Root Collar Excavation, Girdling Roots, and Root System Health

Root collar excavation is the careful removal of soil and mulch from the base of a tree so the root flare and the structural roots beneath it can actually be seen. Done with compressed air it strips the soil away without cutting anything, which is why the same operation serves as both the diagnosis and, when the finding calls for it, the repair. The same tool decompacts a root zone that has been driven over, built on or walked flat, and lets me work organic matter into the soil around a tree that is already struggling. Where a tree is in critical stress, that decompaction and amendment is the urgent part. I am Christopher Hodge, an ISA Board Certified Master Arborist, WE-14385B, and a TRAQ qualified consulting arborist working out of Placerville across El Dorado, Sacramento, Placer and Amador counties. I own an air spade and perform this work myself. That creates a conflict of interest, and rather than bury it I have given it its own section further down this page.

This page is about the health of the root system itself: whether the flare is where it should be, whether roots are strangling the trunk, and whether the soil around them still functions. If your problem is a root lifting a slab or pushing at a footing, that is a different question with different answers.

The root flare should be visible at grade, and usually it is not

Where a healthy trunk meets the ground it widens. That widening is the root flare, also called the trunk flare or the root collar, and it is the structural transition from stem tissue to root tissue. The International Society of Arboriculture's planting guidance is blunt about it: ensure the trunk flare is partially visible after the tree is planted, and remove excess soil before planting if the flare is not visible. The University of Minnesota puts the same rule from the other direction, saying the first true branch roots should be at or barely below the soil surface.

So the first thing I look for is not a symptom, it is a shape. A trunk that enters the ground like a utility pole, straight sided with no visible taper, is telling you that the real base of the tree is somewhere below your feet. The California reference for our oaks, Costello, Hagen and Jones in the UC ANR volume on oaks in the urban landscape, gives the practical decision rule for an inspector: in cases where the trunk flare is not evident, an excavation is likely warranted. That is the entire logic of root flare exposure. You cannot assess what is buried, and a buried flare is itself the finding.

Buried does not always mean somebody dumped fill. A tree can arrive from the nursery already too deep in its own container or root ball. Gary Watson and colleagues, in their review of tree root management for the ISA journal, describe how a cut taproot throws vigorous adventitious roots at the cut end while the natural lateral roots above are lost, so the tree effectively builds a substitute flare deeper down. Their observation is that even when a tree is planted at its original depth with the graft union above ground, that adventitious root flare can sit 30 cm or more below the soil surface.

Stem girdling roots and where they come from

A stem girdling root is a root growing against the trunk rather than away from it, compressing the sapwood as both the root and the stem enlarge. Gary Johnson at the University of Minnesota, whose work with colleagues is the most complete treatment of the subject in North America, describes the compression eventually slowing or stopping the movement of water, nutrients and photosynthates in both directions. The root does not have to encircle the tree to do harm. It only has to press.

Johnson and Fallon separate three patterns, and the distinction changes the prognosis:

• Tangential girdling, where a single root compresses one side of the stem. These are described as the more treatable situations, particularly when caught early.
• Layered girdling, where roots stack against the stem at different depths and often on different sides. This is called the most damaging type, and the guide notes that the deeper a tree is planted, the more likely it is to happen.
• Complete encircling of the stem, most commonly by two or more roots.

The causes are almost entirely things people did. Container stock that sat too long in a pot arrives with roots already circling, and those roots keep their shape after planting. Roots cut for a sidewalk, a trench or a transplant tend to regrow tangentially along the stem instead of radiating outward. Planting too deep buries the stem, and a buried stem grows adventitious roots. Johnson's summary of the mechanism is the sentence I quote most often on site: if tree stems are completely above ground, stem girdling roots are not likely to form, but any practice that puts stem below ground, or below mulch, puts the tree at risk of developing them.

