Save our Water and our Trees

Trees are the most valuable element of our green infrastructure because of the benefits they provide and the investment they represent.

Urban trees combat climate change, enhance public health, and contribute to vibrant, sustainable communities. Additionally, they represent a water-efficient investment; a well-managed urban forest saves more water than it consumes!

5 steps to save water and save trees:

1. Know your trees, know their needs.

Different trees have different water and care requirements. To determine how much and how often your tree needs water, consider:

  • Species. Look up your tree species’ watering needs on Canopy’s tree library. For help identifying your tree, contact Canopy or use a resource like the Urban Tree Key.
    Note: Mature California native oaks should not be irrigated at all in the hot dry months under normal circumstances. See caring for mature trees.
  • Age. Young trees require more frequent watering (but less volume of water) than mature trees. See caring for young trees.
  • Health. A stressed tree may require more water than a vital tree. If your tree is in poor condition, consider hiring a certified arborist.
  • Soil type.  Water needs will also be affected by your soil type. For example, clay soil does not accept water easily, but holds on to water longer.
  • Other site conditions. Factors like sun exposure, surrounding plants, and nearby water sources also influence watering needs.

2. Water Wisely

Not all watering methods are created equal. By applying water where and when your tree needs it the most, you’ll help your tree thrive while using limited water.

Assess soil moisture: Dig or probe the soil to a depth of 12 inches at several locations around the tree’s drip line. Damp soil is good. Soggy soil means your tree is getting too much water, which reduces oxygen in the soil and can kill tree roots. Dry crumbly soil means it’s time to water!

Note: If you have very hard clay soils, consider breaking up the top layer of soil to keep water from running off.

Water gradually and deeply. Apply water slowly and evenly to the tree’s root zone (see diagram), saturating the soil to a depth of 12-18 inches. If applied slowly, water will even soak into hard clay soils. Allow soil to dry out between waterings.

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Click to enlarge

Good watering methods:

Hand-watering: A good choice for establishing young drought-tolerant trees.  Create a soil basin within the drip line and fill with water from a hose or bucket. Allow water to soak in and repeat two more times. See Caring for Young Trees for further instructions on basin watering.

Drip irrigation: To use water most efficiently, install a drip irrigation system or convert your sprinkler system from spray to drip. With these methods, water is applied at a slow rate directly where it’s needed near the tree roots. Soaker hoses can be a cost-efficient alternative.

Watering methods to avoid:

Sprinklers and spray irrigation apply water at a fast rate, resulting in water loss due to runoff and evaporation.

Lawn irrigation does not water trees successfully. It generally reaches only the first few inches of soil, encouraging trees to form weak “surface roots.”

Irrigation notes:

  • It’s a good idea to give your trees their own dedicated irrigation valve, since trees require deeper watering than ground covers or shrubs. For a new system, tap into an existing hose bib. 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.
  • If you plan to keep an existing sprinkler system, replace nozzles with water-efficient models (which have a much lower water application rate), and apply only 1 inch of mulch so water can penetrate easily.
  • If you do use conventional spray heads, or if you are having trouble with water run-off, break water run cycles into smaller periods in the same day—this allows more water to penetrate the soil. For example, use two 20-minute periods separated by a brief rest (to allow water to soak in), rather than one 40-minute period. Most irrigation controllers can be adapted to do two start times automatically.
  • A great source for do-it-your-selfers is the Sunset Publishing book “Sprinklers and Drip Systems.”  For more information, see Irrigation Tutorials.

How to use recycled and ground water in the landscape:

recycled-water-tapRecycled water is a drought-proof source of water that saves potable water and can be valuable in the landscape, as a supplemental source of water.

However, it’s important to exercise caution; most recycled water has a high salt content that can harm many tree species and woody plants.

  • Avoid spraying recycled water directly on foliage.
  • Confine applications of recycled water to lawn and other small landscape plants.
  • Near trees limit application to every other week, ensuring recycled water does not represent more than 1/4 to 1/3 of the water applied to trees.
  • Local companies such as RainDance and Purple Pipe deliver recycled and ground water to residents. Work with them to design the optimal application method and frequency to save both water and trees. 
  • Monitor signs of stress in your trees and plants, such as changes in leaf color, early leaf drop, or wilting. Contact a plant health expert or certified arborist for advice on adjustments to your watering if you notice your tree’s health declining.

