How many times have you gone for a walk and noticed sidewalks cracked or heaved out of place because of tree roots? Unfortunately this problem is all too common. Not only are the damaged sidewalks a potential accident for pedestrians, correcting the situation can be damaging to the tree. Prevention is the best possible way to avoid having to deal with problem tree root systems. When root problems develop, root pruning may be necessary. Root pruning, however, should not to be taken lightly.
Avoid planting trees in areas with less than three feet between paved areas. In areas with 3 to 4 feet between paved areas, plant trees that grow to a mature height of less than 30 feet. In areas with 5 to 6 feet between paved areas, select trees that mature about 50 feet tall. Reserve trees that mature higher than 50 feet for areas with at least eight feet between paved areas. This allows adequate space for the tree roots.
Avoid planting shallow rooted tree species near sidewalks. Norway maple (Acer platanoides), red maple (Acer rubrum), sugar maple (Acer saccharum), ash (Fraxinus spp.), sweetgum (Liquidambar styraciflua), tuliptree (Liriodendron tulipifera), pin oak (Quercus palustris), poplars and cottonwoods (Populus spp.), willows (Salix spp.) and American elm (Ulmus americana) are examples of shallow rooted trees.
Consider installing expansion joints in the sidewalks near trees. This will limit possible sidewalk replacements to just a few sections rather than large areas. Curving and narrowing sidewalk sections near trees might also reduce heaving. Building sidewalks on beds of coarse gravel is also effective at slowing or stopping the heaving. Tree roots will not grow through porous gravel; instead, they grow deeper. The installation of removable or adjustable semi-permanent pavers near trees can serve as alternatives to poured sidewalks. The pavers can be altered when required to compensate for tree root growth.
Install root barriers along the tree-side edge of the sidewalk. The barrier will force tree roots to grow deep below the sidewalk, thus preventing heaving. Barriers can be made of plastic or geotextile fabric. It should extend one foot deep and at least 5 or 6 feet in both directions from the point on the sidewalk edge closest to the tree.
Whenever trees are root pruned, there is always some risk of tree failure. Many factors are involved. Tree species, age, size, site conditions, existing problems, vigor and extent of pruning are just some of the factors. Mature trees are less tolerant of root pruning than young trees, trees on sites exposed to high winds are less tolerant than sheltered trees, and trees with defects or poor general health are not good candidates for root pruning. The closer to the trunk the roots are pruned, the greater the effect on the tree. A rule-of-thumb is to make all cuts at least a distance of three times the trunk diameter from the outside of the trunk. Thus, root pruning of a tree with a trunk diameter of two feet should be done no closer than six feet from the trunk. Make all cuts even farther from the trunk for trees which are judged intolerant of root pruning.
When root pruning mature and intolerant trees use a stump grinder to level the offending roots. Grinding produces less damage than indiscriminate root pruning. After grinding the offending roots, add coarse gravel as a base for a new sidewalk or pavers. Root pruning machines and vibrating plows cause less damage than do trenchers and backhoes. Prune only one quadrant of a tree's root system in a given year; wait at least two years before pruning another quadrant.
Trees tolerant of root pruning include elm, ginkgo (Ginkgo biloba), honeylocust (Gleditsia triacanthos var. inermis), red maple, silver maple (Acer saccharinum), and sycamore (Platanus occidentalis). Intermediate trees include ash, linden (Tilia spp.), Norway maple, oak, and willow. Trees intolerant of root pruning include beech (Fagus grandifolia), birch (Betula spp.), conifers, Callery pear (Pyrus calleryana 'Bradford'), sugar maple, and tuliptree (Liriodendron tulipifera). Many trees intolerant of root pruning exhibit problems with windthrow following pruning.
Because trees can experience tremendous damage following root pruning, have the trees inspected by a certified arborist prior to pruning. He or she can help predict how the trees will respond to the procedure. When contemplating tree planting make sure and consider all aspects of the trees location. Look up for power lines, down for underground lines, and around for potential trouble spots. Many future problems can be avoided by taking a few moments for proper planning.
This article originally appeared in the March 31, 1995 issue, p. 36.