
When, why and how to water
Thinking of all the photosynthesis and respiration going on all day in every single leaf, one begins to think there is a lot of water lost by each tree in a day. Trees usually start showing signs they need water by June of each year. For every 18- to 20-degree increase in temperature, the amount of water lost by a tree nearly doubles.
Evening is the best time to water as the there will be less water lost to evaporation and the tree is not as stressed as it is during the heat of the day. Here in Memphis, we have dense clay soils that can hold a vast amount of water compared to less dense soils. With clay soil, we need to be careful not to over water as the roots will rot or become susceptible to disease. In clay soil, there are tiny air pockets and with overwatering the air pockets become filled with water and the roots die as they do not get the oxygen they need. From July to October, trees should be watered once a week, increasing to twice a week during periods of drought and high temperatures.
A soaker hose is best for watering trees in our area’s dense clay soils. Lay a soaker hose in the area where you lay the mulch, avoiding the tree collar area. The tiny feeder roots go out beyond the drip so water no closer than 3 feet of the tree collar and to the drip line. Watering slowly for 2 or more hours is needed for trees.
Arborists do not recommend irrigation systems for watering trees! Frequent light watering encourages roots to come to the surface and when drought or high temperatures occur, the tree will be weakened or die as feeder roots die off from the high temperatures. Shallow roots are more susceptible to freezing in winter. If you have an irrigation system for your grass, come back at night and use the soaker hose to water your trees.
If you are growing the tree in turf, be aware that the dense, short grass roots grab most of the water. When trees are planted in turf/grass, competition for nutrients, water, oxygen and space occurs below ground between turf roots and tree roots. Turf wins because its dense fibrous root system prevents woody plants from getting their feeder roots into the top few inches of soil. For the sake of the tree’s health and your health, eliminate the turf at the drip line and lay mulch in a donut shape up to the tree base. Check out our resource on mulching.