The root collar of a tree can be defined as an area of the lower trunk that transitions from trunk tissue to root tissue. It is typically associated with the basal flare, or the trunk flare of the tree, which is the wider portion of trunk that insects the ground. In recent years, arborists have spent considerable focus on the root collar, identifying a number of common disorders that can lead to declines in tree health or mortality.
Healthy tree trunks typically have a well-defined trunk flare at the ground level. However, many urban trees don’t have an identifiable trunk flare at the soil surface. Or, in some cases, one side of the trunk may have a normal-looking trunk flare while inspection of the other side of the tree reveals nothing.
One of my first questions for a client with an ailing tree is, “What does the base of your tree look like?”
I’m always interested to learn whether or not a trunk flare is visible because I’ve ran across so many trees over the years in poor health that, upon further investigation, have some kind of root collar disorder.
If you take a look around your neighborhood or a local park you can often find trees that appear to have no trunk flare at all. A seasoned arborist I used to work with referred to those trees as, “telephone poles” since the base of the tree looks very much like a telephone pole, with little or no flare at the ground level.
If your tree looks like a telephone pole going into the ground, the root collar has been buried for one reason or another. Many times, the tree was inadvertently planted too deep to begin with. In other instances, the trunk flare may have been buried after planting. This commonly occurs from soil grading associated with constructions projects, but sometimes is the result of landscaping around the base of the tree.
Regardless of the cause, a buried root collar can impact tree health since soil and mulch are in contact with trunk tissue. Over time, the pressure of soil against the trunk can restrict growth and essentially strangle the tree. This is typically a slow process, but I have seen it occur in young trees planted in very poor soils.
Even in cases where trunk restriction is not an issue, soil traps a large amount of moisture near bark tissue and most trees don’t have bark that evolved resistance to consistent moisture. The result can be a higher mortality rate from common root diseases such as Armillaria and Phytopthora, or simply promotion of rot and decay in trunk wounds present near the soil surface.
The presence of soil against the trunk can also lead to root growth that contacts buried trunk tissue. While roots can successfully grow together, root and trunk tissue cannot. The result is a “girdling root” that restricts trunk growth and strangles the tree over time. In some cases, I have found girdling roots that encircle the trunk entirely, which leads to the slow decline and premature death of the tree. However, most girdling roots only restrict growth on part of the trunk. In these cases, a portion of the trunk flare may be smaller in size or absent entirely.
Certain trees species, such as red maple (Acer rubrum), Norway maple (Acer platanoides), white pine (Pinus strobus), blue spruce (Picea pungens) and others, have a greater propensity for the development of girdling roots. With any of the species that are prone to girdling roots, proper planting depth and monitoring of the root collar over time is incredibly important to reduce the chance that a girdling root may develop.
Arborists perform root collar excavations on young and old trees exhibiting signs of the disorders discussed above. Many times, the simple excavation of soil around the trunk can relive much of the stress on the plant resulting in improved health going forward. Other times, a root collar excavation is used to correctly identify the issue and may not actually help if girdling roots are too large to cut or tree health has been too drastically impacted from past stress.
If you have a tree that may be suffering from a root collard disorder, contact a certified arborist to help with assessment of your plant’s condition. A searchable list of certified arborists is maintained by the International Society of Arboriculture and available by clicking here.