There is a controlled number behind this. Christina Wells and colleagues, publishing in Arboriculture and Urban Forestry in 2006, planted trees at grade and at 15 cm and 31 cm below grade. Two years after transplant, half of the deep planted Yoshino cherries had died while every cherry planted at grade survived. In red maple, girdling roots involved 14 percent of trunk circumference in the controls, 48 percent at the shallower deep planting and 71 percent at the deeper one. Their conclusion was that planting deep by as little as six inches had serious consequences for survival in one species and girdling root development in the other. That is the clearest evidence available that a tree planted too deep is not a cosmetic problem.

Two honest qualifications. Johnson and Fallon note that at least one species, Norway maple, may be genetically prone to chronic stem girdling root formation, and that the problem has been reported on at least 56 tree species in North America, so this is not a single species story. And their estimate that one third to one half of tree losses during windstorms may be attributable to distorted and dysfunctional roots is presented in that publication as an estimate from University of Minnesota data collected between 1995 and 2005, not as a controlled study. I report it as what it is, because the direction of the finding matters even where the precision does not.

The mulch volcano, which is the same mistake wearing a different hat

The consumer facing name for this is the mulch volcano, and the ISA uses the term directly, defining it as an excessive pile of mulch material applied around the base of a tree. The recommended depth in the ISA guidance is 2 to 4 inches, applied near but not touching the trunk, with fine mulches at the shallower end and coarse arborist chips at the deeper end. Where mulch is already piled against a trunk, the instruction is to pull it back several inches so the base of the trunk is exposed.

The harm runs on two tracks at once. The ISA lists both: mulch against the trunk softens the tissue and makes it more susceptible to insects and disease, and mulch against the trunk can also lead to the growth of stem girdling roots, which can reduce growth or eventually kill the tree. Johnson puts it more sharply, writing that piling mulch against the stem is the same as planting too deep, because planting too deep or piling mulch against stems encourages adventitious roots to grow in directions that within a few years compress the stem. The volcano is a slow burial, and the tree responds to burial the only way it knows how, by rooting into it.

The practical version, once soil or mulch comes off, is that it does not go back on. Johnson's instruction after an excavation is explicit: do not replace the excavated soil and bury the stem again, and cover the exposed roots with a light application of mulch, 2 to 4 inches, kept off the stem. The ISA planting brochure asks for a mulch free ring of 1 to 2 inches at the base of the trunk for the same reason, to keep moist material off the bark.

What an air spade does that a shovel cannot

Air excavation uses a supersonic jet of compressed air to fracture and displace soil. Soil particles move; roots, bark and rock do not. That is the entire advantage, and it is why the technique is the standard for this work rather than a preference. Johnson describes the specialized equipment as using pressurized air to displace soil without damaging the roots. The UC ANR oak volume lists a pneumatic excavation tool alongside hand tools and hydraulic excavation as the acceptable methods for exposing a buried root collar, and pairs it with the warning to avoid bark damage in the process.

The UC Statewide IPM Program's guidance for root disease says the same thing in fewer words: use hand tools or compressed air excavators to remove soil from the base and expose the large structural roots and the root flare, never use construction machinery that may wound the root collar or roots, and keep the bark undamaged and intact. A mechanical excavator, a trencher or a shovel in a hurry cannot make that distinction. It cuts whatever is in the way, and what is in the way is the thing being assessed.

This is why air spading tree roots is a diagnostic act before it is a remedial one. A tree root inspection done this way answers questions that no amount of looking at the canopy can settle: where the flare actually is, whether a root is contacting the stem and on how much of the circumference, whether structural roots are present on all sides or missing on one, whether there is decay, and whether Armillaria mycelial fans are present in living tissue at the collar. The UC oak volume says it plainly for our region, that in cases where an oak root fungus infection is suspected, one should conduct a root collar excavation and examination. Watson and colleagues note the same efficiency gain for construction work, that the introduction of air excavation tools has made locating roots considerably more efficient.

