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Tree Irrigation: Water Where Roots Need It

By Joseph H. Fortier, CID, CLIA and Jerry M. Belt, Jr., CLIA, Mojave Water Management, Las Vegas, Nev.

Drip emitters are easily removable and replaceable in this set-up plan, which puts individual emitters on lines attached to main water lines. This toyon shrub gets one outlet that emits 20 gph--larger trees require more emitter stakes. Several algebraic formulas can help planners figure how many units a tree requires.

These nursery-grown redwoods in Southern California show the limits of drip technology. They're watered almost daily, but struggling with much higher sun levels than their native north coast. The California Cooperative Extension Service ( offers info on tree selection in Western climate zones. Photos by Erik Skindrud

Trees' irrigation requirements are very different than those of turfgrass, shrubs and flowing plants. Trees need deep watering, and are much better suited to drip irrigation than more shallow-rooted plants.

These are facts that nursery growers know well. Contractors who don't operate nurseries should keep the importance of irrigation in mind too, to avoid callbacks when planted trees fail.

A basic understanding of the structure of a tree's root system is critical. Like all plants, they "breathe" oxygen from the soil through their roots. The highest oxygen content in the soil is in the first 24 inches, therefore, major feeder roots of trees are usually found in the top 18 to 24 inches of the soil. Since this area contains the most oxygen, the most active and beneficial tree roots are found here. Furthermore, water, nutrients and oxygen are absorbed most readily in this range.

How far beyond the drip line the roots spread horizontally, and where within this vast network of roots the most water is absorbed, are closely related. Generally speaking, tree roots will extend somewhere between one and one-half, to four times the diameter of the tree's canopy. Two-thirds of the water is usually absorbed beyond the drip line.

Heave ho! This olive tree (Olea europea) is headed to an uncertain future as it moves to a new home. Serious problems can develop when trees planted in turf areas are moved, because irrigated turf trains roots to spread near the surface. On the other hand, olive is a drought-hardy species that fares much better in summer heat than other trees. Photo by Guy Nelson

Compaction Poses Problems

A thorough understanding of compaction is critical to designing an irrigation system that meets the water needs of a maturing tree. Compounding this problem are the typical situations found in most urban plantings--narrow beds, parking lot planters and streetscapes. Most tree roots will not grow in the compacted, under-watered, oxygen-poor soils too often found in these situations.

In an ideal situation, the irrigation designer will have input on the planting details for all installations. A sound irrigation design and proper planting will help ensure trees receive sufficient irrigation in order to develop healthy root systems. Well-developed root systems will thus produce healthier trees with fewer diseases and insect problems. Strong roots also provide anchors to prevent trees from blowing over in a strong wind.

While creating a well-developed root system in urban settings is difficult to achieve, there are innovative ways to accomplish the objective.

A more serious problem sometimes exists in a retrofit situation where an existing spray irrigation system (as used for turfgrass) is replaced with a low-volume irrigation system.

Arborists have long debated the appropriateness of planting trees in turf areas. An intelligent irrigation designer will leave this age-old debate to the arborists, and determine whether the sprinklers for the turf will provide sufficient water.

Trees planted in turf areas that receive water from overhead sprinklers are usually shallow rooted and compete with the turf for available water and nutrients. Most turfgrass roots are shallow, fibrous, and absorb water more quickly than the trees. This leaves trees with the table scraps, instead of the main course. To prevent this from happening, a separate irrigation system should be designed for the trees to provide the water to their deeper rootzones.

Two bubblers and two drainage tubes are assigned to each date palm planted in July at a Southern California shopping center. Seen here after a cycle, this example of good installation shows no runoff to adjacent areas and limited pooling as the water soaks in via the 4-inch-diameter PVC drainage tubes.

Dale Skidmore and Dave Lockridge (left) look over drip-system-grown trees at Shadetree Nursery. The cooperative venture produces drought-tolerant species tailored to Southern California. Most of these trees will need irrigation help during an establishment period but should do fine without artificial water after a year or so. Photos by Erik Skindrud

Establishment versus Maintenance

As simple as it sounds, identifying the purpose of an irrigation system is the most important design consideration. Is it designed for establishment, and/or is the system designed to supply regular water to the tree as it matures? The climate typically dictates the type and capacity of the system. When designing an irrigation system simply to help the tree become established, temporary irrigation with overhead spray, bubblers or drip will usually be sufficient.

