Spring Irrigation Tips - April 27, 2011
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

A good definition of irrigation is: the application of water to the soil to satisfy plant survival and growth requirements. To understand the irrigation needs of plants, we must know something about our soil, site, plant species needs and drought adaptations (if any), and climate. In this column, I will attempt to demystify irrigation and give you some basic tools that will help in estimating irrigation needs.

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, they do benefit from periodic watering during times of drought stress. 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, in turn, must be pruned. 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. 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, 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 may have water absorbing roots to a depth of two feet, and trees may have water absorbing roots to a depth of three feet. Saturation depth can be tested with a probe made from any long, thin, smooth piece of steel (a modified irrigation valve key works great). 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 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 a portion 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 plant essentially stops 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/plant system equilibrates and the effects of the previous day's evapotranspiration are no longer evident. This continues until the soil has no more plant available water and must be irrigated again.

Here are a few other irrigation tips. Use organic mulch to reduce water evaporation. Three to four inches of mulch under the plant’s canopy will keep soil cool, reduce water loss, and discourage weeds. Adjust your watering schedule based on: high winds, monsoon rains, winter vs. summer, newly planted vs. established plants. It is best to irrigate early in the morning or in the evening when temperatures are lower than midday, thus reducing evaporation losses.

Follow the Backyard Gardener on Twitter – use the link on the BYG website. If you have other gardening questions, call the Master Gardener line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at cottonwoodmg@yahoo.com and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: April 19, 2011
Content Questions/CApril 19, 2011g.arizona.edu

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