How Long Should I Run My Sprinklers?

Many tree owners and managers are saving water and letting lawns and groundcovers die during this drought and I am seeing many trees suffering from drought stress. When asked how to care for trees during the drought, I typically suggest watering drought tolerant trees every two to three weeks depending upon soil conditions and rooting depth. “How long should I turn on the sprinklers?” is an obvious follow up question.

dieback in tree due to water deficit

Turning off the sprinklers may save water, but it is harming many of our trees. The vast majority of trees in our landscapes require some irrigation to survive.

There is scant research concerning the irrigation requirements of landscape trees. However, studies have shown that aesthetic and functional value can be maintained when irrigated at a rate equal to 50% of the water grass uses (also called reference evapotranspiration or ETo) depending upon a tree’s drought tolerance and other factors (Pittenger, D., et al., Center for Landscape and Urban Horticulture).

If we use 50% ETo as a target and irrigate twice per month, how long do we need to leave the sprinklers on? This will depend upon the application rate of the irrigation system which varies greatly. Rotating sprinklers typically apply between 0.25 and 1 inch of water per hour while fixed head sprinklers apply 1-2 inches per hour. You can conduct a “can test” to determine the application rate of your system, spreading out containers, turning on the sprinklers for an hour and measuring the depth of water in the containers.

If we want to apply 50% of ETo, we have to know these rates for where we live (find ETo for where you live). In Davis, California, ETo for spring and fall are roughly 1.5 inches per week and 2 inches during the summer.

Therefore, in Davis, to apply 50% of ETo every two weeks, we would need to run the sprinklers for the following amounts of time, considering a standard distribution uniformity (a measure of how evenly irrigation water soaks into the soil).

Season Rotating Head Fixed Head
Spring/Fall 2-7 hours 0.75-1.5 hours
Summer 2.5-10 hours 1-2 hours

Since there are many assumptions made above, this would simply be a starting point for scheduling irrigations. You will want to make sure the irrigations moisten the soil to a depth of at least 18 inches and that soil moisture conditions are adequate. In some cases, the run times above may result in water penetrating deeper than 18 inches. In this case, you can shorten the run times.

sprinkler irrigation controller

Program your sprinkler irrigation controller considering the irrigation needs of the trees.

The depth of penetration of the irrigation can be determined by digging, pushing a tile probe (sharpened steel rod with handle) or by using soil moisture sensors. Soil moisture should also be monitored at 12 inches deep for trees. This can be done using soil moisture sensors such as tensiometers or by feeling the soil prior to an upcoming irrigation to determine its moisture level. For more information, see my previous article Practical Guidance for Effective Tree Irrigation.

Thank you to Loren Oki, U.C. Cooperative Extension Landscape Specialist, U.C. Davis for his review of this article.

trees getting sprinker irrigation

The length of time the sprinklers should run is dependent upon the application rate of the system, weather, rooting depth and other factors.

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Your Trees Need Irrigation During This Drought

As this summer progresses, I am seeing more and more trees suffering from drought stress. This is largely due to our irrigating less to save water, comply with governmental regulations and save money. While these are very important reasons to water less, I am finding that many people do not realize that in the majority of cases, the trees in their landscapes require irrigation to thrive.

Even relatively mild drought stress will slow or stop the growth of trees. As drought stress severity increases, leaves wilt, yellow, burn and drop. Twigs and branches die back and debilitating insects or diseases often infest stressed trees. Eventually the tree will die, over months or years.

 

How much water a particular tree will require will depend upon its age, drought tolerance, soil conditions, rooting depth, climate and tree exposure. Recently planted trees will require more frequent irrigation until they become established. Species which have evolved in dry climates can survive with less water than those which are native to areas with more frequent rainfall or ground water. Trees with deep root systems will have access to water longer into the summer than those with shallow root systems. Watering needs increase with greater tree exposure to sun and wind.

When planning for the irrigation needs of your trees, remember that the roots of trees are generally shallow and wide spreading. It is also important to water the majority of the root system of the tree. Therefore, leaving the hose on at a trickle or using a couple of drip emitters at the base of a mature tree will result in only a small percentage of the root system being watered. It is much more effective to spread the water out under and beyond the canopy of the tree. This can be accomplished by using 1) existing in-ground or hose-end sprinklers, 2) soaker hoses with lines two feet on center or 3) drip systems with emitters spaced every couple of feet.

At each irrigation, the water should be left on long enough to wet the soil to a depth of at least 18 inches. This could take up to several hours depending upon the application rate of the irrigation system. Start with your best guess as to how long this will take and then either dig or use a sharpened steel rod (tile probe) to determine whether or not the length of time is adequate to moisten the soil to a depth of at least 18 inches.