If a system is designed to provide regular irrigation, the water needs of the trees as they mature must be considered. In areas that receive little rainfall during the growing season, irrigation systems should be designed to provide sufficient water to the trees during initial establishment, and to encourage development of healthy, vigorous root systems as they grow.

The next challenge is to estimate the water needs of a particular tree. This can be tricky because sufficient scientific data regarding water use is not available for most species. Complicating matters even further, different species have vast differences in their watering preferences.

Soil type and climate conditions will also influence irrigation scheduling and water usage. All too often, enough water is provided for the tree when it is young, but as it matures it becomes stunted or stressed. The California Cooperative Extension Service has spent thousands of hours working with horticulturists and landscapers to develop a simple method for estimating plant water needs in California. Their work is a good starting place for much of the Southwest.

A podocarpus is freshly-planted in a 24 x 24-inch square in a parking lot near the Southern California coast (see below image for details). These versatile plants prefer moderate water and slightly acidic soil.

A close-up of the podocarpus shows the hardware correctly installed and the tree well on its way to survival. Note the water-flow pattern on the soil surface--water is flowing from the bubbler directly over the root crown into the deep-watering tube (its protective grate has been removed in this view). Photos by Erik Skindrud

Other sources include local water utilities, university extension offices, and professional organizations such as The Irrigation Association's book, "Drip Irrigation in the Landscape". Using the following formula is one way to estimate water use if the landscape coefficient, and reference ETo are known.

.623 x RZA x Lc x ETo = EF
RZA = Potential root zone of the plant at maturity
Lc = Landscape Coefficient
ETo = Reference ET
(inches per day)
EF = Emitter and Climate Efficiency

To skip the math and other details, general landscape contractors may want to work with trained irrigation contractors!

After estimating the water needs for plants, design the irrigation system to provide sufficient water to the active root zone area.

The bottom line is, a properly installed drip irrigation system will provide a deep soaking and wet a sufficient area to ensure the development of a sound root system.

Scheduling for Tree Zones

The watering schedule is another important factor. Irrigation systems for trees should be on separate zones from other vegetation in the landscape. If not, they are usually under-watered because they require a slower, deeper watering. Turf, shrubs and groundcovers have individual watering needs that must be addressed as well. Selection of the right controller is as important as the overall irrigation design.

Scheduling point source drip irrigation water management is not addressed often enough. Developing a drip irrigation schedule for point source drip irrigation, like any calculated irrigation schedule it is just a starting point. True water management is monitoring the plants health and soil moisture.

This juvenile sycamore is staked, tethered and hooked up to a water IV (note the bright green root watering system). Trees are an investment--once the contractor has correctly installed them a good maintenance crew is essential for long-term success.

A juvenile orange tree grows by a mature neighbor in a Southern California grove--both are served by the same irrigation line. Mulch covers the surface to limit evaporation and weed growth. Citrus trees are well-suited to mild temperatures across much of the South but depend on a reliable source of water. Photo by Erik Skindrud

In scheduling any type of irrigation there are several pieces of information needed in order to develop a schedule.
The most logical order of questions to ask are:

o what are the water needs of the plant?
o how much water is being applied through the irrigation system?
o what kind of soil will best allow the plant to survive?
o how often and how long to irrigate?

Keep in mind the irrigation system and plant water needs will tell you how long to irrigate, and the soil will tell you how often to irrigate.

Soil types are another important consideration. Sandy, loamy and clay soils will require different watering considerations. When analyzing soils before designing an irrigation system, do not overlook potential "soil interface." If two different types of soil are layered, the top layer must be completely saturated before the water will move to the lower layer.

Typically, the top layer needs to exceed field capacity before water reaches the next layer. When this happens, surface runoff or puddling is evident. The perception that the tree has received sufficient water will present itself, when very little water is reaching the feeder roots. Use a soil probe to find the actual watering pattern. When properly designed and installed, most irrigation systems can be scheduled to provide the correct amount of water.

Bubblers apply water to a limited area and are less vulnerable to dispersion during windy conditions. Workers installed one or two emitters for each tree depending on their size on this project. Trees with two emitters got them placed on opposite sides, between one and two feet from the trunk.

Micro-spray nozzles take care of perennials in this mixed bed that doles out water to staked jacaranda trees via bubblers.

