The monsoons have arrived, and are dumping water in their usual, irregular pattern. For example, the storm on July 22 dropped 0.31 inches on Pusch Ridge, 0.28 inches in Oro Valley, 0.24 inches at Ina & Thornydale, and across the freeway at Ina & Silverbell, 0.04 inches. (Data courtesy of Thomas Helfrich of the Pima County Flood Control District.) This random rainfall means that you may still have to water your plants.
There is no hard and fast rule on when to water your plants. Requirements vary according to plant species, nativity, age, size, health, soil, exposure, location, weather, and more. The good news is that there are some basic watering guidelines which help conserve water while maintaining plant health and beauty.
Water deeply, slowly, and infrequently. This helps your plants grow extensive root systems that are deeper and better able to gather moisture out of the soil. The better developed the root system, the more drought tolerant plants become, since they draw water from a larger area. Frequent, shallow, watering turns your plants into water junkies -- they will need their "fix" every day.
If you are wondering how deep and wide root systems can be, let's look at some wild plants. Grass of the prairies has roots that go deeper than 8 feet. In one study, the roots of mesquite were found at a depth of 30 meters (more than 100 feet), while a saguaro may have roots spread around its base to a distance of twice its height.
There are roots, and then there are roots. Plants have roots that are covered with bark and are needed to support the plant. At the ends of the thicker support roots are water gathering roots. Water gathering roots are slender and not covered with bark. Instead they have numerous fine root hairs that absorb water from the soil. A desert native may conserve water by having no active water gathering root hairs until the rain falls, then they rocket into action and grow hairs within hours to grab the soil moisture.
Desert plants differ in their ability to pull moisture out of the soil. Some continue to pull moisture out of the soil long after other plants have quit. Creosote bush is the prime example. On the other end of the spectrum, the brittle bush gives up the pulling contest early and can turn into a crispy critter without water at least once every few weeks. Just because a plant is a desert native doesn't mean it is drought tolerant. Sabino Canyon is part of the Sonoran Desert, yet if you plant riparian trees that grow there in your yard, they will not survive without a steady source of water.
In all plants, the majority of roots spread out close to the surface to help support the plant. Water gathering roots may also be close to the surface to gather shallow desert rainfalls. It is your job to teach them to go wider and possibly after the water they need. That need will vary by species.
Irrigate trees and shrubs longer and less frequently than shallow rooted plants. New plantings will need more frequent watering. In past lectures on this subject I have been misunderstood on this point. New plantings may need water every single day in the summer, established plants will not. Ideally, any landscape planting should be done in the fall -- September and October, to give plants the winter and spring to become established prior to summer conditions. It may still take two or even three years for plants to become fully established. They may need extra water in their second year in the ground. I have seen plants grow virtually not at all for about two years, then take off. Once plants are established, they may not need supplemental water unless we have another 100 days without rain.
In general, let the soil dry between waterings. Not so dry that it becomes hard as a rock, but dry enough that it doesn't feel moist to your finger when tested just below the surface. Water slowly so the moisture seeps into the soil. As a general guide, check the soil moisture after one hour of watering. Water trees to a depth of 3 feet, shrubs to 2 feet, groundcovers and lawn to a depth of 1 foot. If it takes less time than this to get the water that deep, you are watering too fast.
How to tell if the soil is wet enough? A soil probe will tell you. A soil probe can be an unwound wire coat hanger, a long narrow screwdriver, or other metal bar. You need at least a foot, though three is ideal. My probe is a length of narrow rebar with a bicycle handlebar handgrip epoxyed on. Soil probes sold at nurseries and irrigation stores are ergonomically better for your back muscles and rotator cuffs. The probe should pass through wet soil and become difficult to push when it reaches dry soil.
Alternatively, you may also have to not water your plants. Over-watering can stress plants by physically drowning their roots. This leads to decay and disease problems. Some types of sod are particularly susceptible to over watering. If your part of town got a good soaking rain, shut off your irrigation system for two to three days. The soil probe will help you determine if your yard got enough rain.
For more information on wise water use, the Tucson Water Conservation Office offers free help, even if you live outside city boundaries. Booklets are available by calling the office, or they do make house calls. The Zanjero (zan-heero) program includes a visit by a certified landscape auditor who will check your irrigation system, and pipes and fixtures inside your home. Call 791-4556 (TTY# 791-2639), or visit the website at http//www.ci.tucson.az.us/water/
Jacqueline A. Soule at the Tierra del Sol Institute always welcomes your calls for landscape and gardening questions at 292-0504. She will return calls after August 20 as she is currently out of town.