This is the third article in a series dedicated to sharing our best tips for growing resilient trees that can not only weather intense storms, extreme heat, and drought, but also protect our region from these threats. For over 40 years, the Sacramento Tree Foundation has worked to grow thriving communities through stewardship of our urban forest. In our climate, we depend on trees to make our neighborhoods livable and lovable — but our urban forest also depends on us to help it meet the demands of a changing climate. Here’s our approach, and some tips for residents to grow strong trees today to meet the challenges of tomorrow. 

If you’re wondering about the health of a tree, your first instinct might be to look up at the canopy. However, much of a tree’s health is determined by what happens underground. Healthy, well-developed roots are the anchors that allow trees to stand upright in the face of strong winds. They also help the tree absorb water and minerals, and they store nutrients that allow the tree to grow and defend against stresses.  

There is a common myth that tree roots grow as deep as the tree is tall, creating a mirror image below ground. In reality, most tree roots grow in the top few feet of soil, extending out beyond the tree’s canopy. This image more closely resembles a wine glass sitting atop a dinner plate. Think of the root zone as the entire area underneath your tree’s canopy plus 10 feet beyond that.  

Here are the key elements that roots need to thrive: 

Growing space & soil 

Start by choosing the right tree for the right place – ensure there is ample soil for roots to grow and soil and watering conditions that are suited to the species.   

Root liberation at planting time 

When installing new trees, pause before planting to correct any root issues before your trees get established in their forever homes. Sometimes potted trees can have circling or kinked roots that might grow back toward the trunk instead of outward into surrounding soil. These can become girdling roots that strangle the trunk, preventing flow of water and nutrients. Don’t miss your only chance to liberate the rootball before planting! It might seem harsh, but removing circling roots at this time will encourage new root growth and prevent serious issues in the future. 

Correct planting height 

Buried root flares are also common in potted trees, which can cause trees to be planted too deep. The root flare is where the trunk flares out to meet the first major roots. If the trunk resembles a straight stick in the dirt, the root flare is buried. If needed, remove soil to expose the root flare before planting, and plant the tree so the root flare aligns with the soil line. Trees planted too high will struggle to get established, but trees planted too deep with the root flare buried can rot at the base or develop other underground issues that threaten the tree’s future stability. 

Adequate water 

To absorb water that supports the roots along with the rest of the tree, trees in our climate need occasional irrigation. If trees experience repeated or extended drought stress, their branches will visibly die back. Although we can’t see it, this means the roots are dying back as well, and the tree’s anchors are compromised. Eventually, this stress can make the tree susceptible to other problems and can even lead to tree death.  

All trees need some regular water for the first three years after planting, and after that establishment period, most still need some supplemental deep watering during stretches of dry weather. Learn what types of trees grow in your yard and familiarize yourself with their water needs. 

Lawn sprinklers are not ideal — they only soak the top few inches of soil, but tree roots need water delivered more slowly to reach at least 6 inches below the surface. Deep watering at the tree’s drip line encourages a deeper and more resilient root system.  

No watering stakes or fancy gadgets are necessary, but there are some easy ways to provide a slow and deep soak. Learn our recommended techniques to deep water your trees, where to apply water so trees will benefit, and how to know when it’s time to water. 

A 4-6” layer of natural wood chip mulch (arborist wood chips) spread throughout the tree’s root zone will help conserve moisture in the soil and create an environment where roots can thrive. Keep mulch at least 4 inches away from the trunk.  

Oxygen 

All living cells require oxygen, including the cells that make up tree roots. Healthy soil has tiny pockets called pores that can hold both oxygen and water.  

Waterlogged soil with poor drainage can prevent roots from getting oxygen and allow pathogens to take hold. Allow soil to dry out between waterings so roots have access to oxygen. Some trees tolerate flooding better than others, so if your site has poor drainage, choose species that are suited to occasionally soggy conditions. 

Compacted soil can also block roots from accessing oxygen. To prevent compaction, do not drive vehicles or equipment over a tree’s root zone, and avoid construction of patios or hardscape that requires tamping or compaction for installation. 

Adding soil on top of the root zone after a tree is planted can also be harmful. Oxygen decreases with soil depth, so burying roots deeper might smother them. 

Protection 

Once a tree is planted in its forever home, leave the roots alone. Avoid trenching, digging, removing soil, or grading in the root zone that could cut or injure roots and introduce decay. Just as cutting anchor will cause a ship to become unmoored, cutting roots can threaten a tree’s stability and health.  

Sometimes some root disturbance is unavoidable. Healthy trees can cope with minor damage to their roots, but hire a consulting arborist to make recommendations to protect your tree if the project impacts portions of the root zone or any roots larger than one inch.  

Check back for Part 4 of this series to learn more about tending to your trees aboveground. 

See also:  

• Part 1: Species selection 

• Part 2: Right tree, right place