The entire root flare should be visible at or just above the soil surface. In practical terms, this means the point where the trunk begins to widen and the topmost structural roots emerge from the trunk should sit at grade level or no more than an inch or two above it. There is no benefit to leaving part of the flare buried, and research increasingly shows that even modest burial of this transition zone can quietly undermine a tree’s health for years.
What the Root Flare Actually Is
If you look at a healthy tree growing in a forest, you will notice the trunk does not plunge straight into the ground like a telephone pole. Instead, it widens near the base, and thick roots fan outward from that widened zone. That widening is the root flare, sometimes called the trunk flare or root collar. It is not just an aesthetic feature. The bark on the trunk and the tissue covering the roots are structurally different, and the flare is the transition between them. Trunk bark is designed for air exposure. Root tissue is designed for soil contact. When soil, mulch, or fill dirt covers the trunk bark in that transition zone, the tissue stays perpetually moist, creating conditions that invite decay fungi and interfere with gas exchange.
The trunk flare region also plays a role in how large trees interact with surrounding infrastructure. Research on mature urban trees has shown that understanding the maximum size and typical shape of trunk flares and buttress roots helps managers predict which trees will eventually damage sidewalks and curbs, and select appropriate species for tight planting spaces.1Urban Forestry & Urban Greening. Predicting trunk flare diameter to prevent tree damage to infrastructure That same widening zone you are trying to expose at planting is the zone that will expand substantially as the tree matures.
How Deep Is Too Deep
The standard arboricultural guideline is straightforward: the topmost structural root should sit within the top two to three inches of soil after planting. If you cannot see any flare at all when you look at the base of a newly planted tree, the tree is almost certainly too deep. Deep planting, where the root collar sits below the soil surface, is one of the most common mistakes in both residential landscaping and commercial nursery production. Trees frequently arrive from the nursery with several inches of excess soil above the root flare, having been potted too deeply during production or having settled in the container over time.
Research confirms that burying the root collar below soil grade impairs tree development and establishment, affecting root architecture and long-term survival.2Forests. Impact of Planting Depth on Urban Tree Health and Survival The problems are not always immediately visible. A tree planted four or six inches too deep may leaf out fine in its first season and even grow respectably for a few years. But the buried trunk tissue slowly deteriorates, and adventitious roots that form above the original flare often grow in circles around the trunk rather than radiating outward. These circling roots can eventually girdle the trunk, strangling the tree’s own vascular system.
Not All Species React the Same Way
One of the complications in giving a single universal answer is that tree species vary in their sensitivity to burial. A ten-year study on red maple and northern red oak trees planted about 12 inches below grade found that deep planting generally slowed growth in red maple over the observation period, but there was no clear evidence it affected red oak growth over nine years.3Urban Forestry & Urban Greening. Growth and stability of deep planted red maple and northern red oak trees and the efficacy of root collar excavations Red oaks appeared more tolerant of the buried condition, at least within the timeframe researchers were watching.
This does not mean you should deliberately plant red oaks deep. It means that if you discover a red oak with a buried flare, the situation may be less urgent than it would be for a maple, which tends to suffer earlier and more visibly. Maples, beeches, lindens, and dogwoods are among the genera that arborists generally consider most sensitive to deep planting. Oaks, elms, and certain conifers seem to tolerate some degree of burial for longer, though they are not immune to the long-term consequences. The safest approach for any species is to plant at the correct depth from the start.
How to Find the Root Flare Before Planting
Before you put a balled-and-burlapped or container-grown tree in the ground, you need to locate the actual flare, not just the top of the root ball. These are often not the same thing. Nursery soil, burlap, excess potting mix, and even wire baskets can all obscure the true flare by several inches or more.
To find it, carefully remove soil from the top of the root ball with your hands or a blunt tool, working from the trunk outward. You are looking for the first woody root that extends laterally from the trunk at a roughly horizontal angle. This root should be thick enough to feel structural, not a thin hair root. On a typical two-to-three-inch caliper nursery tree, you might need to remove anywhere from one to six inches of excess media before you hit the first real root. Once you find it, that root should end up within about two inches of the finished soil surface when the tree goes in the ground.
If you are planting a bare-root tree, the job is easier because you can see the entire root system. Set the tree so the uppermost lateral root sits just below grade. For container trees, be especially suspicious. Trees that have been shifted from one pot to a larger one during nursery production sometimes have two or three inches of accumulated soil above the original root collar at each repotting. You may find circling roots that have already formed in the container and should be straightened or pruned before planting.
