Large Trees and Surface Roots
Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist
Have you ever noticed those big, gnarly roots of trees that come to the surface and create all sorts of problems with sidewalks, driveways, and lawns? They can make mowing almost impossible. What can you do to solve the problem... short of cutting down the trees?
How do those roots get there? The woody roots of most trees and shrubs are only about 12 to 18 inches deep in the soil. These roots grow and radiate out from the tree trunk in a horizontal network.
As a tree grows older and larger, their branches and trunks grow in girth or diameter and so do their roots. As they grow in diameter, the roots “come to the surface,” but actually they’ve always been there, they’ve just gotten bigger. Surface roots become a problem sooner or later on most large trees. However, they become a problem more quickly with fast growing tree species, such as silver maple, Norway maple, cottonwood, sycamore, elm, and willow.
So what can be done about these roots? Cutting them all off right at the trunk isn’t a good solution, if you want to keep the tree. These larger roots are the major “pipelines” for the transport of water and nutrients to the tree. When you sever the roots, you are preventing the uptake of water and nutrients. We should also keep in mind that the roots help to anchor the tree.
How about covering the roots with soil? Most tree species will not tolerated a significant change of grade. Extra soil placed on top of the roots leads to the suffocation of roots and the eventual death of the tree. The least drastic approach to take with “surface roots” is to mulch the problem area with a three to four inch layer of mulch. A coarse mulch that allows for the free flow of air and water into the soil is best. Some good choices include coarse bark mulch, wood chips, coarse compost, or pine needles. Keep the materials six inches or more away from the trunk to deter rodents and to prevent damage from freezing and thawing.
Another option would be to cover the area with a shade tolerant groundcover. Many times the root area directly beneath a tree is bare because the dense shade is not conducive to grass growth. Soil erosion from wind and water often follows, making the area increasingly difficult to mow and quite unattractive. Groundcovers adapted to shady conditions can be used to hide the roots and to avoid the need for mowing. Consider the use of ajuga, vinca, pachysandra, or other shade tolerant groundcovers.
It’s okay to plant a groundcover, but don’t come in and mound soil around the base of the tree so you can plant flowers or shrubs. This soil around the base of the tree can lead to rot in that area and cause the gradual death of the tree.
Mulch or groundcovers aren’t going to solve the problem of roots that are damaging your foundation, driveway, or sidewalk. Something has to be done! If the tree is already large you may want to consider the use of root pruning. Root pruning will hurt the tree, but if done properly you can attempt to keep the damage to tolerable levels.
WSU Cooperative Extension of Spokane County recommends pruning the roots over a period of at least three years. They suggest in their bulletin “Surface Roots” to slice straight down into the soil at regular intervals along part of the length of the encroached structure or feature, marking where the cuts were made. The following year make more cuts and continue yearly until the entire length has been pruned. “If more than 1/3 of the tree’s roots are severed at any one time, there will likely be noticeable damage to the tree, such as dieback of limbs and branches, stunting of growth, and leaf drop. Every year, you should also re-cut the previous years’ cuts to keep the roots from reentering the area.”
Keep in mind that you are compromising the structural integrity of the tree when you cut any of it’s major roots off, especially when they’re close to the trunk. The more roots you cut, the bigger the compromise. If a major portion of a root system must be pruned to save a structure or feature, it may be better to simply remove the tree. If you don’t remove the tree, it may come down in a windstorm.
If you have a younger tree that you realize was planted in the wrong place and will eventually be the cause of a root and structure clash in the future, you can perform root pruning earlier to prevent damage from occurring and to avoid significant damage to the tree later in life. This type of root pruning is done by slicing straight down into the soil about 12 inches away from the structure or feature. You are slicing through small roots instead of large major roots of mature trees. To prevent encroachment in the future, the roots must be cut along the same line every year. This same procedure can be used if tree roots are encroaching on garden areas. The earlier you start and the smaller the roots cut, the less damage that will occur to the tree.
The real cause of troublesome surface roots is planting a large tree species that is very fast growing and placing it where it will become a problem. Select the right size tree and situate it where it won’t be a problem as it grows larger.