The scale is usually smaller than people expect. Johnson describes a typical root collar inspection as excavating 6 to 12 inches out from the stem and 4 to 12 inches deep. The UC oak guidance for fill soil against a trunk asks for soil removal within a distance of at least 12 inches of the trunk and down to the depth of the trunk flare, exposing the bark to the air so it can dry. A diagnostic excavation is a small, careful hole, not a trench around the tree.

The foothill version of this problem: native oaks, summer water and grade

Everything above is general arboriculture. What makes it a local page is which trees it happens to, and here it is overwhelmingly the native oaks that were already on the lot when the house went in. Valley oak, blue oak and interior live oak get landscaped around, and the landscape brings three things at once: fill or a grade change at the trunk, an irrigation system, and compaction from whatever drove across the root zone.

The USDA Forest Service field guide to California oak insects and diseases connects those inputs to an outcome in one passage. Severe Armillaria root rot commonly develops in native oaks exposed to prolonged periods of irrigation in the summer, and this typically occurs when irrigated landscaping is installed around existing oaks. It goes on to name the other factors that stress or damage roots and favor severe disease: soil compaction, addition of fill soil, and cutting or crushing of roots by construction equipment. In undisturbed stands the same fungus is described as normally a minor pathogen, typically attacking only oaks already in severe decline. The organism did not change. The site did.

The UC oak volume reaches the same conclusion from the soil side, noting that Armillaria root disease and Phytophthora crown rot are both favored by moist soil conditions, especially when soil is in contact with the lower trunk, and that placing fill soil against a trunk is ill advised for exactly that reason. UC's own root disease guidance adds two lines worth memorizing: keep the root collar dry through the warm summer months, meaning do not let a sprinkler hit the base of the trunk, and note that deeply planted trees, or trees with soil covering the root collar area, are often more susceptible.

Geology sharpens this in El Dorado County. On the foothill lots I work, soil depth over decomposed granite or metavolcanic bedrock is often modest, so there is less vertical room for roots than the tree's size suggests and more of the root system is running near the surface. The ISA's own planting guidance notes that the majority of a tree's roots develop in the top 12 inches of soil, and where bedrock sits close, that concentration is not a tendency, it is a ceiling. Six inches of fill at the trunk is a much larger fraction of the available profile there than it would be elsewhere. Out on the Sacramento Valley floor the constraint is different but not gentler: heavier clay drains slowly, holds applied water longer, and keeps a buried collar wet for weeks rather than days. Placer and Amador lots tend to sit somewhere between the two, and in practice the diagnosis follows the soil more than the county line.

Construction, trenching and driveway cuts

Tree protection during construction is mostly a root problem wearing a hard hat. The UC oak volume lists the disturbances that cause significant injury and asks an inspector to check for every one of them: trenching, excavation, soil compaction, paving, fill soil installation, and changes in drainage patterns. It also flags that during site development, excavations for foundations, trenching for utilities, soil compaction and changes in hydrology can all adversely affect the structural stability of root systems, with cutting of lateral, heart and sinker roots a common occurrence.

The stability half of that is the part homeowners tend not to see coming, because a tree that lost roots on one side can look completely normal until a storm arrives. The same volume states that root pruning which removes lateral roots close to the trunk can substantially destabilize a tree, particularly if the roots are cut on the windward side, and that wounding from root pruning and mechanical injury can lead to decay, which further reduces the ability of those roots to carry load. A trench for a water line on the uphill side of an oak is not the same event as the identical trench on the downhill side, and nobody trenching it will know that.

If a root is already lifting a walkway or a driveway slab, the question of which root can be cut and how far out is answered on the

rather than here, because there the structure is the client. On this page the tree is.

Root pruning: what the distance guidance actually says

There is real guidance on how close to a trunk a root can be severed, but it is narrower than the confident rules of thumb that circulate. The most specific published figure I am comfortable quoting comes from Watson and colleagues' review, which concludes from the available studies that vigorous trees less than 30 cm in diameter may be able to tolerate roots being severed on one side as close as three times the trunk diameter without major loss in stability. Note every condition in that sentence: vigorous, under about 12 inches of trunk diameter, one side only, and stability specifically rather than health.

