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                                                   Root Pruning
                                              Edited by Len Phillips

Root pruning occurs in four basic situations: if a tree is being transplanted, if construction is occurring near the tree, if the roots are being removed from under a curb or sidewalk, or to develop a highly fibrous root system for nursery stock.

Prior to root pruning, one must first decide whether or not the tree is worth risking the effects of root cutting. Tree inspectors must be knowledgeable in hazardous tree evaluation and know what to look for, especially when it comes to root crown examinations. A thorough examination of the tree's health and vigor, structure, shape, and balance should always be performed and documented. Remember, some trees will survive root pruning without any additional care while others will die no matter what.

Reasons to Root Prune

When a tree will have a substantial portion of its root system removed, a thorough site analysis should be performed. This would include data being taken on wind potential, direction, and velocity. The distance and location of potential targets should also be noted. The site should also be examined for drainage and excessive moisture that can play a significant role in windthrow potential and root rots.

A critical concern when dealing with root pruning is whether or not the tree will survive. Arborists must have knowledge of the root spread, growth habit, and root aggressiveness of the species. Root pruning should be done as infrequently and as far away from the trunk as possible. Low-vigor trees may require substantial increases in the minimum distances required, or tree decline and/or liabilities may result. Severe root pruning should only occur on one side of the tree in any one year or tree stability and anchoring will be jeopardized. Furthermore, repeatedly severe pruning on a tree's roots can result in a hazardous situation.

Construction Standards

When construction occurs within 10 feet (3 m) of a tree, the following rules should apply:

• For trees measuring 12 inches (30 cm) in diameter or less, soil excavation work or root cutting should not occur closer than 3 feet (1 m) from the outer bark of the tree.

• For trees measuring greater than 12 inches (30 cm) in diameter, soil excavation work or root cutting should not occur closer than the distance measured by the circumference of the tree or a distance of 6 feet (2 m), whichever is less.

• Soil excavation work may be done closer than the distance parameters established under the above two categories, provided all excavation of soil is accomplished by hand and no roots greater than 2 inches (5 cm) in diameter are severed. If it is not feasible to perform any of the requirements established above, strong consideration should be given to removing the tree.

If removal of the tree is required under life-threatening or related emergency situations, remove it carefully but immediately. If more than 50% of the tree's roots have to be removed, or if 30% have to be removed from one side, the tree should be removed. The same rules should apply when pruning roots from under a curb or sidewalk.

Root pruning can be accomplished by two methods:

1) Selective root pruning - excavate the soil away from the roots and carefully select the roots to cut and prune like the branch of a tree.

2) Non-Selective root pruning - no real excavation required; get an effective piece of equipment and start cutting.

Cuts

Roots that have been ripped and torn with backhoes typically leave a large amount of root surface exposed to pathogens and unnecessary drying. Clean cuts are an absolute necessity if quick compartmentalization of decay is desired. After pruning, it is essential that cut roots be backfilled as quickly as possible. Small feeder roots can die in less than 10-15 minutes, with larger roots dying in less than an hour. Hot, dry, and windy conditions warrant extreme expediency in backfilling.

Timing

Root pruning should be avoided during environmentally stressful times such as droughts, floods, or times of active bud break and shoot growth, due to the water and nutrient demands which are placed on the root system at this time. From the standpoint of wound closure and health, the best time to conduct root pruning would be just prior to active root growth, which occurs in early spring before bud break and late fall in temperate climates.

Porous Species

Due to the differing vascular system of many species, it is important to know the difference between ring-porous and diffuse-porous species of trees.

Ring-porous trees (such as an elm, ash, oak, chestnut, and black locust) are angiosperms that have large diameter vessels in the first portion of the growth increment and vessels of smaller diameter later in the growth increment. The vessels of a ring-porous tree are generally larger and concentrated in the outermost layer of sapwood. These vessels are produced early in the season and laid out in concentric circles. They copiously absorb water in the early growth season, after which they close down. Ring-porous vascular systems are very efficient but are vulnerable to blockage. For many ring-porous trees, severe root pruning on one side of the tree may result in a loss of major branches on the same side that the roots were cut, rendering the tree aesthetically unsightly. In this situation, if retaining aesthetics are paramount, root pruning should not be done.

Diffuse-porous species (such as a birch, maple, cherry, poplar, beech, sycamore, honey locust, tulip tree, pine, spruce, ginkgo, and fir) may also be affected by root pruning, but any effects will bee seen throughout the entire tree. Diffuse-porous wood has vessels, parenchyma, and fibers of about the same size arranged equally throughout the entire growth increment. Vessels are produced regularly during the growing season. They take up water during the entire growth period.

Follow-up Care

There will need supplemental care and observation after root pruning. Maintaining adequate soil moisture, nutrition, and aeration following root pruning is critical if quick wound closure and root regeneration are desired. The duration of continued maintenance should depend on the tree's root reestablishment rate. Though the roots of many trees can grow up to 15 feet (5 m) per year under ideal conditions, this is rarely the case in most urban soils. Hence, monitoring of vigor should be conducted to determine when supplemental care is no longer needed. Whenever public trees are being severely root pruned, it is wise to provide periodic inspection, usually for 1 – 5 years, using twig growth and other vigor indicators to determine adequate reestablishment.

Root pruning or trenching to correct a problem is costly and often severely affects a tree. Research that has attempted to determine how much of a tree's root system may be safely removed has yielded mixed results, probably due to differential sensitivities among tree species. It is well established however, that major root removal can cause tree decline and often mortality.

Sources

• Carlson, Chris, "Factors to Consider Prior to Root Pruning", City Trees, November, Vol. 30, No. 6, p.20-22, 1994.

• Elmcare.com, "Important Facts About Trees", ArborScience Inc., 2007.

• Harris, Richard W., “Arboriculture”, Prentice Hall, p. 174-184, 273-276, 324-327, 1992.

• Keslick and Son, "Technical Tree Biology Dictionary", Modern Arboriculture Associates, 2007.

• Keslick Jr., John A., "Environmental Drought", Keslick & Son Modern Arboriculture, 1997.

• Rose, Mary Ann, “Tree Roots and the Hardscape” Landscape and Nursery Dialog, The Ohio State University, December, 1996.

• Trowbridge, Peter J. and Nina L. Bassuk, "Trees in the Urban Landscape", John Wiley & Sons, Inc. 2004.

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