Girdling Roots: The Strangler That Can Kill Your Trees

There are many reasons why a tree may not look healthy. Insect pests, bacterial and fungal diseases, lack of water or nutrients, winter or storm damage, nearby construction, and improper pruning are some common reasons that many people are familiar with. But there’s one cause you may not know about – girdling roots.

Learn what these roots are, how they form, why they’re dangerous, and what you can do to prevent or correct it to save your trees.

What Is Girdling?

Root girdling is when a tree’s own roots strangle its growth and cut off the flow of water and nutrients from the soil up into the tree’s crown.

You mean the tree kills itself?

Well, not intentionally! But yes, the roots wrap around the tree, growing tighter and stronger with each passing year, and will eventually kill the tree unless the problem is dealt with.

Do all roots have the capability to strangle a tree?

A tree has different kinds of roots for different purposes.

Some tree roots are fine, making an almost invisible network of roots that search for water. Others are woody, rigid anchoring roots that store energy reserves, regulate growth, and stabilize the tree to keep it upright. You’ve probably encountered both kinds if you’ve tried to dig a hole too close to an existing tree.

The large, woody roots that grow laterally from the base of a tree’s trunk start becoming woody as soon as a tree develops its trunk wood and expands its branching structure. Depending on how well a tree has been planted and how it develops, some of these woody, lateral roots can grow partially above the soil surface.

It’s the strong, woody roots that can cause girdling problems.

So what, exactly, does girdling do to my tree?

Girdling causes two noticeable problems: an unhealthy-looking tree that doesn’t grow as it should and a tree that suddenly falls over, sometimes for no obvious reason.

Because it can’t take up enough water and nutrients to support healthy growth, girdled trees fail to thrive. Over time, they lose their leaves and, eventually, they die.

Circling roots also prevent a tree from anchoring itself securely in the surrounding soil, automatically making it a hazard. Winter storms can easily uproot a girdled tree or blow it over in an instant. A girdled tree can also fall over at any time and with no warning.

If your tree is within range of vehicles, property, or people, it is a hazard and you may be held responsible for any damage or injury it causes.

A tree with girdling roots can also limp along for some time, with slowing growth and noticeable stress. Stressed trees are also attractive to insect pests and more susceptible to disease, so your tree might be a target. This puts your other, healthy trees at risk by creating a location or host for pests and diseases that can spread.

Image courtesy of Joseph OBrien, USDA Forest Service, Bugwood.org

Why Does Girdling Happen?

The goal of a tree’s woody roots is to grow out and down, but they can sometimes be thwarted in their efforts. Here are the usual reasons why girdling roots occur.

Improper Planting

Poor planting can cause root girdling. If a tree is planted too deep, the soil that covers lateral roots can encourage girdling by smothering them and preventing their natural development. If a tree is jammed into a hole that’s too small for its rootball, roots will begin girdling.

Learn How to Plant a Tree the Right Way

Container-Grown Trees

Container-grown trees can develop roots faster than their nursery container size allows, leaving the roots nowhere to grow but around and around the container. This starts a process that can doom a tree before it’s transplanted into your yard. As tree roots grow in length, they also increase in girth, or diameter. When this happens, the circling roots expand inward and can push against the tree’s trunk flare (the base of a tree where its trunk ends and its roots begin). This expansion starts the strangling that can kill the tree, as woody and rigid roots don’t move or bend.

Poor Root Pruning

Root pruning, if not done correctly, can create girdling roots. Root pruning is done to remove dangerous or poorly formed roots to make room for newer or better-formed roots. A properly pruned root ball will provide room for new roots and encourage their lateral growth. But if a root is pruned where its direction can still steer roots to encircle the trunk or grow inwards, the pruning won’t prevent girdling.

Small Planting Holes

Planting trees in small planting holes is a common cause of girdling. It’s especially bad when planting holes are surrounded by inflexible concrete or paving. There’s nowhere for the lateral roots near the surface to go, so the natural flaring at the base of a trunk is compressed. Roots meant to grow laterally hit the inflexible edge of the planting hole and change their direction to circle around in the limited volume of open soil. Urban street trees are common victims of this, as their planting holes are notoriously undersized.