That leaves most of the trees I actually get called about outside the tested range. A 30 inch valley oak is not a vigorous 10 inch street tree, and the guidance does not scale by simple multiplication. So my honest position on tree root pruning is that the published distance guidance exists, is species and size dependent, and thins out fast for large mature trees, which is why the decision on a big oak comes from what the excavation shows about how many structural roots there are and where they run, not from a formula applied to a tape measure. Anyone quoting you a single universal multiplier for every tree is going past the evidence.

Removing a girdling root is the same judgment in miniature. Johnson's threshold is that girdling roots may be removed if less than half the stem circumference is compressed, and that where removal would involve severe wounding of the stem, the root should not be removed but instead cut at both ends where it contacts the stem to stop its growth. He is equally direct about the ceiling on what treatment can achieve: if stem compression has already occurred, there is often not much that can be done, and removing the root may take an extensive branch root and its fine roots with it, potentially accelerating decline rather than reversing it. Prevention outperforms treatment here by a wide margin, and saying so costs me work.

Decompaction and soil amendment, and when it cannot wait

If a tree is in critical stress and the root zone is compacted, decompaction and amendment are not work to schedule for next season. They are the immediate work. A root system that cannot get air and water through the soil is losing ground every week it stays that way, and decline in a mature tree is not a straight line. By the time the canopy shows it plainly, the tree has usually been running a deficit underground for years. So when I find compaction on a tree that is already declining, the recommendation is to open the soil and amend it now, and to take up the slower questions afterward.

The work itself is direct. I use the air spade to break up the compacted zone under and immediately around the tree without cutting roots, which is the entire reason a pneumatic tool is used here rather than a machine or a shovel. Once the soil is open, organic matter goes into fine textured soils, water settles it, three to four inches of mulch goes over the top, and access is restricted afterward so the ground does not simply get packed down again. UC's oak volume sets out that same sequence. The last step is the one people skip, and it is the one that decides whether any of it holds.

Compaction is measurable rather than a matter of opinion, which is what makes it worth acting on with confidence. Watson and colleagues put the critical limit of soil strength for woody plant roots at 2.5 MPa on a standard penetrometer. Bulk density matters as well, but there is no single threshold for it, because it depends on texture: 1.60 grams per cubic centimeter would restrict roots in a clay loam and would not in a sandy loam. So I measure in the compacted zones and read the result against published densities for that soil class. Anyone quoting one number for every soil has skipped a step.

Two specific techniques deserve a word of caution, because they get sold harder than the evidence supports. Vertical mulching, which is augered holes backfilled with sand and bark, and radial trenching both give mixed results in the literature. Day and Bassuk found no growth difference two years after vertical mulching around established trees, while radial trenching did help Callery pears planted into compacted clay loam. Their conclusion was that no universally successful technique is available. I use either one where the soil and the situation genuinely call for it rather than as a default, and I will tell you which you are getting and why.

On root collar excavation itself, the diagnostic value is not in question and I will stand behind it without qualification. Exposing the flare shows you what is actually there, and you cannot treat what you have not seen. The corrective side is more variable: one published study of street trees found no measurable growth effect over four years, and early removal of girdling roots on young Norway maples did not stop multiple roots reforming. That is a reason to keep watching a tree after the work is done. It is not a reason to leave a girdling root in place around a stem it is strangling.

A disclosure, because I now perform the work I diagnose

For most of my practice the answer to whether I do the work was simply no. I do not prune, I do not remove trees, and I run no removal crews, so I have never had a reason to talk anyone into a cut. All of that is still true. What changed is that I bought an air spade, and I now perform root collar excavation, root pruning and soil remediation myself.

That creates a conflict I would rather name than have you discover. When I excavate a root collar and find a buried flare or a girdling root, the corrective work I describe is work I am able to perform and be paid for. So where a report of mine identifies excavation or root work I can carry out, I disclose that in the report, so you can weigh the recommendation with that in mind and hand the work to someone else if you would rather. The finding does not change either way, and neither does the fee for the finding.

