   Irrigation - May 31, 2000 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County How should I water my plants? How often should I water? Where should I water? Clients ask me these questions frequently. However, they cannot be answered with a simple one-line response. 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 needs. The dictionary defines "irrigation" as: supplying water to land through artificial means. I like this following definition better: application of water to the soil to satisfy plant growth requirements. Too little irrigation reduces yield and quality of crops. Too much irrigation can leach nutrients from the root zone and displace soil oxygen needed by living roots. In my experience, most drought tolerant landscapes are over irrigated. Once drought tolerant or native plants have become established, they seldom need regular irrigation. However, many irrigation systems are on automatic timers scheduled to water these landscapes every day. This defeats the purpose of using low water use plants and causes excessive growth which they must prune. The result is excessive use of water and a plant that has lost its natural form. Conversely, vegetables, flowers, and other poorly adapted plants require regular irrigation. For these types of plants, soil texture determines frequency and quantity of irrigation applications. Sandy soils have larger pore spaces causing water to soak in quickly. Sandy soils need frequent irrigation 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, water it thoroughly to an appropriate depth for that type of plant keeping track of amount of water applied. Generally, annuals and small 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 into the soil to determine the depth at which the soil appears dry. It is time to irrigate annuals and small perennials when the soil is dry to a depth of one inch. For shrubs, the dry soil depth is two inches, and for trees it is three inches. You now know the irrigation frequency and quantity for 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 stops using water and evaporation ceases as well. During the night, deeper water moves upward through the soil by a force called capillary action. At night the soil/plant system 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. Tree roots often grow two to four times the total tree height away from the trunk. For example, a thirty foot-tall tree could have roots from 60 to 120 feet away from the trunk. This is just not feasible to irrigate and truthfully, these types of tree are growing these roots to mine water from this large area. In practical application, one should certainly irrigate beyond the drip line and definitely not irrigate on the base of the trunk. Shrubs follow similar rules to trees. This long winded explanation of irrigation only scratches the surface (so to speak). Seriously, the method presented here is highly simplified, but it works in most cases. Water is a valuable commodity and we should use it wisely. The University of Arizona Cooperative Extension has publications and information on irrigation and vegetable crops. If you have other gardening questions, call the Master Gardener line (extension 14) in our Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. The Yavapai County Cooperative Extension web site can be accessed at http://ag.arizona.edu/yavapai/. |