Mulch Volcanoes and Gradual Burial
Even trees planted at the correct depth can end up with buried root flares over time. The most common culprit is mulch piled against the trunk in a cone shape, sometimes called a mulch volcano. Landscape maintenance crews often add fresh mulch annually without pulling back the old layer, and after a few years the trunk can be buried under six or more inches of decomposing organic material. This is functionally identical to deep planting. The trunk bark stays wet, decay organisms colonize the tissue, and adventitious roots may grow into the mulch itself.
The fix is simple: pull mulch back so that it does not touch the trunk. A ring of bare soil two to four inches wide around the base of the trunk is ideal. Mulch should extend outward from there in a flat or slightly doughnut-shaped ring, ideally covering the root zone but never mounding against the bark. If you inherit a property where mulch has been piled against trees for years, carefully remove the accumulated material down to the soil surface and look for the flare. You may need to do a root collar excavation if the flare is no longer visible even with the mulch removed.
Root Collar Excavation on Established Trees
When the root flare of an existing tree has been buried for years, whether from deep planting, mulch accumulation, grade changes, or construction fill, the corrective procedure is called a root collar excavation. This involves removing soil from around the base of the trunk down to the level of the first structural roots. On small trees, you can do this by hand with a trowel and patience. On larger trees, arborists typically use an air spade or air excavation tool, which uses compressed air to blow soil away from the roots without cutting or damaging them.
The goal is to expose the entire flare and all major roots radiating from it, usually in a zone extending roughly 12 to 18 inches out from the trunk, though more may be necessary if girdling roots are present. Any roots found circling or pressing against the trunk are typically removed with a sharp saw or chisel. The excavated area is then left open or filled with a coarse material that allows airflow, not backfilled with dense soil.
Research on deeply planted Turkish hazels showed that trees with excavated root collars performed slightly better during flooding stress. Photosynthetic rates declined in all trees during a flooding event, but there was an apparent slight delay in that decline for trees with excavated root collars and those planted at grade.4Urban Forestry & Urban Greening. Growth, survival, and root system morphology of deeply planted Corylus colurna 7 years after transplanting and the effects of root collar excavation The benefit was modest, but it suggests that even years after planting too deeply, correcting the depth gives the tree a better chance of handling environmental stress.
In fruit tree production, root collar excavation has shown more dramatic results. Peach orchards on sites where Armillaria root rot was a problem saw reduced tree mortality when preventative root collar excavations were performed. The delayed onset of disease-related death and lower annual mortality in excavated trees translated to measurably higher profitability over the life of the orchard.5PubMed. Preventative Root-Collar Excavation Reduces Peach Tree Mortality Caused By Armillaria Root Rot On Replant Sites Keeping the root collar exposed and dry reduced the conditions that favored the fungal pathogen, essentially removing the welcome mat.
How Aggressively Can You Excavate
One common concern when exposing a buried root flare is whether the process itself harms the tree. Root collar excavation done with an air spade is generally low-risk because it removes soil without cutting roots. But sometimes the flare has been buried so deeply, or for so long, that significant root cutting becomes necessary to remove girdling roots or to access the original flare beneath layers of adventitious growth.
Research on root severance provides some useful context. A study that tracked two urban tree species for over four years after trenching, which removed up to 70 percent of the root system, found that tree survival was not significantly affected even at the most severe damage levels. However, growth and physiology were measurably impaired: stem diameter growth dropped by roughly a sixth to a quarter, and shoot elongation declined by about a third to two-fifths compared to undamaged trees. The trees’ ability to photosynthesize also dipped, and even after more than four years the recovery was incomplete. Uprooting resistance remained 20 to 66 percent lower in damaged trees compared to controls, depending on severity.6Urban Forestry & Urban Greening. Evaluating the effects of trenching on growth, physiology and uprooting resistance of two urban tree species over 51-months
That study examined trenching, which is more aggressive than a typical root collar excavation. It involved cutting major roots at a set distance from the trunk, not just removing soil. But the takeaway is still relevant: when you remove roots, even with good reason, there is a real and lasting cost to the tree’s growth and structural stability. This is why arborists generally recommend removing only roots that are actively girdling the trunk or clearly causing harm, rather than cutting every root exposed during excavation. A root that crosses over another root but is not pressing into the trunk can often be left alone.
