More than 94 percent of U.S. paved roads are covered by asphalt, and many parking lots, driveways, walkways and tennis courts also are topped with the material. An asphalt driveway can stand up to heavy loads, requires little maintenance, lasts for years and can be recycled. As sturdy as asphalt is, tree roots growing under it can cause its surface to bulge, buckle and break.
Most tree roots that cause problems under asphalt are within 12 to 18 inches of the asphalt surface. One way to kill a tree's roots under asphalt is to sever the offending portion from the tree. You could dig down to the root by using a shovel and then cut the root with an ax or chainsaw, but using a gas-powered trencher may be an easier, quicker way to cut the root. Be sure to follow all safety precautions when using any potentially dangerous tools. Familiarize yourself with operating manuals and wear protective eyewear to protect your eyes from flying debris of wood.
Chemical Kill Zone
Certain chemicals, such as copper sulfate, kill tree roots upon contact. Roots absorb copper sulfate but transport it only a short distance along themselves. That factor provides the advantage of killing the problem portion of a root without killing the parent tree. Although copper is an essential mineral for life, it is considered a heavy metal that is toxic to microbes that are necessary in the sewage treatment process. Widespread use of such chemicals could contaminate the environment.
After you've cut or killed the offending root, establishing a root barrier between the root's tree and the asphalt surface can help to prevent a recurrence of the problem. Root barriers are in a variety of configurations, ranging from physical barriers, such as sheet metal, to chemical barriers, such as synthetic fabric soaked in copper sulfate. No root barrier is 100 percent effective, according to a University of Georgia, Warnell School of Forest Resources Extension publication. Eventually, nature finds a way around it.
If you already have a tree and asphalt surface in place, trying to control the tree's root growth is the only option short of removing the tree. If you are considering installing an asphalt driveway or other asphalt surface, proper planning can keep you from having to deal with a tree root problem under the asphalt. Choose tree species that don't exhibit surface root tendencies. Take into consideration that roots can grow to the crown edge of a mature tree, and then plant new trees an appropriate distance from your asphalt surface, basing that distance on the expected width of each tree's crown at maturity.
- Friends of the Little Miami State Park: Root Encroachment
- Virginia Cooperative Extension: Trees for Parking Lots and Paved Areas
- University of Georgia, Warnell School of Forest Resources Extension: Methods for Root Control
- The Baltimore Sun: Blacktop and Tree Roots Don't Go Together
- National Asphalt Pavement Association: History of Asphalt
- University of California, Statewide Integrated Pest Managment Project: Sewer Line Root Control
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