How often irrigation is required will depend upon the tree’s age, drought tolerance, rooting depth and soil conditions, climate and tree exposure as mentioned above as well as existing sources of water. As a starting point, for most urban situations, I would recommend watering mature trees that are not drought tolerant every 7-10 days and drought tolerant trees every 2-3 weeks.

Proper irrigation is the most important tree management concern in Northern California. Call Tree Associates at (530)231-5586 and I will prescribe an effective and efficient irrigation strategy based on your landscape conditions and tree needs.

Callery pear and London plane suffering drought stress

Figure 1. Callery pear and London plane suffering drought stress symptoms in early August. Many tree owners have stopped watering lawns and the trees are suffering as they are not getting the water they need .

Drought stress symptoms

Figure 2. Drought stress symptoms of wilting, early fall color.

Leaf burn from drought stress

Figure 3. Leaf burn from drought stress on an ornamental cherry.

unirrigated Chinese hackberry trees

Figure 4. These unirrigated Chinese hackberry on Russell Boulevard in Davis have lost some to all of their leaves and are under extreme stress.

Coast Redwood trees

Figure 5. Coast Redwood trees at the Davis Post Office. These trees have suffered from a lack of irrigation and a fungal disease (redwood canker) has caused significant dieback from which the trees are unlikely to recover.

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Successful Tree Preservation in Davis, California: The Result of Professional Cooperation and Effective Communication Considering Tree Biology and Architecture

John Lichter, M.S.
Owner, Tree Associates
ASCA Registered Consulting Arborist, ISA Board Certified Master Arborist

Steve Pro, Owner of Morningstar Builders, asked me to prepare an Arborist Report, including a tree evaluation, impact assessment and preservation plan, to meet City of Davis requirements prior to approval of his construction project. The project involved demolition of an existing residence and construction of a new one on a narrow lot. A concrete slab foundation with an 18-inch-deep stem wall was to be built 11 feet from the trunk of a mature valley oak and 13 feet from a London plane (Figure 1). A paved driveway was to be constructed between the trees and the foundation (Figure 2).

Figure 1. Looking southwest at property in Davis, California, post-demolition. The valley oak is on the left and the London plane is in the right side of the photo.


Figure 2. Site plan of property in Davis, California project. Subject trees are located to the left (south) of the ribbon driveway in the lower left side of plan.


I concluded that there was a high likelihood that the trees would topple or decline in health as a result of construction, considering the proximity of the trees to the proposed foundation and the likely damage caused by typical construction methods. This rating assumed an even distribution of roots at typical depths below grade. However, root systems vary considerably in size, depth, and spread, even among trees of the same species. I recommended that the driveway be installed on grade, with no scarification and minimal soil compaction, to avoid root injury and allow for adequate moisture penetration to the roots. In addition, because the foundation could not be located farther away from the trees, I recommended that the soil be excavated to expose tree roots where the foundation would be located. The information gained would help me to refine my prognosis for the tree and guide a more “tree friendly” foundation design.

With our Air Knife, a pneumatic excavator that uses compressed air to pulverize and remove soil safely and efficiently preserving roots as small as 1/8 inch diameter, I created a 38-foot-long and 14- to 20-inch-deep trench (Figure 3). The excavation uncovered several roots of one inch diameter and six larger roots. The larger roots were between 2 and 3.25 inches in diameter and from 4 to 13 inches deep. All of the larger roots were at least 9” below grade, except for one 2-inch diameter root.

Subject trees and pneumatically excavated trench

Figure 3. View looking southwest at subject trees and pneumatically excavated trench. Exposing the roots allowed us to provide a more accurate prognosis and recommendations for effective tree preservation.


The proposed foundation construction plan would have required severing the six large roots. I discussed my concerns with the contractor, and we came up with a plan to “bridge” the foundation over the six roots so that they would not need to be cut and had adequate space for future root growth to occur without damaging the foundation (Figure 4).

Driveway was installed on-grade without scarification and soil compaction

Figure 4. Property in Davis, California post-construction with subject trees on left. Note the driveway was installed on-grade without scarification and soil compaction. The foundation “bridged over” all roots greater than 1” in diameter.


The tree preservation plan also called for avoiding construction vehicle traffic within as much of the trees’ root zones as possible, installing mulch over a geotextile fabric in areas where traffic was necessary, and irrigating the trees regularly during the construction process.

Approximately two and one half years after the project was built, the trees were thriving and exhibited no signs of stress (Figure 5). As this example shows, effective tree preservation that is compatible with building construction requires an understanding of tree biology and root architecture, as well as effective communication and cooperation between development professionals and the Arborist.

