Preserve Trees, Conserve Water

Trees’ watering needs vary greatly and can fall into low, moderate or high categories. By learning about your trees’ water needs and by adapting trees’ irrigation, you can preserve your trees while conserving water.

Some things to know about irrigation:

  • Most trees need irrigation during dry months.
  • Native oak trees do not need summer water and can actually be harmed by it. See our section on caring for mature trees.
  • Use drip irrigation wherever possible – water is applied at a slow rate exactly where it’s needed. This is the most effective and most water-efficient irrigation method.
  • Spray irrigation is wasteful. It applies water at a fast rate, often running off the top soil and evaporating into the air
  • Water runoff is also wasteful, but easy to solve; reduce the rate at which you apply water to allow time for water to soak into the soil.

Some things to know about tree roots:

  • A complex network of small absorbing roots take in most of the tree’s water and nutrients
  • Many absorbing roots are in the top few inches of soil
  • Deep watering, to a depth of 18 – 24 inches, will encourage deeper rooting
  • Only a few tree species have deep tap roots
  • Absorbing roots are not present directly around the trunk of mature trees
  • Roots need oxygen to grow; they do poorly in compacted soil

Trees will often show signs of water related stress such as browning of leaf edges, poor or stunted new growth in the spring, or dieback of leaves, twigs and branches. Sunburn on the leaves can occur during hot spells when the tree cannot take up water as quickly as it is losing it through the leaves and twigs. This shows itself with brown patches in the middle of the leaf.

To find out if your tree is getting adequate water, dig or probe the soil down 6” – 12” at the dripline. An 18” tubular steel soil sampler is ideal for checking soil moisture. If the soil is damp, you are probably doing an adequate job. The soil should be moist, but never soggy and squishy for any length of time. Excess soil moisture excludes oxygen from the soil, creating an environment where small absorbing roots die off.

To give your trees adequate moisture while conserving water, follow the step-by-step procedure below.

1. Find out what kind of trees you have and look up their watering requirements in the Canopy Tree Library.

Newly planted trees have their own unique considerations (see our section on Caring for Young Trees). Mature California native oaks should not be irrigated at all in the hot dry months.

2. Consider the environment for your tree. What kind of soil do you have? Where will you water?

Is the soil very hard and compact, especially in the summer? Is it almost sticky, breaking up in very large blocks when you dig in moistened soil? Most likely you have soil that has a strong clay component. Compacted clay soil does not accept water easily, so break up the top layer of soil to keep water from running off the top. Water will soak into clay soil if applied slowly.

Don’t bother watering near the trunk of a mature tree; there are few absorbing roots there. Look at the outer edges of where the leaves of the tree canopy reach; this circle around the tree is called the dripline. Irrigate the soil from half-way between the dripline and the trunk out to 10 feet or so beyond the dripline. Lawn and other groundcover competes for water with tree roots; remove them underneath the canopy of the tree where practical. At a minimum, keep them away from the trunk.

3. Consider the method of irrigation: Is there existing irrigation in place? Is it suited for my situation?

Discuss irrigation plans with your gardener or licensed landscape contractor. Incorporate goals to reduce water use and preserve trees by deeply watering them. If you decide to hand water your trees, create a soil basin within the dripline and fill with water from your hose. Allow water to soak in and repeat two more times. See Caring for Young Trees for further instructions on basin watering.

4. Install or change the irrigation.

To use water most efficiently, install a drip irrigation system or convert your sprinkler system from spray to drip.

Your trees should have their own dedicated irrigation valve, as they require deeper watering than groundcovers or shrubs. For a new system, tap into an existing hose bib or connect to the water pipe coming in from the street to your home. Consult with your irrigation contractor or bring a layout diagram of your property to your local hardware store or irrigation supply house to get advice. A great source for do-it-your-selfers is the Sunset Publishing book “Sprinklers and Drip Systems”.  For more information, see Irrigation Tutorials.

If you plan to keep your sprinkler system, replace each nozzle with water-efficient nozzles which have a much lower water application rate. This is more efficient because there is less evaporation, and the water will percolate into the soil with little run-off. If you don’t replace the nozzles, it is best to split the watering into two sessions so the water will penetrate the clay soil.

5. Consider the frequency of irrigation: How often do I need to irrigate?

Check the Canopy Watering Guidelines to determine how frequently to run your irrigation system. Adjust if the weather is hotter or cooler, or if your watering area is shaded or particularly hot.

6. Consider the duration of irrigation: How long will I water each time?

Check the Canopy Watering Guidelines to determine how long to run your irrigation system. These are guidelines only; you may need to adjust if your system is slightly different.

Whenever using conventional spray heads or if you are having trouble with water run-off, break water run cycles into smaller periods in the same day. For example, use two 20 minute periods, rather than one 40 minute period. Most irrigation controllers can be adapted to do two start times automatically.

7. Mulch the top of your soil to reduce evaporation.

Applying a layer of organic material (like bark chips) over the soil will reduce evaporation and keep the soil temperature cooler. The bark chips will slowly break apart and add organic material to the clay soil.

Mulch deeply with drip and bubbler systems: 4” – 6” mulch depth applied over the drip tubing.

Mulch more shallowly with spray heads, only to 1” depth, so that water will still penetrate the mulch and get to the soil.

8. Maintain your system as your tree grows.

Inspect your irrigation system every spring to look for leaks or clogged sprayheads or drippers. Move drippers outward away from the tree as it matures, always applying water under the dripline of the tree and slightly beyond. Replenish the mulch layer every spring just as the natural rains taper off; you will capture nature’s water, reducing the amount of irrigated water you will need to apply later in the dry months.