Summer temperatures are here! How are your plants doing?
Every year a large number of local trees, shrubs and other plants are heavily damaged from the heat and dryness of the summer months. Arizona’s hot, dry summers are tough on plants, especially on non-native trees and shrubs that are not particularly adapted to desert conditions. Even native trees and shrubs, if they are not properly cared for, can be injured by the desert heat. There is hope, however. Proper planning coupled with good plant care can go a long ways in preventing much of the damage. Here are a few suggestions to help protect plants from hot weather injury.
First, the best defense against heat injury is a healthy, actively growing plant. In this climate, proper watering is an absolute key to maintaining the health of trees and shrubs during the summer.
Most water absorbed by plant roots is used for transpiration, the loss of water vapor through the leaves. At the end of its trip through the plant, water enters open spaces between the cells of the leaves where it evaporates and exits the plant through tiny holes in the leaf called stomata. This movement of water vapor out of the plant acts much like an evaporative cooler by removing excess heat and leaving the leaf tissue cool to the touch. Supplying enough water to keep the transpiration process going is a critical step in protecting landscape plants in the desert. Insufficient water at any time during the growing season can seriously damage plants.
To make sure that plants receive enough water, irrigate properly. Schedule irrigations frequently enough to maintain sufficient water for the plant at all times, but infrequently enough to let air return to the soil and give roots a breath of fresh air. It is especially critical that regular, deep irrigations begin when the thermometer approaches 100 degrees F. on a regular basis.
All roots require both water and air to properly function. During the summer months, trees and shrubs should be flood irrigated with the hose every 5 to 10 days depending upon their water use needs. High water users like mulberries and ash trees may need to be watered more frequently than those with moderate requirements like citrus or low water requirements like mesquite and palo verde. With a drip system, irrigations will have to occur more frequently, perhaps every other day or so depending upon the type of emitters used.
The best way to determine irrigation frequency, which is influenced by the temperature, type of soil and the evaporation rate of water from the soil surface, is to dig down about six inches into the ground and pick up a handful of soil. If the soil remains in a hard ball after it has been squeezed, it probably is still moist enough to support plant growth. If, however, the ball begins to crumble when the hand is opened or if the soil is starting to feel dry, it is time to water. With this simple test, anyone can properly determine the correct irrigation frequency for the specific conditions in their own yard. Don’t forget that the irrigation frequency that supports proper plant growth in the winter and spring will probably not be sufficient in the summer when temperatures go up dramatically.
Make sure that the water applied to the plant wets the entire area from the trunk or stem out to at least, and perhaps beyond, the drip line of the plant. Adding enough water to move the lower edge of the wetted zone deep enough to reach the lower roots is also critical.
Fertilizing during hot weather should be done cautiously. The increased activity of life processes during the warm months can use up nutrient reserves faster, but a too-rapid uptake of fertilizers by summer-active roots could result in fertilizer burn. A good rule of thumb for fertilizing during the hot season is to increase the fertilization frequency, but decrease the amount of fertilizer applied each time. Fall fertilization helps plants recover from summer exhaustion, while spring fertilization helps encourage strong growth to better withstand summer stress.
Second, protect the roots of heat sensitive plants by mulching the surface of the soil with an organic mulch. Mulch spread over soil surfaces under plants provides a practical insulation against summer heat. Mulches also reduce evaporation of water from the surface of the soil. Not only does this contribute to a more efficient use of water resources, but it also helps minimize the buildup of toxic salts at the soil surface.
Mulching can pose a danger, however. Since surface mulches retain soil moisture longer, the intervals between irrigations must be lengthened to ensure the proper balance of water and air in the soil. If not, over watering problems like root rot and iron chlorosis could occur. These problems could seriously damage the health of the tree. Again, the hand feel method of testing soil moisture levels is a great way to help determine the timing of irrigations.
Finally, avoid the temptation to use drainage water from evaporative coolers and swimming pools as well as water that has been through a water softening system. Each of these water sources carry excessive salts that could damage tender plants.
Protection of plants from summer injury generally boils down to proper irrigation and fertilization. Just a little extra care now can save serious injury later on.
For more information on how to protect trees and shrubs from summer injury, ask for a copy of the Cooperative Extension leaflet, “Hot Weather Effects on Landscape Plants”. If you have questions, you can reach Rick Gibson or one of the Master Gardener Volunteers at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande. The telephone is (520) 836-5221. The author’s email address is gibsonrd@ag.arizona.edu.
The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.
Rick Gibson
Extension Agent, Agriculture
University of Arizona Cooperative Extension
820 E. Cottonwood Lane, Building C
Casa Grande, Arizona 85222
Voice: (520) 836-5221
Fax: (520) 836-1750
email: gibsonrd@ag.arizona.edu
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