Cochise County Cooperative Extension Home Horticulture
Environmentally Responsible Gardening & Landscaping in the High Desert 

Research conducted over the past two decades has caused the University of Arizona, the International Society of Arboriculture, the American Forestry Association, and other arborist and horticultural organizations to revise guidelines for irrigation and planting techniques for trees and shrubs. Knowing when to water and how to water plants is one of the most important aspects of gardening and also one of the most complex. A great portion of Arizona is either arid or semi-arid and ever-increasing demands are being placed on our limited water resources. As water for landscapes and gardens utilizes roughly 60% of the total water consumed by residents, it behooves us all to make wise decisions about the use of this water for irrigation purposes.

Irrigation has four key components:

bulletWHERE to place the water
bulletHOW MUCH to apply
bulletHOW OFTEN to apply

The factors that effect these components include:

bulletPlant Maturity
bulletPlant Type
bulletSoil Type

Where: Most of a plant's absorbing feeder roots spread 1 1/2 to 3 times the width of the plant's canopy. It is important to water the entire root zone each time irrigation is applied to maintain a healthy, well-distributed root system. Most of the water that plants absorb is outside the canopy drip line not at the base or trunk of the plant. The best time to water is in the morning or evening when air temperatures are lower than at midday, reducing evaporation. In the evening do not wet foliage; this can encourage the growth of fungus or mildew, making plants unsightly and jeopardizing their health.

How Much: Apply enough water to wet soil at the proper rooting depth in the entire root zone of the plant. Use a pointed metal rod, commonly called a soil probe, to test how deep water has been applied by inserting the probe into the ground soon after irrigation. The rod will easily slide through the wet soil and become difficult to push when it reaches dry soil. Once it has been determined how long it takes to wet the soil to the proper rooting depth, water this same duration every time irrigation is required. Frequent, shallow watering encourages a shallow root system, an unstable plant, and wastes water. Studies show that the rooting depths for the following plants are: 12 inches for annuals, vegetables, and lawns; 12-24 inches for perennials and shrubs; and 28-36 inches for trees.

How Fast: Water should be applied only as quickly as it can be absorbed by the soil. Applying water too quickly causes erosion, wastes water, and compacts the soil surface.

How Often: The soil should be allowed to briefly dry out between watering to supply plant roots the oxygen they need. Use the soil probe every few days to test the soil until the probe will not penetrate any deeper than 4 6 inches of soil then reapply water to the proper rooting depth. Three to four inches of mulch applied under the canopy will keep soils cool, reduce water loss through evaporation, and discourage weeds that compete for the plant's water.

Weather: Plants use more water during the hot, dry spring & summer months. Dry winds also increase a plant's need for water. Winter rains usually supply sufficient moisture to sustain plants through the winter months unless its unseasonably dry. In addition, the high humidity and rain during the monsoon season reduce the need for irrigation. Be sure to adjust the watering schedule accordingly.

Plant Maturity: Young and newly planted plants should be watered more often than older and established plants. Initially, water should be applied to the root ball and as the roots mature the water should be applied out at the drip line and beyond. After they become established, in one to two years, allow a slight drought between watering. The plants will adapt to the stress and become more drought tolerant. Generally, established plants that are properly irrigated and mulched should not require watering more than once every two to four weeks.

Plant Type: Research and select plants according to their horticultural requirements that match the site. Fast-growing plants need more water than slow-growing plants. Many plants are specifically adapted to arid climates. They may have small leaves, gray foliage, photosynthesizing trunks, leathery or fuzzy leaves, or other characteristics that enable them survive with less water. These plants need considerably less water than less well-adapted species with large, dark green leaves.

Soil Type: Sandy soils absorb water more quickly and drain more rapidly, therefore, water can be applied faster and must be applied more frequently. Clay soils absorb water more slowly and retain water more effective; therefore, water must be applied more slowly and less often. Water applied too rapidly to clay soils is wasted and causes erosion as it runs off. If a berm has been built around the root zone of a plant to contain water, then water may be applied quickly without fear of runoff.

Feature article for the Sierra Vista Herald/Bisbee Daily Review
Sunday, July 11, 1999

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