Established Trees With Deep Roots

As concern over deep roots grows, more and more established trees with deep roots are being identified in the landscape. Lack of visible root flare can be an indication of deep roots, especially if the root flare is missing on all sides. Lack of roots on just one side may be just a result of poor root development on that side. Probing with a surveyor’s chaining pin or larger tile probe can pinpoint the depth of the structural roots. If all else fails, soil excavation may be necessary.		View larger

What Can Be Done?
Secondary Problems

What Can Be Done?

If the roots are deeper than normal, the first question to ask is “Is the situation severe enough to require remedial action?” There is not always an clear answer.

If the tree has been established for many years, and looks as good as nearby trees without deep roots, there may be no need to do anything. If a tree has symptoms of stress, such as reduced twig and leaf growth, off-color leaves, a thin crown, or dieback, deep roots may be the problem if it lacks a well-developed root flare. Be careful not to overlook other causes for these symptoms, though some may be secondary as a result of the stress that the deep roots may be causing. There is no one symptom that can conclusively diagnose a deep root problem. Actual root depth, site conditions, and species tolerance must all be considered. Above all, there is no guarantee that the tree will improve if remedial action is taken.

Short of raising the tree by transplanting, root collar excavation is the only option available to treat an established tree with deep roots. Removing the soil from around the base of the tree provides better aeration to the soil and roots. Typically, soil is removed just wide enough to expose the root flare and create a slope that will keep the soil from falling back into the excavation. Air excavation tools are fast and minimize damage to roots. The depression left by the excavation may need to be filled with another well-aerated material to avoid creating a hazard. Pea gravel is sometimes used in order to keep the base of the trunk as dry as possible. Sand and wood chips provide good aeration, but may hold too much moisture. (photo by: T. Smiley)		View larger View video of excavation [1,212kb]

Arborists are reporting good success revitalizing trees with root collar excavations. This Japanese Black Pine in Long Island, N.Y. was excavated and began to show healthier new foliage the next season. (photos by: T. Smiley) The only research report available showed mixed results. Shingle oak (Quercus imbricaria Michx.), blue spruce (Picea pungens Engelm.) and honeylocust (Gleditsia triacanthos L.) failed to respond to root crown excavation while ash (Fraxinus spp.) and lacebark elm (Ulmus parvifolia Jacq.) responded favorably. Root crown evacuation often includes the removal of potential girdling roots and this was detrimental to maple (Acer spp.) species (Rathjens, LBG [pdf]).		More images: Before \| After \| Closeup

Root collar excavations did appear to offer some short-term protection from the stress when trees with deep roots were flooded. Research data is starting to reveal physiological explanations for the success of these treatments. Photosynthesis rates appeared to be higher for excavated trees during the first week of flooding than for non-excavated trees. (See Day [pdf]) Sap flow was lower in deep planted trees than in correctly planted trees. (See Fare [pdf]) (photo by: D. Fare)		View larger

No one really knows why removing such a small amount of soil can be so effective. Soon after planting, it is common to see regenerated roots growing up toward the soil surface. It could be that regenerated roots have all grown back towards the surface to their normal depth, and only the original root ball roots are too deep. This small section of roots is a vital connection to the trunk, and could be a ‘bottleneck’ if handicapped by low oxygen. A root collar excavation could provide better aeration and a simple solution.		View larger
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Secondary Problems

Girdling roots—When trees are planted with the roots too deep, regenerated roots from near the edge of the root ball sometimes grow back towards the trunk and develop into girdling roots. Four years after planting, root collar excavations revealed that deeply planted maples had, 50-75% of their trunk circumference encircled by girdling roots, compared to < 20% in control trees. (See Wells [pdf]) These girdling roots can be confused with the structural roots when probing. They may also make treatment more difficult. (photo by: C. Wells)

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Disease—There is always concern that the soil in contact with trunk tissue could eventually lead to decay. Certain diseases have been associated with deep roots in practice—Phytophthora, Armellaria, Procera or Leptographium. (photos by: T. Smiley)

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In a more formal study, trunks of white oaks and sweetgums buried 8 inches deep for 8 years showed no signs of tree decline. Visually the bark appeared blackened and rotting, but biopsies indicated no pathogenic fungi present, only saprophytic fungi decomposing the outer bark tissues. (See Day [pdf]) There did not appear to be any effect on bark tissues of sweetgum trees from soil contact. In another site or with another species, it is possible that decomposing bark may predispose a tree to infection by pathogens, but there was no direct evidence of this. (photo by: J. Homyk)

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Insects—Several insects have been associated with deep roots, including Pine Root Collar Beetle (Hylobius radicis), Turpentine Beetles (Dendroctonus), Peach Tree Borers (Synanthedon exitiosa). (photo by: T. Smiley)

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