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
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
it’s unseasonably dry. In addition, the high humidity and rain during the
monsoon season reduce the need for irrigation. Be sure to adjust the watering
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.