Two related limits belong in the same breath. I hold a California Qualified Applicator License and can apply control materials, but I am not a Pest Control Adviser, so writing a pesticide recommendation is not a licensed function of mine. And I do not offer tree appraisal or valuation. If a matter needs a dollar figure placed on a tree, I refer that out to someone qualified to do it.

Being explicit about this matters more in my case than it might in another, because a real share of my work is documentation that other people read: reports that go to a county, an insurer, an attorney or a court. A finding is only worth what the reader can trust about how it was arrived at, and a disclosed interest is a manageable one. An undisclosed interest is the thing that gets a report thrown out.

What it costs, and what it is being weighed against

A site assessment with a written report starts at $450. That includes the inspection, the diagnosis, the evidence behind it and the realistic options, in a document you can act on or hand to a contractor. The site visit on its own is $250 and covers up to one hour on site, with no report included. Excavation, root pruning or soil work is quoted separately once I know what the site actually requires, and always before any work is scheduled.

Set that against the number it is competing with. Removal of a medium sized tree near a house commonly runs $1,500 to $4,000, and it is the one decision on this list that cannot be reversed. A great many of the declining trees I get called to look at turn out to have a buried collar, a girdling root, a mulch volcano or a grade change at the base, and those are conditions with something to do about them. Finding out which case you are in costs a fraction of the removal and is the step that logically comes first.

If a tree is thinning from the top down, if the trunk goes into the ground without a visible flare, if mulch or soil is heaped against the bark, if a landscape and an irrigation system went in around an existing oak, or if a trench or a driveway cut went through the root zone, a root collar excavation is the assessment that answers the question rather than guessing at it. Call or text (530) 391-6100.

Frequently Asked Questions

What is root collar excavation?

Root collar excavation is the removal of soil and mulch from the base of a tree to expose the root flare and the structural roots underneath so they can be inspected. Done with compressed air it displaces soil without cutting roots or damaging bark, which is why it works as both a diagnostic and a corrective procedure. UC guidance for root disease specifically calls for hand tools or compressed air excavators and warns against construction machinery that may wound the root collar or roots.

What is a root flare and why does it matter?

The root flare, also called the trunk flare or root collar, is the widening where the trunk transitions into the root system, and it should be at least partially visible at ground level. When it is not visible, the base of the tree is buried, and buried stem tissue is what allows stem girdling roots to form and holds moisture against bark that is not built to stay wet. A trunk that enters the soil straight sided with no taper is the single most useful thing a homeowner can learn to notice.

How do I know if my tree has girdling roots?

Above ground you may see a trunk with no visible flare, a flat spot or constriction on one side of the base, undersized or off color leaves, early fall color, dieback at branch tips, or thinning canopy density. None of those is proof, because drought and root disease produce similar symptoms. Confirmation requires exposing the root collar, which is why the University of Minnesota guidance treats above ground symptoms as a clue and a root collar inspection as the final diagnostic step.

Is a mulch volcano really that bad for a tree?

Yes, and the mechanism is well documented. The ISA states that mulch applied against the trunk softens the tissue and makes it more susceptible to insects and disease, and that mulch against the trunk can also lead to stem girdling roots that reduce growth or eventually kill the tree. University of Minnesota guidance treats piling mulch against the stem as equivalent to planting too deep, because both bury stem tissue and encourage adventitious roots. Correct depth is 2 to 4 inches, applied near the trunk but not touching it.

Can girdling roots be removed safely?

Sometimes, and it depends on how much of the trunk is already compressed. The University of Minnesota threshold is that a girdling root may be removed when less than half the stem circumference is compressed, and that where removal would severely wound the stem, the root should instead be cut at both ends where it contacts the trunk to stop its growth. Where compression is already advanced, removal can take a major structural root and its fine roots with it and accelerate decline rather than reverse it, so the honest answer is often that prevention was the treatment and it was missed.

How close to the trunk can tree roots be cut?