Root Barriers

Root barriers can also encourage girdling, even though most people think they prevent it. Poorly installed root barriers or ones in undersized planting holes can behave the same way an inflexible perimeter can, forcing roots to circle instead of growing down and outward. It’s a good idea to have a professional evaluate whether or not you need a root barrier and, if so, install it properly. Some landscape architects and some cities still use standard tree-planting construction details that call for root barriers. In many cases, they haven’t updated or changed the details in a while or evaluated the benefit of root barriers based on research or local examples. Always ask if a root barrier is really necessary.

How can I tell if my tree has girdling roots?

It’s not always possible to identify girdling roots, but here are some clues to look for:

• A straight trunk, with no base flare. Young trees with small trunks don’t always have visible trunk flares, but as a tree grows it develops this flare naturally. If your tree doesn’t have a flare at its trunk base, it could simply be buried too deep. Or it could be that girdling roots have stunted its growth by encircling the trunk base. To check, you’ll need to carefully scrape away soil to expose the root system near the trunk.
• A stunted tree. Because girdling roots cut off water and nutrient movement, your tree may stay small from lack of nutrients and energy. Other signs of stunting include small, yellowing leaves, scant canopy leaf growth, canopy dieback, and undersized branches.
• A loose tree. If you grasp your tree’s trunk and rock it, you might see movement in the soil around its trunk, the rootball may emerge from the soil, and the tree might lean. This happens when a tree’s roots have not grown out into the surrounding soil and instead have circled the rootball.

Image courtesy of Joseph OBrien, USDA Forest Service, Bugwood.org

Can I prevent or correct girdling?

Sometimes there’s nothing to be done for a girdled tree. Depending on its age and size, a tree’s roots may have grown too large and too rigid, and may have formed a tangle of roots that can’t be undone.

But often a girdled tree can be helped.

Newly-Planted & Potted Trees

For young trees in containers, the first thing to do is remove it from its can and expose the rootball. This should be done during dormancy, when a tree is not growing and there’s no risk of water stress.

Wash the soil from the rootball and examine it to evaluate how many roots you can prune off and which should stay. First, trim or move aside fibrous roots. Any flexible roots should be gently coaxed into better positions where they can grow laterally. For rigid roots that can’t be moved or repositioned, prune them back to a point before they begin to encircle. This is the best way to encourage new growth that won’t kink or grow circularly.

Then position the remaining roots over the soil in either a much larger container, or over the base of soil in a planting hole. Make a mound at the bottom of the planting hole and compact the soil with your foot so that the newly planted tree won’t sink as the soil settles. Position the roots over this mound and backfill the planting hole. This is usually a two-person job, as the tree needs to be held in position while the hole is backfilled.

NOTE: This is the same process you’ll use for newly planted trees that are showing signs of girdling, usually within a year or two of transplanting.

Established Trees

For trees that have already established themselves the process is more complicated and should be done by professionals. First, the soil around the tree’s root ball must be excavated so that the roots can be examined. This is done by hand or with an air spade. Although air spades may look ferocious, they are the safest method for exposing roots, as well as for remedying compacted soil.

The roots of established trees are more complex, so an experienced tree care professional should be the one to evaluate the condition of the girdled roots and the likelihood of root pruning success. If root pruning can help, the rootball will be carefully pruned. Larger roots are tricky to remove and even girdled roots may still be the main source for water and nutrient uptake, and overall stability. Removing large, vital roots can stress a tree so much that it can’t recover.

And sometimes the only solution to a girdled tree is to remove it and plant a new one.

Silver Linings

While it can be devastating to have to remove a tree, leaving a declining, girdled tree in place can be worse. You won’t be able to predict its behavior from looking at it, and if its crown still puts out green leaves you might think it’s going to be ok. But it’s a ticking time bomb just waiting to go off.

What Should I Do If I Think My Tree Has Girdled Roots?

If you have a struggling, stunted, or leaning tree, get it evaluated by a tree care professional. An evaluation will narrow down the possible reasons for your tree’s loss of vigor, and a professional will give you realistic options about what to do. No professional will encourage you to leave a problem tree in place if it threatens life or property. Instead, they will explain the likely outcomes of treating a girdled tree and when removal is the best option.

Our work revolves around trees and we want them to be safe, strong, and long-lived. Give us a call if you have any questions about girdling roots that may be affecting your trees. We’ll do our best to keep your trees in good health, and we’ll also advise you to remove any tree that’s unsafe or unlikely to survive.