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Ch. 13, pp. 13 - 15

Irrigation has four key components:
  • WHERE to place the water
  • HOW MUCH to apply
  • HOW FAST to apply
  • HOW OFTEN to apply

The factors that effect these components include the weather, plant maturity, plant type, and soil type.
Where: Most of a plant's absorbing roots spread 1 1/2 to 3 times as wide as the plant's canopy and are within 1 foot of the soil surface. It is important to water the entire root zone each time you water. Most of the water is absorbed outside the canopy drip line.
How Much: Apply enough water to wet soil at least 1 foot deep in the entire root zone of the plant. Use a pointed metal rod to test how deeply you have watered by inserting it into the ground soon after you irrigate. The rod will easily slide through the wet soil and become difficult to push when it reaches dry soil.
How Fast: Water should be applied only as quickly as it can be absorbed by the soil. Applying water too quickly causes erosion, wastes the water, and compacts the soil surface.
How Often: The soil should be allowed to briefly dry out between watering. The weather, plant maturity, plant type, soil type, and irrigation method are all key factors determining where to place the water; how much to apply, how quickly to apply, and how frequently to apply water.
Plants in the low deserts of Arizona use 3 to 5 times as much water during the hot, dry summer as they do during the winter. Dry winds also increase a plant's need for water. On the other hand, plants at higher elevations may not need any supplemental water for many months during the winter. In addition, the high humidity and rain during the monsoon season reduce the need for irrigation. Be sure to adjust your watering schedule accordingly.
Plant Maturity:
Young and newly planted trees 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.
Plant Type:
Fast-growing plants need more water than slow-growing plants. Many trees are specifically adapted to arid climates. They may have small leaves, grey foliage, photosynthesizing trunks, leathery or fuzzy leaves, or other characteristics that enable them survive with less water. These trees 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 dike has been built around the root zone of a plant to contain water, then water may be applied quickly without fear of runoff.
Watering Guide

  More Frequent Watering Less Frequent Watering
Plant Maturity/Type    
Growth Rate
Newly planted
Small, narrow, resinous, fuzzy, succulent or leathery

Irrigation Methods

The type of watering method used will determine how long it will take to sufficiently water your plants.
Drip Irrigation:
Be sure to have enough well-placed emitters to cover the entire root zone. Program the system to run long enough to apply enough water to penetrate two feet.
Be sure the basins are level and extend beyond the edge of the canopy. Avoid wetting the root flair by building an inner dam.
Flood Irrigation:
This is an excellent technique for developing deep-rooted trees.
Hoses: A perforated soaker hose spread throughout the root zone is an effective device for watering trees, and can be placed under mulch.
Tips for efficient watering
  • Three to four inches of mulch applied under the canopy will keep soils cool, reduce water loss through evaporation, and discourage weeds.
  • 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.
  • In areas with high salts, leach the salts out of the root zone once or twice a year by watering twice as long as usual.
  • Control weeds and turf grass that compete for your plant's water.
  • Expand the area you water as the plant grows.

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