Maintaining Landscapes During Heat and Drought
University of Arizona Cooperative Extension
Arizona’s desert regions are a challenge for cultivating plants in the best of times. When a heat wave or drought comes to the desert, many plants can be pushed over the edge. These are times when some extra attention and care can get them through the worst of the weather.
Prioritize your trees and shrubs. Extreme heat is a form of abiotic stress and will negatively impact plant health and the efficiency of normal processes such as metabolism and respiration. Flowers and vegetables will wilt and turn yellow, but don’t exhaust your time on short-lived plants. Trees and shrubs are long-lived investments. If your trees and shrubs are established in the landscape, it shows they have weathered a few summers already. If they are suffering now, it means the weather has pushed them to their limit, or something else has changed in their environment. If trees and shrubs die back or die completely, they are not easily replaced. Removal can be difficult and costly. For these reasons, it is important to monitor your woody species such as trees and shrubs during periods of intense heat and/or prolonged drought.
Water Effectively. Apply water until the soil is wet to the depth of your plant’s root system. Wet the soil to a three-foot depth for trees. Wet the soil to a two-foot depth for shrubs. Water to a one-foot depth for smaller plants. Use a soil probe or a rod to measure water infiltration into the soil. Push the probe into moist soil and it will pass through smoothly until it reaches dry soil. This measures the depth to which you have watered. Avoid taking measurements directly at the crown of any plants so that you don’t inadvertently injury primary roots. For vegetables and wildflowers you want to be able to probe the soil easily to about 12 inches after irrigation. For shrubs and vines the 18-24 inch range is a good depth for water infiltration to reach the roots and be better stored in the soil. For trees it is best to ensure that your irrigation is delivered slowly enough to reach 24-36 inches into the ground. Water the correct area. Tree roots do not gather water close to the trunk of the tree. The feeder roots are farther out, at and beyond the reach of the tree’s branches. Large trees may extend roots into neighboring yards.
A critical element of proper irrigation during our hot summers is timing. It is advisable to water early in the morning, pre-dawn even. If you have an irrigation system on a timer this is easily programmable. It is important to apply the water to the soil before the heat of the day, when rapidly warming soil surfaces can begin to dry and wick moisture out of the root zone. Watering early ensures that the water moves deeper into the root zone, where it can be retained and is accessible by plant roots for longer. For established trees and shrubs you might begin irrigating anytime after midnight and may need to apply water at a slow and steady rate for multiple hours to ensure water infiltrates to the roots at a depth of 2-3 feet.
Water for the season. The amount of time needed to wet your soil to the appropriate depth will vary depending on your soil type and the way you deliver water (by hand or by irrigation system). The amount of time needed to wet your soil will stay the same in all seasons. The frequency you must do your watering will change by season. You must water more frequently (but not for a longer time) during the summer, and less in the spring, fall, and winter. If you use an irrigation system, the irrigation clock should be adjusted at least four times a year to reflect this. In an average year the end of the monsoon season occurs in mid-September and will require adjusting your watering practice to compensate for the tapering rains. In 2020 there has been a record shortage of monsoon precipitation.
Follow Landscape watering guidelines. With irrigation to home landscape and gardens accounting for up to 70% of average residential water consumption, there is never a bad time to make sure that you are delivering water judiciously and effectively to your plants. With lingering heat and diminishing monsoons this is a great time of year to check the efficiency of your irrigation schedule, and the working condition of your system itself. Simple guidelines are available to show how frequently to water your landscape plants:
Bear in mind that some trees and shrubs need more frequent watering than others. Fruit and nut bearing trees have high water needs, while desert-adapted Arizona native trees generally require less watering.
Check your irrigation system. If you use an irrigation system, check that it is running correctly. Examine emitters and bubblers to observe they are delivering water. Drip emitters are great for precisely and conservatively delivering water to crops and plants, but can become clogged by soil, mineral deposits from the water itself, or chewed by rodents. Examine the clock to ensure it is running and scheduled correctly. Look for leaks in the system. If your landscape plants have grown significantly since they were planted, your irrigation system may need to be upgraded to support the water needs of larger plants. Drip emitters may require re-positioning over time too, to accomodate the expanding root zone of a maturing plant.
Don’t ignore plants beyond your irrigation system. You may have trees and shrubs on the edge of your property beyond your irrigated zone. These may be desert-adapted plants, but consider giving them a watering by hose to help them through a hot dry summer.
Be wary of reclaimed and gray water. If you water with reclaimed or gray water (such as output from a washing machine) the extra salts in the water are not beneficial when plants are under heat stress. Do not discontinue applying the reclaimed or gray water, but consider supplemental watering with tap water (or rain water if it is collected) to help wash away salt buildup.
Don’t work on your plants. Do not plant, transplant, prune, trim, shear or deadhead plants during a heat wave. All such disturbances damage plant tissues and may expose plants to an increase in sun exposure. Do not move plants in pots unless to a shadier site. Do not dig or trench the soil around plants, as this can damage roots. Apply no chemicals, neither fertilizer, herbicide nor insecticide to plants when they are heat stressed.
Move pots into the shade. Plants in pots may be moved to shadier areas, but be wary if the plants have grown roots through the drain hole of their container and into the soil. It is best not to break such roots in the summer. Instead bring shade to the potted plant by using shade cloth. You may consider bringing special potted plants indoors, but this can result in too little light for their needs. The indoor strategy works better for plants which have a natural summer dormancy, such as winter-growing bulbs.
Use shade cloth. Horticultural shade cloth may be draped over plants to provide extra shade. This is especially helpful for cacti and succulents which are turning yellow, and extra water is not the answer for these plants. Use 30% shade cloth, which will create 30% shade and allow 70% of the sunlight to pass through. Heavier shade cloth will deprive plants of light and may result in sunburn when the cloth is removed later in the season. Consider constructing a framework to hold the shade cloth over the plant to allow air circulation between the cloth and the foliage. Some gardeners use white nylon tulle mesh fabric, both as shade and as protection for seedlings that might be eaten by birds.
Horticulture shade cloth protecting garden plants
Tulle fabric can substitute for shade cloth
Creosote branches can be used to shade small plants
When growing plants in the Sonoran Desert remember that irregular rainfall patterns are a natural characteristic of the desert climate. However, recent decades have shown a notable increase in summer temperatures and a decrease in yearly precipitation. These long-term changes in climate may render some growing choices no longer feasible in a warmer drier climate. Desert cities are growing more aware of the demands on water supply as cities grow and precipitation dwindles. Watering more is not always the answer. The University of Arizona Cooperative Extension and many Arizona municipalities are offering guidance for plant choice and landscape management strategies that can keep our desert cities green, while continuing to conserve precious water.