The published guidance is narrower than most rules of thumb suggest. A review of root management research concluded that vigorous trees less than 30 cm in diameter may tolerate roots severed on one side as close as three times the trunk diameter without major loss of stability. That does not extend cleanly to large mature trees, and cutting roots close to the trunk, especially on the windward side, can substantially destabilize a tree. For a big oak the decision should come from what an excavation shows about the structural roots, not from a formula.

Does an air spade damage tree roots?

Used correctly, no. Compressed air fractures and displaces soil while leaving roots, bark and stone intact, which is the whole reason the tool exists for this work. The risk is not the air, it is the operator, and both UC guidance and the UC oak volume pair the recommendation with an explicit warning to avoid bark damage during excavation. Mechanical digging cannot make the same distinction, because a blade cuts whatever is in front of it, including the roots you were trying to assess.

Can root collar excavation save a declining oak?

It can identify why the oak is declining, which is a different and more defensible claim. Excavation is very well supported as a diagnostic procedure and only thinly supported as a cure: one review notes that dramatic improvements have been attributed to it in practice, while the only published research study found no influence of root collar excavation on street tree growth over four years. What genuinely helps a declining native oak is correcting the causes that excavation reveals, chiefly summer water at the root collar, fill soil against the trunk and compaction in the root zone.

Why do so many oaks around here decline after landscaping goes in?

Because the landscape usually delivers summer irrigation, a grade change and compaction to a root system that evolved without any of them. The USDA Forest Service field guide to California oaks states that severe Armillaria root rot commonly develops in native oaks exposed to prolonged summer irrigation, typically when irrigated landscaping is installed around existing oaks, and names soil compaction, added fill soil and roots cut or crushed by construction equipment as additional factors. In undisturbed stands the same fungus is usually a minor pathogen. The site changed, not the tree.

My tree is declining and the soil around it is compacted. What happens now?

Decompaction and soil amendment, and not on a leisurely timetable. A tree in critical stress with a compacted root zone is losing ground continuously, because roots cannot push through soil past about 2.5 MPa of resistance and cannot breathe in soil that has no pore space left. I open the compacted zone with the air spade without cutting roots, work organic matter into fine textured soils, water it in, mulch three to four inches over the top and keep traffic off it afterward. On a tree that is already showing crown decline, that is the work I would do first and the slower diagnostic questions second.

Can compacted soil around an established tree actually be fixed?

Yes, and pneumatic tools are the reason. Compacted soil under an established tree used to be close to untreatable, because loosening it meant digging through the root system that was already struggling. Compressed air breaks the soil apart and leaves the roots intact, so the root zone can be opened and amended without paying for it in severed roots. The part that decides whether it lasts is what happens afterward: mulch over the treated area and keep vehicles, storage and foot traffic off it, or it packs straight back down.

How much does root collar excavation cost?

A site assessment with a written report starts at $450, which covers the inspection, the diagnosis and the options in writing. The site visit on its own is $250 for up to one hour on site and does not include a report. Excavation, root pruning or soil work is quoted separately once the site is understood, and always before anything is scheduled. Call or text (530) 391-6100.

Do you do the excavation yourself, or only write the report?

Both, and that is why this page carries a disclosure. I do not prune or remove trees and I run no removal crews, but I do own an air spade and I perform root collar excavation, root pruning and soil remediation. Where a report of mine identifies excavation or root work I am able to carry out, I disclose that in the report so you can weigh the recommendation accordingly and hire someone else for the work if you prefer.

