The health and longevity of any tree begins with correct planting depth, which is determined by the root flare. This distinct, transitional area where the trunk widens as it meets the root system is foundational to a tree’s long-term survival. Ensuring this anatomical feature is properly exposed is one of the most important tasks for a homeowner or landscaper. By focusing on this detail, you establish a structure that supports the tree for decades to come.
Defining the Root Flare
The root flare is the visible, outward curve at the base of the trunk where it transitions into the main supporting roots. This area is also commonly referred to as the root crown or root collar. A healthy, mature tree typically displays a noticeable “flare” or taper, resembling a graceful slope leading into the ground, rather than a telephone pole going straight into the soil.
To identify the true root flare, especially on young trees from a nursery, you may need to gently remove excess soil or potting medium from around the base of the trunk. The flare is marked by the point where the first main structural roots begin to emerge laterally from the trunk. In nursery stock, the flare is often buried several inches deep due to “potting-up,” which makes this pre-planting inspection necessary.
In some cases, small, pencil-like roots may be found growing higher up the trunk; these are adventitious roots, which grow in response to excess moisture and soil against the trunk. The actual structural roots that form the flare are significantly larger and anchor the tree. Locating the point where these substantial roots emerge determines the correct planting depth.
Biological Role of Proper Exposure
The practice of keeping the root flare exposed is based on the distinct biological needs of the trunk tissue compared to the root tissue. The tissue of the trunk, covered by bark, is aerial tissue, meaning it is designed to be above ground and requires air exposure. Bark contains lenticels, which are pores that facilitate the necessary exchange of oxygen and carbon dioxide for the tree’s health.
When the flare is buried by soil or piled-up mulch, the bark remains constantly moist, which inhibits gas exchange and creates an environment conducive to decay and opportunistic pathogens. This decay can compromise the tree’s vascular system, which is concentrated in the flare area for transporting water and nutrients. A lack of oxygen can also suffocate the root system, causing the tree to enter a state of stress.
A buried flare also weakens the tree’s physical stability and encourages the formation of damaging roots. The lack of air causes roots to grow vertically toward the surface in search of oxygen, often circling the trunk and becoming girdling roots. These roots constrict the trunk as they grow thicker, effectively choking the tree by restricting the flow of water and nutrients, which can lead to decline and premature failure.
Practical Guide to Correct Exposure
The root flare should be completely visible and positioned slightly above the finished grade of the surrounding soil. For most planting scenarios, the top of the root flare should sit approximately 1 to 2 inches higher than the surrounding soil level. Planting slightly high accounts for the inevitable settling of the disturbed soil in the planting hole.
Before digging the planting hole, you must first locate the true root flare on the root ball. Use a hand tool or your fingers to carefully remove soil from the top of the root ball until the first main lateral roots are found. This establishes the true top of the root ball and the required planting depth.
The planting hole should be dug wide—two to three times the width of the root ball—but only as deep as the distance from the bottom of the root ball to the exposed root flare. Once the tree is positioned, the backfill soil should only come up to the level of the flare, ensuring it remains exposed. Mulch should then be applied in a broad, thin layer, but kept several inches away from the trunk itself to prevent moisture buildup against the bark.
Diagnosing and Correcting Buried Flares
Trees that were planted too deeply or have had soil or mulch accumulate around the base will often show signs of stress and decline. A primary visual symptom of a buried flare is the absence of the characteristic trunk taper, where the trunk appears to enter the ground like a straight pole. Other indicators include premature leaf drop, dieback in the canopy, and a general lack of vigor despite adequate watering.
The tree may also exhibit swelling or cracking of the bark near the soil line, which is a sign of rot and decay due to constant moisture. Furthermore, the presence of girdling roots—roots that circle the trunk just beneath the surface—is a common consequence of a buried flare, as the roots grew upward to find air. These roots can sometimes be seen pushing up the soil or bark.
To correct a buried flare, a process called root collar excavation is performed to remove the excess soil and mulch. This is done carefully using hand tools or, for more extensive issues, an air spade that uses pressurized air to remove soil without damaging the roots and bark. The goal is to expose the entire root flare and the bases of the main structural roots.
During the excavation, any girdling roots that are constricting the trunk must be carefully pruned away. These roots will not correct themselves and will continue to choke the tree if left in place. The soil grade should be permanently lowered around the tree to match the exposed flare, and mulch should be pulled back from the trunk to prevent the problem from recurring.