The consequences of planting a tree too close to a sidewalk, driveway or even a patio can be considerable. When we plant a new tree we typically consider where the tree will cast its shade, but not where and how far its roots will grow.
Basic guidelines tell us not to plant large growing trees within 20 feet of sidewalks and driveways, and small growing trees (under 20 ft.) no closer than 10 feet. Suppose, however, that a tree in question is already established closer than these guidelines suggest.
Tree roots grow comfortably under paving as long as the soil is not compacted and oxygen is available. As roots under a paved surface become more extensive they can cause paving material to buckle or heave. Typically the result calls for pavement repairs and action regarding the offending tree.
Generally you can estimate how far a tree’s roots extend from a tree by measuring the trunk’s diameter about four feet above the ground. Multiply the diameter by one and a half feet. That will provide you with an educated guess which informs you the distance a tree’s roots have spread.
Decades old trees are irreplaceable for the beauty and services they provide. Their roots are no longer a hazard because they have finished their aggressive growth. However, aggressively growing young trees can damage close by sidewalks and driveways. Heaving pavement presents a danger to pedestrians and a liability to homeowners.
Removal is an option for dealing with an offending tree, so is moving it if it is not too old or large. A contractor may recommend one or more methods that deal with damaged pavement; however, short of cutting down a tree, it is wise to bring in an arborist to assess the situation before you act.
Pruning encroaching tree roots may be the first thing that comes to a homeowner’s mind, but it is rarely a solution. Cutting a tree’s roots can make a tree unstable or even ultimately kill it.
Cutting roots more than three inches in diameter leaves the remaining ends vulnerable to insects and disease—including rot. Roots under paving actually add to the structural integrity of a tree. Pruning away such roots can make a tree less structurally sound and more likely to blow over in heavy wind. When tree roots are pruned there is always some danger of tree failure.
Roots close to a tree should remain untouched because they provide the mechanical support that holds the tree upright. The closer cuts are to the trunk the more impact on the tree. The more roots that are cut the more stress a tree endures. Leaning trees should not be root pruned. Neither should old trees or those in poor health.
Know that roots can regrow. That means a recurring job pruning tree roots every couple of years unless a barrier is installed to direct root growth downward.
Root barriers made of plastic or geotextile material are used to physically direct root growth downward to a depth where roots will not physically affect paving.
Concrete reinforced with rebar is used to prevent cracking and heaving in some cases by rendering the weight of the concrete too heavy for roots to lift.
Pervious concrete allows water to pass through it to the soil. In certain conditions it may deter shallow root growth by allowing water to penetrate more deeply into the soil, thereby drawing deeper root growth.
A gravel sub base at least six inches deep under cement discourages root growth because roots will not grow in gravel.
There are a number of building methods and alternative sidewalk materials, each with varying positive and negative features. All are most effective if employed before roots grow under paving and cause a problem.
A Certified Arborist is your best advisor when dealing with serious tree related problems. Arborists know tree varieties suited for tight planting areas, growth rates for different trees, which species tolerate root pruning, how and if a tree can be moved and much more. They will guide you to plant wisely and prevent future problems.
Reach Debbie Menchek, a Clemson Master Gardener, at firstname.lastname@example.org.