By Don Shor, Greg McPherson and Yael Franco
Special to The Enterprise
One of the simplest actions you can take to improve your neighborhood, increase your property values, lower your energy bills, and help sustain our urban forest is to plant a tree.
We all know trees are beneficial. They help to reduce the urban heat island effect by shading reflective surfaces. They moderate the local climate by blocking wind and providing shade. Direct cooling of buildings reduces energy use. They filter pollutants from the air, provide habitat for wildlife, sequester carbon, and are well known to have social, emotional, and cultural benefits.
Just how beneficial are trees?
Dr. Greg McPherson, retired Urban Forest Researcher with the USDA Forest Service, has done research to quantify some of the benefits of trees. For example, a mature shade tree in Davis can reduce annual air conditioning costs by $15-20. The same tree removes 10 pounds of air pollutants with annual health human benefits valued at $40-50.
By intercepting 700 gallons of rainfall, this tree reduces stormwater runoff and pollution, valued at $6. The same tree can sequester 200-300 pounds of carbon dioxide each year, helping to offset our emissions. It increases property values, harbors wildlife, and promotes human health and well-being.
Just the act of planting and caring for trees can strengthen our connection to nature and the community in which we live. Research tells us that the annual value of environmental services provided by a mature tree in Davis is $60 or more, but our hearts tell us that the value of a large, healthy tree is priceless.
Selecting a shade tree should be done with care.
You have site-specific factors that will help a plant professional guide you toward suitable species. If your home has solar panels, you want your tree to be positioned far enough away to avoid shading them, especially in winter. Neighborhood with higher densities may require that trees not exceed a certain size. Additional important factors include your irrigation plan, soil pH, drainage issues, other landscape uses such as vegetable gardens, presence or absence of turf, and more. Avoid trees with known pest or disease problems, poor branch structure, tendency toward surface rooting, and invasiveness.
One obstacle to realizing the benefits of trees is the limited supply of tree species that are well-adapted to climate stressors such as heat, drought, salinity, extreme wind and pests. Dr. McPherson and Tree Davis have been evaluating trees from desert cities since 1999. Emily Griswold, a Tree Davis Board member and Director of UC Davis Arboretum Horticulture, has extended this research to include many species from Texas.
In partnership with the city of Davis, the Tree Davis Community Canopy program is planting a variety of trees that are relatively new to the area and proven to be climate-ready. Evaluation of their performance is on-going. Examples of these trees are listed below and more information can be found at the website: https://climatereadytrees.ucdavis.edu/.
* White Shield Osage Orange — Pest free, drought tolerant, with glossy foliage.
* New Harmony and Triumph elms — Disease-resistant, with excellent form and foliage.
* Texas Red Oak — Drought tolerant, rapid growth, and excellent structure.
* Red Push Pistache — Red foliage in spring and fall, no fruit, compact crown.
* Kentucky Coffeetree — The cultivar espresso was propagated from a tree in Davis and is podless, heat and drought tolerant, and relatively fast-growing with a large, vase-shaped crown.
* Shoestring Acacia — Striking specimen, evergreen with attractive flowers.
* Desert Willow — Beautiful summer flowers and fall foliage.
* Oklahoma Redbud — Drought and soil tolerant with gorgeous pink flowers.
* Texas Live Oak — Evergreen with attractive foliage and excellent structure.
* Thornless Mesquite — Lush foliage but tolerates drought or turf once established, requires frequent pruning.
How to plant a tree
Once you’ve selected a tree, it’s important to know how to plant it, and how to care for it during the first year or so while it’s getting established.
Yael Franco supervises the planting of trees for Tree Davis. She has planted and directed crews installing hundreds of shade trees in Davis and other communities in Yolo County. She gives a few pointers.
How big a hole should we dig?
A common misconception is that roots grow deep down, when in reality, roots grow out wide rather than deep. The hole should be two to three times the width of the root ball and about the depth of the root ball. The root ball starts where you see the first wooded root at the base of the trunk. The biggest mistake you can make when planting a tree is planting too deeply. Look for where the trunk starts to flare and ensure that the first wooded root is level with the ground.
Do we need to check the root system and do anything to the roots?
Definitely. Sometimes the roots will have grown in circles and matted inside the container. You should pull these roots apart as much as you can to encourage the roots to start growing out instead of continuing to grow in a circle. When large roots continue in this pattern, they are called girdled roots.
This can choke the tree, leading to decline and eventual death of the tree. This happens less in trees that are smaller in size as they’ve had less time to be in a pot. If possible, opt for smaller trees to avoid this issue. In bare root or balled and burlapped nursery stock this is less of an issue, but you should still inspect the root ball for damage, foul soil odor or girdled roots.
Do you add anything to the soil? Fertilizer?
Research has shown that soil amendments are not advantageous in tree establishment or growth. Since the root system of a newly planted tree is fairly limited, fertilization is not recommended when the tree is planted. Adding a lot of fertilizer, which can be high in salt, can actually lead to increased water stress.
The most helpful thing you can do for your tree is add a thick layer of mulch around the base of the tree after it’s been planted. Make sure it’s about 4 to 6 inches deep and installed in a “doughnut” shape, leaving 3 to 4 inches clear around the base of the tree. Add more mulch seasonally as it decomposes.
What do you backfill the hole with?
Backfill with the native soil you dug as well as the soil that came with the tree.
Do trees always need to be staked? Is there any special way to do that?
In most parts of Yolo County we can experience strong North to South winds. To ensure your young tree develops strength in its roots, trunk and canopy, the best practice is to install stakes on the East and/or West sides of the tree about 6-8 inches away from the base of the tree. You can use a Reddy Stake or wooden lodgepoles with tree tape. Make sure you remove the nursery stake that came with the tree and install stakes that allow the tree to move slightly. This movement is what allows the tree to develop strength.
When can you take the stakes off?
Remove the stakes in 1 to 3 years after the tree has been planted assuming it can stand firmly on its own.
How much water should you give the newly planted tree, and how often in the first year?
Water should be applied as soon as the tree is planted. Note that trees require infrequent, deep watering which is different from lawns, shrubs and perennials. If your tree is planted in the lawn, make sure your tree is getting additional deep watering. In the first few years, your tree should receive about 15 gallons of water every week unless it’s raining. If temperatures are cooler, you can alternate to watering every other week as the soil will retain moisture better. During extreme heat waves, add 5-10 gallons to your weekly watering as needed.
A well-chosen, properly planted tree can live for many decades, providing benefits to you and to future generations. For more information, ask a local plant professional, and visit www.treedavis.org.
— Don Shor owns Redwood Barn Nursery. Dr. Greg McPherson is a retired Urban Forest Researcher with the USDA Forest Service. Both serve on the board of Tree Davis. Yael Franco is an ISA Certified Arborist and Program Manager for Tree Davis.