Tree Emitter Location. Placement of emitters impacts future root growth and tree health. Suggested pattern for emitters includes removing emitter at base of tree in second year. Diagram: Joseph Fortier.

Trees are the foundation of any landscape setting. Without trees, modern urban landscapes would lack character and interest. Try to imagine your favorite park, golf course, or backyard without trees. It does not present a pretty picture. Few people would dispute the importance of trees in an urban landscape where they provide cooling shade, help filter the air, and allow people to escape the maze of asphalt and concrete momentarily.

If we all agree how important trees are why do we often neglect their water needs? Designing an irrigation system to meet the unique needs of each plant group, while meeting budget constraints can cause many an irrigation designer to lose sleep. Perhaps the most difficult plant group to irrigate is the "majestic giants" of the landscape, trees. Due to their extensive, deep root systems, size, and water needs, trees require special irrigation. All too often, the long term health and well being of a tree is sacrificed to design and install a simpler, less expensive irrigation system. Many factors need to be considered when designing an irrigation system to water trees.

A basic understanding of the structure of a tree's root system is critical. The major feeder roots of a tree are usually in the top 18 to 24 inches of the soil. Of course there are exceptions, and many trees will root deeper than two feet, but these trees are usually seeking available subsurface water, and irrigating to this depth is typically a waste.

Trees, like all plants, "breathe" oxygen from the soil through their roots. The highest oxygen content in the soil is in the first 24 inches. Since this is the area containing the most oxygen, the most active, beneficial tree roots are found at this depth. Water, nutrients and oxygen are absorbed most readily at this depth.

The next factors to consider are closely related. How far beyond the drip line will the roots spread horizontally, and where within this vast network of roots will the most water be absorbed? 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.

Urban Conditions Create Problems

A thorough understanding of these concepts is critical to design an irrigation system that meets the water needs of the tree as it matures. Compounding the 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 these circumstances, the irrigation designer needs to work as closely as possible with the planners and designers to make sure the watering needs of the tree are properly addressed.

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 the tree is receiving sufficient irrigation to develop a healthy root system. A well-developed root system will produce a healthier tree with fewer disease and insect problems. The roots also provide a strong anchor to prevent the tree from blowing over in a strong wind.

In many urban settings this is difficult to achieve, but there are still creative ways to accomplish this objective.

A more serious problem sometimes exists in a retrofit situation where an existing spray irrigation system is replaced with a low-volume irrigation system. Careful consideration must be given to the area that received the most irrigation from the old system. This area will have the most active roots. Ignoring this fact has caused the slow demise of numerous mature trees when turf was removed from the area under and around the tree, and if emitters or bubblers were only placed near the trunk. Visible stress and even death can occur within one year.

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 or if supplemental irrigation is required. In an arid climate with a deep water table, supplemental irrigation is usually required.

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 the poor 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.

Designing Irrigation Systems for Trees

As simple as it sounds, the most important irrigation design consideration is to identify the purpose of the irrigation system. 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 system needed. When designing an irrigation system simply to help the tree become established, a temporary irrigation system with overhead spray, bubblers or drip will be sufficient.

If the system is designed to provide regular irrigation, you must consider the water needs of the tree as it matures. In areas that receive little rainfall during the growing season, irrigation systems should be designed to provide sufficient water to the tree during initial establishment, and to encourage development of a healthy, vigorous root system as the tree grows.

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.

Other sources include local water utilities, Cooperative 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.

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

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

Irrigation systems for trees should be on separate zones from other vegetation in the landscape. Turf, shrubs and groundcovers have individual watering needs that must be addressed. Providing the correct amount of water to these plants when they are irrigated together is virtually impossible. And, selection of the right controller is as important as the overall irrigation design. Trees on the same zone as other plants are usually underwatered because they generally require a slower, deeper watering.

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.

Let's examine the most common types of irrigation systems. Conventional irrigation systems include spray and bubbler irrigation. Most conventional systems have a high application rate. The application rate of these systems usually exceeds the infiltration rate of the soil. This causes excessive runoff, and therefore water waste. When selecting spray irrigation around a tree, you need to minimize how much water is directly spraying the tree trunk. Many tree species, such as oaks or palms, are very susceptible to disease and decay if the trunk is constantly moist.

Bubblers avoid the problem of spraying the tree trunk, but their high application rate usually requires a well to be constructed around the tree to contain the water. Increasing the size of this well is important as the trees increase in size. Keep in mind that the feeder roots extend well beyond the canopy of the tree, and a well of this size may not be acceptable.

Tree roots can extend one to four times the width of the canopy. Diagram: Joseph Fortier.

Low-Volume Systems

Conventional systems deliver water at a flow 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. The growers claim the feeder roots develop more vigorously when micro-sprinklers are used instead of point source drip systems. Some disadvantages of micro-sprinklers are:

• High visibility invites vandalism;
• Small droplets are susceptible to wind drift;
• Components are easily damaged by animals, people, and maintenance equipment; and
• Micro-sprinklers have a higher evaporative loss in arid climates.

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. The most common types of drip irrigation systems are conventional point source drip and in-line drip irrigation. In-line drip irrigation differs from conventional drip irrigation because the emitters are pre-installed in the distribution tubing.

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 wet 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 a lesson in the real world has taught us to resign for the mature size from the beginning. Even the most diligent maintenance contractor or property owner will neglect to install sufficient emitters in a couple of years. 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 to use.

The emitter placement in the drawing will provide water to a large area and encourage the tree to develop a vigorous, healthy root system. Most nursery stock is grown in a highly-organic medium. To minimize the problems of soil interfacing between the growing medium and native soil, remove the emitters over the root ball after the roots begin to develop. This will also minimize the risk of the development of crown rot at a future date.

How critical is it to water our majestic giants properly? Just look at the face of any homeowner who has lost a mature tree in a moderate wind storm because of an an inadequate root system. If the tree was in a turf area, it was most likely caused by shallow watering from the lawn sprinklers only. As previously discussed, trees need water to be applied deeper than grass. If the tree was in a xeriscape, the irrigation was probably installed from the base of the tree to the edge of the original box, and the roots never developed much further. As the canopy extended beyond the root zone area, the tree became unstable, and the wind just pushed it over. Roots can also be confined if the tree is planted incorrectly in tight, compacted soils.

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 about $50 to $250 to purchase, and as the tree matures the value of the tree increases tenfold. 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, NV. The authors would like to thank and acknowledge the late Jimmy Tipton for his research and inspiration.