Regional Gardening Reports :: National Gardening Association

In the Garden:
Southwestern Deserts
February, 2009
Regional Report

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The two drip emitters are too close to this mature tree trunk. The feeder roots that absorb water have grown far beyond the canopy edge.

Effective Plant Watering and Drip Irrigation, Part I

Did you know that more landscape plants in the desert die from overwatering than underwatering? Some folks think that because they have a drip irrigation system, they must of course be conserving water. Unfortunately, that isn't always the case. The design and installation of an irrigation system must work within the framework of recommended plant watering guidelines. In addition, the irrigation system needs to be monitored and adjusted as plants grow and seasons change.

The following info is a basic primer on how to water plants for maximum health without overwatering. My next report will describe specific components of a drip irrigation system and how they are interconnected with efficient watering.

There are three basic questions to answer when determining how to water plants effectively: how deep to water, how frequently to water, and where to apply water. All depend upon the drip irrigation system's design and capabilities (highlighted in parentheses with each heading below).

1. How deep water should penetrate (related irrigation design: controller's scheduling flexibility, number of valves, and number of emitters)

Water must soak deep enough through the soil to reach a plant's "feeder" roots, which are small, hair-like roots that absorb water. Depending on plant type, the majority of feeder roots grow in the top 1, 2 or 3 feet of soil. An easy guideline to remember is the 1-2-3 Rule. Water should soak:

--1 foot deep for shallow-rooted plants (groundcovers, annuals, perennials, cacti and succulents)
--2 feet deep for shrubs.
--3 feet deep for trees.

A soil probe can be used to determine how far water has penetrated within a set irrigation time.

2. Where water should be applied (related irrigation design: number and placement of emitters)

Water should be applied around a plant's entire circumference, at the canopy edge (also called the dripline). As a plant matures, feeder roots grow outwards horizontally, keeping pace with the expanding canopy. Thus, drip emitters need to be moved outwards as a plant grows. Emitters left at the base of a maturing plant are of no use because the feeder roots are not there to absorb the water.

3. How often to water (related irrigation design: controller's scheduling flexibility and number of valves)

Horticulture and water conservation experts recommend at least 4 seasonal changes for irrigation schedules. The basic guideline regardless of season is to always water deeply (1-2-3 feet), but as infrequently, as possible. Drip irrigation systems are often mistakenly programmed to do the opposite: they run frequently for short time periods (i.e, shallow watering).

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