University and Forest Service resources

• UC IPM Pest Notes: Armillaria Root RotUC guidance to remove soil with hand tools or compressed air excavators to expose structural roots and the root flare, never with construction machinery, and to keep the root collar dry in summer.
• USDA Forest Service Field Guide to Insects and Diseases of California OaksPSW-GTR-197 by Swiecki and Bernhardt, source for severe Armillaria root rot developing in native oaks under prolonged summer irrigation, and for compaction, fill soil and construction root damage as contributing factors.
• University of Minnesota: Stem Girdling Roots, The Underground Epidemic Killing Our TreesGary Johnson and Dennis Fallon, the source for how stem girdling roots form, the three girdling patterns, root collar inspection dimensions, and the limits of removal.
• UC ANR: Oaks in the Urban Landscape, Chapter 7, Biotic and Abiotic DisordersCostello, Hagen and Jones on exposing a buried root collar to the original soil line, pneumatic excavation tools, fill soil against trunks, and measuring soil bulk density in compacted zones.
• UC ANR: Oaks in the Urban Landscape, Chapter 8, Structural Failures, Defects, and Risk AssessmentThe root collar inspection checklist, the rule that an excavation is likely warranted where the trunk flare is not evident, and the destabilizing effect of cutting lateral roots close to the trunk.
• UC Cooperative Extension: Urban Oak CareThe UC Oaks hub for landscaping under native oaks, summer irrigation of established oaks, and the chapters above.
• University of Minnesota Extension: Planting and Transplanting Trees and ShrubsUM Extension guidance that the trunk flare should sit above the soil surface at planting, that container stock outgrowing its pot can produce girdling roots, and where to find the practitioner's guide to stem girdling roots.
• ISA: Proper Mulching TechniquesThe International Society of Arboriculture brochure defining mulch volcanoes, setting mulch depth at 2 to 4 inches, and stating that mulch against the trunk can lead to stem girdling roots.
• ISA: New Tree PlantingISA planting guidance that the trunk flare must be partially visible after planting, that excess soil should be removed if it is not, and that most roots develop in the top 12 inches of soil.

Peer reviewed research

• Watson and colleagues (2014), Arboriculture and Urban Forestry 40(5)Review of strategies to mitigate human impacts on tree roots: the three times trunk diameter root severance finding, the adventitious root flare, and the single published study finding no growth effect from root collar excavation.
• Watson and colleagues (2014), Arboriculture and Urban Forestry 40(4)Review of soil influence on root growth, including the 2.5 MPa soil strength limit for woody roots and why a bulk density of 1.60 limits roots in a clay loam but not a sandy loam.
• Wells and colleagues (2006), Arboriculture and Urban Forestry 32(6)Effects of planting depth on survival and girdling root formation, with deep planted Yoshino cherry mortality and red maple girdling root percentages by depth.
• Day and Bassuk (1994), Journal of Arboriculture 20(1)Review of soil compaction and amelioration treatments, the source for the mixed results of vertical mulching and radial trenching and for the conclusion that no universally successful technique is available.

Sources

• Johnson, G.R. and Fallon, D. Stem Girdling Roots: The Underground Epidemic Killing Our Trees. University of Minnesota, Department of Forest Resources, with support from the USDA Forest Service Northeastern Area.
• Johnson, G.R. and Hauer, R.J. A Practitioner's Guide to Stem Girdling Roots of Trees. University of Minnesota Extension Service, BU-7501-S.
• Costello, L.R., Hagen, B.W. and Jones, K.S. 2011. Oaks in the Urban Landscape: Selection, Care, and Preservation. University of California Agriculture and Natural Resources Publication 3518.
• Swiecki, T.J. and Bernhardt, E.A. 2006. A Field Guide to Insects and Diseases of California Oaks. General Technical Report PSW-GTR-197. Forest Service, U.S. Department of Agriculture.
• Watson, G.W., Hewitt, A.M., Custic, M. and Lo, M. 2014. The Management of Tree Root Systems in Urban and Suburban Settings, Parts I and II. Arboriculture and Urban Forestry 40(4) and 40(5).
• Daddow, R.L. and Warrington, G.E. 1983. Growth Limiting Soil Bulk Densities as Influenced by Soil Texture. USDA Forest Service Watershed Systems Development Group, cited by UC for critical bulk densities by textural class.
• International Society of Arboriculture. Proper Mulching Techniques and New Tree Planting, consumer brochures, 2021 revisions.