Getting to the root of the problem

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

When we think about the effects of street and landscape trees on our environment, we tend to think of positive effects such as energy savings, improving aesthetics and property values and cleaning the air by absorbing greenhouse gases and pollutants. Trees, however, can also have negative impacts on our lives and property. One of the most obvious is storm damage to structures and utilities. This was dramatically illustrated by the recent storms in St. Louis where thousands were left without electricity in the searing summer heat and the National Guard had to be called in to help remove downed trees and debris. A less spectacular, but nevertheless costly impact of trees, is the damage caused by roots to sidewalks, sewers and foundations. A survey of 15 cities nationwide indicated that the average annual cost to repair sidewalks damage by trees averaged over $3 per tree. Sewer repairs cost cities an additional $1.66 per tree every year. For individual homeowners damage to foundations can run into thousands of dollars.

Reducing root damage

The extent to which root damage can be mitigated depends of a number of factors. The most obvious factor is whether the situation deals with an existing tree or planting new trees. Clearly, planting new trees offers many more options to contain tree roots and mitigate damage.

Tree species

Other factors being equal, trees that grow fast above ground also grow fast below ground. Researchers in England compared the “how often tree roots caused damage to buildings relative to the species’’ frequency in the landscape. Oaks made up 2.1% of the tree population but caused over 11% of the damage (5.5:1 damage to tree ratio). Roots of Populus (3:1), Fraxinus (2.5:1) and Robinia (1.7:1) also caused a disproportionate share of damage to buildings. Trees that caused comparatively little damage were Cupressus (3% of damage but 10% of tree population), Sorbus (0.2:1), and Prunus (0.3:1). In general, fast growing species should not be planted near sewer lines or sidewalks. These include silver and Norway maple, boxelder, sweetgum, cottonwood, aspen, tuliptree, and sycamore and planetree.

Proximity to target

As one would expect, the further a tree is away from a sewer or sidewalk, the less likely it is to cause a problem. A study in street trees in Cincinnati found that the likelihood of finding a tree root under a sidewalk decreased by 13% for every 1-yard increase in the width of the tree lawn.

Cultural practices

Roots proliferate where soil resources are most favorable for growth. Therefore, it is possible to encourage root growth where you want it and discourage it where you don’t want it. If you know you want to keep tree roots away from your foundation or garage, delineate areas where you don’t water or don’t fertilize. Roots will not grow in dry soil. Keeping soil dry under awnings or roof lines can create root-free zones around buildings.

Tree root barriers

There is considerable debate over effectiveness of tree root barriers. Common materials used for barriers include plastic (either in sheets or thicker layers) and Bio-barrier. Bio-barrier a landscape fabric impregnated with an herbicide (trifluralin) that helps to stop or slow root growth through the barrier. Dr. Ed Gilmann, at the University of Florida, conducted a widely cited study that showed roots can grow under a barrier and grow upward on the other side. Either with or without a root barrier, Gilman found nearly the same distribution of roots 3 feet away from the base live oak or sycamore trees three years after planting. It is important to note, however, that the barrier used in this study extended only one foot below the soil surface. Current manufacturer’s specs recommend installing Biobarrier to depths up to 60” depending upon the application.

Installing a root barrier system involves trenching between the tree and target to be protected. Trenching around existing trees needs to be done with extreme caution both to prevent excessive root loss and tree dieback and also to protect the structural integrity and wind firmness of the tree. This is usually best done by a professional arborist or landscape service company.

Rubber sidewalk panels

Several cities are now experimenting with sidewalk panels made from recycled plastic material. The rubber sidewalks are billed as better for jogger’s joints as well as resistant to buckling from tree roots. They are still relatively new and time will tell how effective they are over the long-term.

Dr. Cregg's work is funded in part by MSU's AgBioResearch.

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