Reclaiming “dewatering” water: In Palo Alto, the City (and some residents) have also used “dewatering” water—groundwater pumped out from local basement construction sites—to water urban trees. This water from dewatering stations is not as high in salinity as recycled water, but still higher in salinity than most potable water sources. Again, treat it as a supplemental water source for your landscape by mixing or alternating it with potable water.

Use gray water for plants and trees: Note that recycled water is not the same as gray water, which is potable water leftover from household use such as baths, dish washing, washing machines, etc. It is perfectly safe to use gray water to water your trees and plants, as long as biodegradable soaps are used.

Decide on watering frequency and duration.

Water is needed most during hot, summer months and prolonged dry spells in other seasons.

  • Water mature trees once a month or every other month, and young trees weekly or every other week.
  • See detailed watering guidelines based on the size and species of your tree. You may need to adjust these guidelines when the weather is hotter or cooler, or if your watering area is shaded or particularly hot.
  • The best time to water is early in the morning or in the evening, when less water will be lost to evaporation.

Maintain your system as the tree grows.

  • As your tree grows, move nozzles farther out from the trunk, and consider removing additional lawn.
  • Adjust watering frequency and duration. Water thoroughly, but less frequently, allowing the soil to dry out between waterings.
  • Inspect your irrigation system every spring to look for leaks or clogged spray heads or drippers. Move drippers outward away from the tree as it matures, always apply water at or near the drip line of the tree and slightly beyond.

3. Mulch to conserve moisture

Besides proper watering, the best thing you can do for your tree is to apply mulch over its root system.

Properly applied, mulch:

  • Insulates the roots from extreme temperatures
  • Slows soil moisture evaporation
  • Reduces water use 
  • Suppresses weeds
  • Reduces soil compaction
  • Feeds nutrients into the soil as the mulch breaks down
  • Provides a buffer, protecting the tree from mowers

mulch-volcano4

Image courtesy of Madison Tree Care & Landscaping


Mulching Tips:

  • Use organic matter such as wood chips, with or without leaf matter. Trees actually prefer wood chip mulch and the “duff” created by their own leaves.
  • Apply mulch layer 3-5 inches deep (or 1 inch deep if using spray irrigation),  extending at least as far as the drip line.
  • Say no to mulch volcanoes! Mulch should have a “donut” distribution, not a “volcano” shape. Mulch should be a few inches away from the base of the tree; too much moisture around the trunk can lead to decay.
  • If weeds persist, place a layer of newspaper, cardboard, or a biodegradable weed cloth fabric underneath the mulch.
  • Replenish the mulch layer every spring just as the natural rains taper off; you will capture nature’s water, reducing the amount of irrigation needed in the dry months.

A note about fertilization: Under normal circumstances, most non-fruit trees do not require fertilization. During drought conditions, fertilizing is actively discouraged, as it promotes growth that the tree cannot sustain in water-scarce conditions.

4. Conserve water throughout your landscape

Remove your lawn or let it go golden. More than half of outdoor water is used on lawns. Many cities offer rebates for lawn removal.

But help your trees transition! Trees in lawns develop shallow roots and are accustomed to frequent shallow watering. They can adapt, but will need time to adjust to a less frequent, deeper watering pattern. Use mulch liberally to help with this transition. Consider installing a drip system. Learn more about protecting trees during turf conversion.

5. Plant trees to replenish the urban forest

It may seem counter-intuitive, but planting trees is even more important during a drought. Trees lost to drought need to be replenished and the urban forest renewed.

Additionally, trees provide many water-related benefits:

  • Leaves and branches intercept rainfall, helping slow stormwater runoff
  • Trees reduce soil erosion and improve soil quality
  • Organic material from trees increases carbon in the soil, which increases its water storage capacity
  • Increased water storage and water penetration in the soil helps recharge groundwater supplies
  • Shade provided by trees helps reduce the need for irrigation in other landscape plants

To maximize these benefits and conserve water, it’s important to plant the right tree in the right place:

  • Choose climate-appropriate, drought-tolerant species. Visit the Canopy tree library for help choosing the right tree for your site.
  • Match the tree to its location. A properly sited tree will be healthier and longer-lived. Consider factors including: existing access to water, soil type, shade, and overhead wires.

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