What are Girdling Roots?
Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist
There are many diagrams that show trees with a big tap root directly beneath the trunk. This concept has been proven wrong by many different research studies of tree roots. Most trees do not have tap roots, especially when the soil is compacted or where the there is a high water table. You will not find tap roots on most of the trees in your yard. The exception to the rule might be pines and oaks which as young trees may have tap roots in sandy, well-drained soils.
If there aren't deep tap roots in the soil, where are the roots? Most tree roots are in the top 6 to 24 inches of soil. Tree root systems consist of larger, woody perennial roots and smaller, fine feeder roots. The woody perennial roots grow mostly horizontally in a network that radiates outward from the base of the trunk. Most of these roots will be found in the top six inches of soil, but vertical anchoring roots along this horizontal network may grow several feet deep or more. The function of the woody perennial roots is anchorage, water conduction, mineral transport, and food storage.
The fine feeder roots are much smaller than the perennial roots. They grow outward and upward from the perennial roots and are predominantly located in the top several inches of soil. Feeder roots die and are replaced on a regular basis. Their function is water and nutrient absorption.
It’s not unusual to find young nursery trees with kinked, entwined or circling woody perennial roots. Once planted in the landscape, these roots grow in girth and can "girdle" or strangle the tree, preventing water and nutrients uptake. Girdling or circling roots also compromise the structural integrity of a root system and can lead to wind-throw in severe winds.
Sometimes circling roots on established trees can be cut with a sharp knife or chisel, but often the problem is noticed too late for this corrective action. If you suspect that your trees have girdling or circling roots, consult an arborist before taking any action. Keep in mind that large woody roots are your main framework of the root system. Cutting large roots can further compromise their anchoring ability.
Avoid girdling root problems when purchasing trees. Check the root systems before planting and only accept trees with well-distributed root systems, ones without kinked or circling roots.