Outdoor plants must thread their way through many hazards during the hot months of summer in order to survive, but perhaps the most common, and most deadly, is simple dehydration.
Plants, as well as animals, must maintain vital levels of water within their systems to sustain life functions. Both desert adapted and imported plants will benefit from the addition of water although different species, according to how well they are adapted to desert environmental conditions, will require differing amounts of the life-giving fluid. Since excesses can be just as damaging to plants as deficiencies, proper irrigation becomes a top priority for desert gardeners at this time of the year.
Plants require water for several purposes. First, it is important to always remember that all living things, plants as well as animals, are largely composed of water. If water levels become deficient in living plant tissues, plant vitality will be, at best, diminished. In our busy world, it is easy to forget to turn on the faucet or properly adjust the timer in time to prevent serious injury to plants. Because of this, many plants suffer serious summer water stress damage.
Second, plants require water for cooling. The process of transpiration allows the plant to absorb water from the soil through the roots and transport it to other parts of the plant through special water conducting tissues. Some of the water reaching the leaves eventually evaporates and drifts out through tiny holes, called pores, into the outer atmosphere. This process of evaporation and diffusion cools the plant. Water deficient plants will have a warm feel to the leaves while non-deficient plants will feel cool to the touch.
Third, water is essential for proper plant nutrition. If root absorption and transport of water and nutrients doesn’t keep pace with the demands and consumption of the plant, the plants begin to heat up, wilt, and starve. Water is the medium that transports the nutrients from the roots to the leaves where most of the nutrients are used in the process of growth and in the formation of seeds.
The most common symptom of dehydration in Pinal County is leaf dessication, or wilting or drying. Symptoms include browning, blackening, and sometimes bleaching of leaf tissue that gradually progresses from the tips and edges to between the veins of affected leaves. In most cases, little can be done to improve the appearance of these tissues once damage has occurred. However, new damage can be prevented by maintaining good soil moisture and fertility in the future.
Most trees and shrubs will endure moderate amounts of leaf desiccation before they begin to show symptoms, although unseen damage in the form of loss of plant vigor and vitality can occur. Generally a light fertilization followed by deep watering will restore plant vigor.
Leaf wilting or drying usually appears first on inadequately-rooted or marginally-adapted plants.
Shallow or limited rooting is very common in many landscapes and plants with this problem will show dessication symptoms quickly. The quick onset of these symptoms can be noticeably worsened by the hot, dry winds of summer.
Shallow-rooted trees such as the cottonwood and the mulberry, tender-leaved species such as the silk oak and the ash, and marginally adapted plants such as the rose are among the first to show dessication symptoms. Plants sitting on dry, compacted, salty, infertile, or chemically toxic soil are also often affected. Root-binding and caliche layers are other factors that can enhance these problems. Mature, older leaves usually show the symptoms first because of their longer exposure and because plants give nutrient priority to new growth.
When correcting plant dehydration, it is important not to go overboard. Over-watering can be just as hazardous as under watering. The most common disease problem caused by over watering is root rot caused by the water mold root rot fungi. Most of the water mold root rot fungi, like Pythium, Phytophthora and Rhizoctinia, do not work as fast as their first cousin, Texas or cotton root rot, but all can be devastating to plants. These fungal diseases can occur at any time in any season, although they are most commonly seen during the warm summer months. In moist soil environments, the disease organism proliferates and quickly engulfs the roots of susceptible plants. Many types of plants can be affected and death can occur within days of initial infection.
Root rot can, in most cases, be prevented by proper watering techniques. Do not water on a set schedule year round because plants require less water during the cool months than they do during the warm months. Watering at a summer rate in the winter will give the plant too much water than can lead to disease. Watering plants in the summer at a winter rate can quickly lead to dehydration. It is important to change the length of time that an irrigation period will run, and the number of times each week that water is applied, as the seasons change.
Varying the irrigation schedule so that plants get more water in the heat of the summer and less water in the cool of winter will go a long ways in preventing disease problems. For the same reason, it is also important to know the soil moisture conditions around trees and shrubs before the next irrigation is applied. If the soil is still wet when the next irrigation is made, disease may occur. Check the soil with a soil auger, shovel, or screwdriver to a depth of six inches and feel the soil with a hand to judge the amount of water in the soil. If the soil is still moist, hold off watering. If it is dry, give the plant a deep watering sufficient to fill the entire rooting zone and to leach out harmful salts.