Earlier observations on the same experimental setup found that root loss induced mild water stress in the trees, even in years with ample rainfall. The growth reduction after root severance appeared to be driven by reduced gas exchange and less favorable water relations in the canopy.7Acta Horticulturae. EFFECTS OF ROOT SEVERANCE BY EXCAVATION ON GROWTH, PHYSIOLOGY AND UPROOTING RESISTANCE OF TWO URBAN TREE SPECIES Essentially, removing roots reduces the tree’s ability to supply water to its leaves, and that bottleneck ripples through everything from photosynthesis to branch extension. The practical lesson: be conservative with any cutting during a root collar excavation, and time the work for periods when the tree is not already drought-stressed.
Signs That a Root Flare Needs Attention
If you are looking at an existing tree and wondering whether the flare is buried, there are a few visual clues. The most obvious is the “telephone pole” look: the trunk enters the ground at the same diameter, with no visible widening at the base. A healthy, properly planted tree should have a visible taper at the soil line. If the trunk looks like a post driven into the earth, the flare is almost certainly buried.
Other signs include bark that looks abnormally dark, wet, or soft near the soil line. You may notice fungal fruiting bodies (mushrooms or shelf fungi) at the base of the trunk. Canopy symptoms are less specific but can include smaller-than-normal leaves, early fall color, branch dieback starting at the tips, and a general thinning of the crown. These canopy symptoms can have many causes, but when combined with a missing flare at ground level, buried root tissue is a strong suspect.
Trees with buried flares are also more prone to developing stem girdling roots, which can slowly strangle the trunk over decades. If you see a trunk that appears to have an indentation or a flat side near the base, that flat area may be where a girdling root beneath the soil is pressing inward. Exposing the flare and inspecting for girdling roots sooner rather than later gives the tree a better chance, since girdling roots caught early and small are easy to remove, while those that have been compressing the trunk for 20 years may be too large to cut without creating an unacceptably large wound.
Timing and Aftercare
Root collar excavations can be performed at any time of year, but most arborists prefer to do the work in late winter or early spring before the tree leafs out. This gives the tree a full growing season to respond to any root cuts that were necessary. Avoid excavating during drought, heat waves, or in the middle of summer when the tree is already working hard to keep its canopy hydrated.
After exposing the flare, the excavated area should be left open. Resist the urge to fill it back in with soil or compost. The whole point is to get trunk bark out of contact with moist material. Some arborists install a root collar guard, which is basically a physical barrier to prevent mulch or soil from being pushed back against the trunk during future landscape maintenance. If the excavated zone is deep enough to create a noticeable depression around the base of the tree, a ring of coarse gravel can help with drainage while discouraging organic accumulation.
Monitor the tree for two to three years after excavation. You may see a temporary slowdown in growth as the tree adjusts, especially if girdling roots were removed. New growth should gradually improve in vigor if the flare exposure was the limiting factor. If the tree continues to decline despite a successful excavation, there may be additional problems at play, such as compacted soil over the broader root zone, ongoing drainage issues, or disease that was already well established before the flare was uncovered.
When Professional Help Is Worth It
For a small, recently planted tree that is clearly a few inches too deep, you can often excavate the flare yourself with hand tools. If the tree is young enough and light enough, you might even be able to lift it, remove excess soil from the bottom of the hole, and replant at the correct depth. This is far easier than waiting years and then doing a corrective excavation on a mature tree.
For established trees with trunks larger than about six inches in diameter, or trees where the flare has been buried for many years, hiring a certified arborist with an air spade is the safer route. Air excavation is fast, minimizes root damage, and allows the arborist to assess what is happening beneath the soil before deciding which roots to remove. The cost varies widely depending on the size of the tree and the extent of burial, but it is typically a fraction of the cost of removing and replacing a mature tree that has declined because its flare was never addressed.
One scenario where urgency matters more than timing: if you see mushrooms growing directly from the trunk at the soil line, or if the trunk bark feels spongy and gives way under thumb pressure near grade, decay may already be advanced. Exposing the flare in that case is diagnostic as much as therapeutic. It lets the arborist see how far the damage extends and whether the tree is still structurally sound or has become a hazard. A tree with extensive basal decay may need to come down regardless of what you do with the soil around it, and knowing that sooner is better than discovering it during a storm.