   Summer Landscape Irrigation - June 14, 2006 Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Proper irrigation of landscape plants is both an art and science. Irrigation distribution, frequency, and amount should all be considered to effectively satisfy plant growth requirements. Since water is both a valuable and limited resource, we should also be mindful of water conservation. Unfortunately, there is not a simple, one-size-fits-all answer to this question. To understand the irrigation needs of plants, we must know something about our soil, site, plant adaptations, and climate. In this column, I will attempt to demystify irrigation and give you some basic tools that will help in estimating irrigation frequency. Many landscape plants used in our area are heat tolerant, but are not entirely drought-tolerant. These plants require moderate levels of irrigation to keep them healthy. Drought-tolerant plants are either native desert plants or plants that have been selected from hot dry ecosystems in areas other than the desert southwest. Once these plants are established, they require little or no irrigation to survive, but can look poor during extended drought periods if not irrigated. There are also native plants that cannot survive without regular irrigation because they are adapted to wetter sites. Soil texture determines frequency and quantity of irrigation applications. Sandy soils have larger pore spaces causing water to soak in quickly. Sandy soils need more frequent irrigation consisting of smaller quantities. Clay soils have small pore spaces that fill very slowly. They should have less frequent irrigation but require greater amounts of water to saturate all the small pore spaces. To decide when and how much water to apply to a given soil, irrigate it thoroughly to an depth appropriate for that type of plant keeping track of amount of water applied. Generally, annuals and shallow rooted perennials have water absorbing roots to a depth of one foot, shrubs have water absorbing roots to a depth of two feet, and trees have water absorbing roots to a depth of three feet. Saturation depth can be tested with a probe made from any long, smooth piece of steel. After the water has soaked in and equilibrated, the tiny pores (micropores) should be filled with water and the larger pores (macropores) should be filled with air. At this time, the soil is said to be saturated to field capacity. To decide the irrigation frequency (time between watering), check the soil each morning before the sun has come up. The check is made by poking your finger or the probe into the soil to determine the depth at which the soil appears dry. Annual and shallow rooted perennials will require irrigation when the soil is dry in the early morning to a depth of one inch. This is the longest period of time that these small plants can go without subsequent irrigation. For shrubs, the dry soil depth is two inches, and for trees it is three inches. You now know the irrigation frequency each type of plant in your garden at that time of year. Why should the soil moisture be checked at predawn? Consider where the irrigation water you apply is used. The plant uses much of it to satisfy its internal needs. This process is called transpiration. The remaining portion is evaporated by the sun. The combination of transpiration and evaporation is called evapotranspiration. At the end of the day, the plants essentially stop using water and evaporation decreases as well. During the night, deeper water moves upward through the soil by a force called capillary action. At night, the soil water equilibrates and the effects of the previous day's evaporation are no longer evident. Irrigation water should be applied to the root zone. We can easily tell the diameter where small plant roots are located. Knowing where a tree root system is located is not nearly as straightforward. Trees that are part of planted landscape and irrigated by homeowners usually only have roots where irrigation water has been applied or where natural water is concentrated. Native-grown trees may have roots a considerable distance beyond the drip line and may be difficult to effectively irrigate. These tree roots can grow two to four times the total tree height away from the trunk and follow routes where water from natural precipitation concentrates. Water is a valuable commodity and we should use it wisely. The irrigation frequency method described above requires thoughtful investigation, but should get you thinking about effective irrigation in the context of your soil and plant species. Remember, to reduce irrigation provided to drought tolerant plants once they are established. This “tough love” approach will conserve water and create a more natural looking landscape. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |