Issue: June 23, 2007

Patio and trees

I am planning to extend my patio, but I want to retain the two globe willows in the area. What would be a good product for the patio and how much room should be left around the trees? The trees are about ten feet tall and still young. I will be removing my lawn in the area and use some kind of base for the patio. I would think that that is the critical product as far as the trees are concerned.

Answer:

The base material is important, but significant disturbance of the roots is also be a major concern. When removing the lawn, minimize soil disturbance (scraping, digging, etc.) within 5 to 10 feet of the trees to avoid damaging the tree roots. The base material should be as coarse as possible. This will allow oxygen and water from the patio to penetrate to the roots of the trees that WILL grow under the paving if nothing is done to stop the roots. I also suggest a porous type paving (dry-set pavers without mortar and without a concrete layer under the pavers). This is best for the trees, but perhaps not for the patio.

The tree roots will grow under the patio. Paved areas, such as the patio, serve as mulch for the roots, as long as the trees aren't killed by the construction of the patio. The materials I described above will speed the growth of roots and perhaps allow them to grow at greater depth (where they will do less damage to the patio or the damage will be delayed longer). However, the patio (even if a non-pervious surface) will serve as mulch for roots that will grow under it. If the soil is compacted, the roots will grow through the base material and as the roots enlarge they will break, or at least raise, portions of the patio.

It is possible to create a barrier to prevent or reduce root growth under the patio if that is desired. There are materials designed to serve as root barriers. One such material is Typar biobarrier (TM). This is a spun bonded polypropylene fabric containing plastic dots of treflan herbicide. If this material is placed vertically in a trench between the patio and the tree, it will inhibit growth of roots toward the patio without killing the tree (if installed far enough from the tree to avoid damaging existing roots). Experience has shown that roots will often grow down through the loosened soil beside the root barrier, then under the barrier, and back up the other side toward the patio. To prevent root growth under the barrier, place the barrier vertically in the trench, but then also fold the bottom portion of the barrier to form a horizontal barrier toward the tree. When the roots grow down to the bottom of the trench, they will encounter the horizontal barrier at the bottom of the trench and will be less likely to grow under the barrier to reach the patio side.

If you prefer to avoid the treflan herbicide, there are rolls of solid plastic (like the plastic used to make large nursery containers) that may be used in a similar manner. Remember to make the horizontal barrier at the base of the vertical barrier. The thick plastic barrier may be more difficult to bend at the base, but this step is important to prevent the roots from growing under the barrier toward the patio.

If this root barrier is installed, the base material and the type of paving are less likely to impact the tree as long as the trees are far enough from any area of soil disturbance. Remember the roots are very near the surface, so scraping only 3 or 4 inches of soil will damage the roots. If the disturbance is at least 5 feet from the tree, the roots should recover.

There are other possible solutions to the problem, but they will have to wait for another article.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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