There are so many reasons to love trees! We not only love them for their beauty, but we also need them; they provide oxygen, improve air quality, store carbon, reduce stormwater runoff, stabilize soil, and cool our environment; so, we need to provide them with what they want. Unfortunately, ours have had a stressful decade (or more) as far as weather. Watering them correctly is the best thing we can do to help them grow and be healthy.
One of the challenges with watering trees is when they are growing in a lawn which is irrigated frequently. While it might not have appeared to be a problem in the past, it is different when the trees are stressed. Trees which are watered frequently do not do well – over time it leads to disease. For the trees to be happy, the grass might have to be allowed to get a little dry, but it can handle it, think of all the unirrigated grass areas around the area. Established grass should be watered no more often than once a week and should be able to go for a couple of weeks without any problem. It might not be as green by the end of the second week but the benefit to the trees is worth it. Besides, we are in Stage 2 water restrictions and are currently allowed to water every other week.
If you are watering more often, gradually increase the days between irrigation – but increase the amount of time each area is watered. Deep watering will result in deep root growth and development leading to plants, including trees and grass, which are more drought tolerant. It is important to wet the soil deep down because roots will not grow into dry soil, only into moist soil. Plants in the ground should not be watered daily. Roots need oxygen and if the soil is always wet due to frequent watering, all the little spaces in the soil that would hold oxygen will be filled with water. For all plants it is important to water deeply but not frequently.
Established trees only need deep watering every three to four weeks in the summer and every couple of months for the rest of the year. Two inches of rain would count as deep watering. Experiment with your irrigation system – turn it off and see how long the grass looks good, or at least decent. When it needs water, grass blades will curl a bit (to reduce water loss), turn a slightly bluish color, and footsteps will remain visible for a while after someone walks on it. None of these symptoms are irreversible and it is fine for your grass to show them. Irrigate deeply so the roots will grow deeper and then turn the system off again until you figure out a schedule. Of course, this will vary throughout the year, so plan to adjust it a couple times each year.
We know it is going to continue to be a scorcher of a summer; the shade from trees significantly lowers the temperature. The Texas A&M Forest Service did testing in several areas of the state and showed what a difference a tree could make on surface temperatures. For example, at 2:00 pm in College Station, when the air temperature was 95 degrees, the surface temperature on asphalt pavement was 137, on a lawn in full sun it was 122, and on a lawn in the shade of a tree it was 99 degrees.
We have all experienced the cooling effect of trees just by walking from a full sun area to an area with many trees. This is just one of the reasons people get upset when large trees are cut down – and it takes so many years to grow a large tree! A city is cooler if it has trees and other plants interspersed with buildings and paved streets. One effect of the heat is that the heat dome over a city can prevent rain from falling there – the heat pushes the potential rain clouds up and away from the city.
In addition to keeping our trees happy, we can plant more. Plant medium to large trees 15-20 feet from the house on the west side to shade the walls and roof of a one-story house. Shading the roof can reduce the indoor temperature by 8-10 degrees. Smaller trees, or large shrubs, can be placed closer to provide additional shade to the walls and windows. Plantings that provide shade for the outdoor part of an air conditioner also save energy, just be sure to allow space for service access.
Bottom line? Take good care of your trees by watering them correctly and think about planting more!