View of trees two and one-half years after completion of project

Figure 5. Looking west at property in Davis, California on April 24, 2012. The trees are healthy two and one-half years after the project.

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Tree Risk Assessment and Formative Pruning Can Improve Safety and Reduce Liability

acacia tree broke at the trunk during a storm

This acacia tree broke at the trunk during a storm, damaging a house in San Jose, California. Had an Arborist examined the tree prior to the failure, they would have seen fruiting bodies (conks) of a decay fungus on the trunk and likely recommended a more detailed inspection to determine the extent of decay and ultimately recommended that the tree be removed.

Posted by Kelly McGlothlin, M.S., Associate Arborist

The structural development of trees in unpopulated areas is not a problem since there are usually not any targets of concern when a tree falls or loses a limb.  But in urban settings, trees left to develop on their own often develop structural defects, which make the tree more susceptible to failing (breaking or falling).  Tree Risk Assessment involves a systematic identification of structural defects within the canopy, trunk and/or roots of trees; assessment of the likelihood of tree failure (loss of tree part or toppling); and identification of the likely target for the failure.  In addition to the above, we at Tree Associates also assess the probability of costly tree root damage to structures such as foundations, patios, walk and roadways.

Over the years of conducting Tree Risk Assessments, I have noticed that many defects could have been eliminated in the trees’ early development.  Trees that were not trained during their early developmental stages are more likely to have structural defects that increase the probability that part or the whole tree will fail.  Therefore, I encourage all to invest a little time to select trees for planting that have a good beginning structure, as well as time to train them in the first few years of their development. This time investment will ensure a safe, structurally sound tree and decrease future maintenance costs by hundreds or thousands of dollars.

broken limb on this Monterey pine

A large limb on this Monterey pine in Menlo Park, California broke during a windstorm as a result of the limb's large size and windsail. The failure could have been prevented if the defect had been identified and the limb shortened.

TreeQualityNurserySelectionBrian Kempf, with the Urban Tree Foundation, and Edward F. Gilman, Professor of Environmental Horticulture at University of Florida, put together some valuable resources related to tree selection and training:

YoungTreeTraining

Tree Associates Arborist-experts in Davis, California can help you by objectively identifying potential defects and specifying mitigation alternatives.  For clients who have large numbers of trees, such as Homeowners Associations, School or Park Districts or Municipalities, we can provide Tree Risk Assessments with risk-prioritized hazard abatement recommendations.  These recommendations can be used to obtain and compare tree work bids for the same scope of services.  Please contact us if you have any questions about this article or would like to schedule a Tree Risk Assessment for your home, business, or government entity.

limb split away from the main trunk of this valley oak tree

A large scaffold limb split away from the main trunk of this valley oak tree in Citrus Heights, California. The blackened area (oxidation) on the bark indicates that a crack had existed previous to the failure. While the crack may not have been visible to a lay person, an arborist would have identified it during a Tree Risk Assessment and recommended mitigation measures (e.g. limb removal) to avoid the failure.

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Pruning for the Structural Improvement of Trees

This gallery contains 9 photos.

While the majority of my work is as a Consulting Arborist, I have been pruning trees professionally since I started working for a tree service in 1992. I enjoy many things about pruning, including being outdoors, the climbing, the workout, … Continue reading

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Should we water our trees?

This gallery contains 2 photos.

Kelly asked me recently if we should write an article about watering our trees.  Now that a storm is just about here, I wonder if I should post this; but I don’t know for sure how much rain we will … Continue reading

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Bare Root Planting Season is Here!

This gallery contains 6 photos.

The following is a list of some of the benefits of planting bare root trees as opposed to container-grown or balled-and-burlapped stock. The root system is visible and girdling roots (circling roots near the trunk) are not common (Figure 1). … Continue reading

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Horticultural Considerations for Utilizing Coast Redwood in Northern California Landscapes

This gallery contains 7 photos.

By John Lichter, M.S., and Richard Evans, PhD. I want to share my observations concerning a malady that recently has caused widespread dieback and decline of coast redwood in Northern California. Since the pathogens associated with this malady seem to … Continue reading

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Correcting Leaf Yellowing in Alkaline Soils

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Many plants growing in our region of California suffer from the chronic occurrence of leaf yellowing, or chlorosis. Chlorosis is the result of the loss of the green pigment, chlorophyll, which is essential to the process of photosynthesis. In effect, … Continue reading

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Practical Guidance for Effective Tree Irrigation

This gallery contains 9 photos.

John Lichter and Kelly McGlothlin, Tree Associates and Richard Evans, Evans Associates In our experience, irrigation is the most important management concern for trees in most of California. Unfortunately, myths and misinformation abound in this area, leading to much confusion … Continue reading

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