Samuel Rodriguez of John Vander Geest Landscape cuts a hole in a valve box at The District shopping center in Tustin, Calif. in July. Getting the system up and running as soon as trees were in the ground was essential for tree survival following this summer installation. Photos by Erik Skindrud

Low-volume Drip Basics

Conventional systems deliver water at a rate measured in gallons per minute (GPM), whereas low-volume irrigation systems deliver water in gallons per hour (GPH). The most successful low-volume systems are micro-sprinklers, conventional drip irrigation and in-line drip irrigation. Micro-sprinklers are similar to conventional sprinklers, but deliver water at a much slower rate, generally about 5 to 6 GPH, allowing deeper watering with minimal runoff. Micro-sprinklers are widely used in fruit orchards.

Drip irrigation systems that trickle water through emitters generally deliver water .5 to 2 gph. Drip irrigation is perhaps the most efficient means of irrigating landscape plants if it is designed and installed correctly.

Some of the basic hardware used with any drip system is seen in this view taken at Shadetree Nursery in Irvine, Calif. The soda-can-sized appendage on the line at left is a filter housing. The black collar around the line (center) is a pressure regulator that takes water down to the system's 15 psi requirement.

The recycled water system employs a simple screen filter that removes grit from the lines. With their small openings, "you don't want the emitters getting clogged," Shadetree Nursery's Dale Skidmore said. Photos by Erik Skindrud

Emitter Placement is Key

One of the hottest debates in the industry today is determining the number and placement of emitters needed to develop a healthy, well-rooted tree. Properly-installed drip irrigation will provide a deep soaking, and saturate a sufficient area to ensure the development of a sound root system. You should evenly space emitters to cover the area underneath the tree and extend them a few feet beyond the mature canopy.

Some people believe it is better to add emitters as the plant grows, but real world experience shows it is best to design for a mature size from the beginning. Even the most diligent maintenance contractor often neglects to install sufficient emitters. Wetting a larger area also encourages the tree to develop an extensive root system quickly, reducing the chance of uprooting by the wind. For trees requiring a large number of emitters, in-line drip irrigation is cost effective method to use.

Considering the cost of replacing a large tree, it is more than worth the cost and effort of installing adequate irrigation at the time the tree is planted. No matter what type of irrigation system is used, be sure the irrigated area is large enough to support the establishment and sustainability of a healthy root system.

A tree costs $50 to $500 (and up) to purchase, and as the tree matures the value of the tree grows too. Proper irrigation is the best insurance policy for urban trees.

Joseph H. Fortier, CID, CLIA, and Jerry M. Belt Jr., CLIA are principals with Mojave Water Management in Las Vegas, Nev. The authors would like to thank and acknowledge the late Jimmy Tipton for his research and inspiration.

Multiple drip emitters are essential for the survival of big trees in arid settings. Some municipalities and water districts prohibit surface watering but allow drip application.

Tree roots can extend up to four times beyond the drip line of a canopy, depending on the species. Tailoring irrigation to a species' growth habit is an evolving art--nursery growers and horticulturists are good sources of detailed information on specific varieties. Diagrams courtesy of Joseph H. Fortier

Avoiding The 'Seven Deadly Sins' of Drip Irrigation

We don't want to sound like holy rollers (we live in Las Vegas!) but there are Seven Deadly Sins of Drip Irrigation.

Each "sin" creates its own insidious problems, many of which take years to rear their ugly heads. Cutting corners to save money on installation is a very shortsighted savings and actually costs a client more money in maintenance and repair.

1. Placement - The placement of emitters is one of the most critical and often overlooked factors of a successful drip irrigation system.

2. Quantity - Use the correct number of emitters to supply the mature plant with sufficient water to meet its needs within the allowed watering window.

3. Flow - Overlooking system hydraulics due to the typically low volume and pressure of a drip system is a common error. Size all piping and tubing to deliver adequate water, at an adequate pressure to the last emitter on a lateral.

4. Pressure - This one is closely related to its sibling, flow. Know the pressure at the source, pressure after the backflow preventer, and loss through the valve, filter, and pressure regulator to determine the available pressure.

5. Filtration - All drip systems require filtration. Use the correct type of filter for the situation.

6. Zoning - "Not all plants are created equal." We repeat, "Not all plants are created equal." Different plants require varying amounts of water and frequencies of irrigation. Always separate trees from shrubs. Trees require deeper, less frequent irrigation compared to shrubs.

7. Scheduling - This "Sin" deserves an article in itself. So many factors affect irrigation scheduling. Climate, soil type, root zone, plant selection, watering windows, and equipment used contribute to the magical item known as the irrigation schedule. To keep things simple let's just say that deep, infrequent watering is better.

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December 14, 2019, 9:49 am PDT

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