Industry News: Tree Risk Assessment

Q: How can we stop a root system from damaging property, but keep it alive and in its place?

Thursday, December 31, 2015   (0 Comments)
Posted by: Shannon Sperati

Q: One of my neighbors has a beautiful 30-40 foot California pepper tree planted about 2 feet from the fence line. It's messy, but the much appreciated shade is the tradeoff for us. The neighbors do a great job of pruning it, including the growth that hangs over onto our property. However, the roots from the tree are lifting the pavement (walkway) along the side of our home. In addition to the damaged pavement, the roots are buckling my shed that once stood on an even surface. The paved walkway also has a drain, but because of the change in grade due to the roots, there is a standing water issue. These are all concerns, but my biggest fear is the root system of the tree. With the tree only 12 feet from the nearest room of our home, which happens to be a full bathroom, I'm very concerned about the potential damage to the plumbing.

A: First, let me say how nice it is that both you and your neighbor appreciate and value this tree. All too often, property line trees can be seen as a nuisance, ignoring the benefits provided to all. As to the displacement of the infrastructure, some amount of root pruning is tolerable, especially if the tree is healthy and well maintained. The real question is how close to the tree the root pruning will occur, how big the roots are that require pruning, and how many roots will be pruned.

In general, the roots are concentrated in the top 2 to 3 feet of soil (depending on soil type and irrigation regime). So, excavating and pruning some roots in the top 8-12 inches of soil in order to level the infrastructure should not compromise the health or stability of the tree. Of course, this is a general recommendation, and without seeing the situation firsthand, I cannot be certain of the number, size, or location of the roots and what their removal would mean to the tree.

One of the most critical elements for the tree's survival will be how you or your contractor approaches the situation. The more careful the excavation, root pruning, and construction practices, the more likely the tree will tolerate the impacts.

So, if it were me or one of my clients, this is the process I would recommend:
1) I would remove the existing infrastructure carefully to avoid damaging the roots. Injury can be direct (physical injury) or indirect (e.g., through soil compaction), so it will be important to protect those roots and the soil that will be preserved below the depth of excavation. Soil can be protected by placing 4-6 inches of mulch, pieces of plywood, or steel plates to help displace the load.

2) To expose the roots requiring pruning, I would dig a trench BY HAND behind the limit of the infrastructure (walkway, shed, etc.) to 8-12 inch depth. The goal is to expose those roots that are causing the displacement without damaging the portion that will remain and/or those you want to save beneath.

3) Once the roots are exposed, cut them square and clean with a SHARP saw beyond the limit of the infrastructure so they are not in the way when it comes time to frame and pour the new concrete. Give yourself/your contractor enough room to work so the root is not in the way and won't get damaged by the construction process.

4) You do not need to paint or otherwise treat the roots following pruning. However, if this is done during the dry months, irrigating the tree prior to and following the root pruning will help it recover from the root loss. I would also not recommend the tree be pruned within six months of the root pruning, as the loss of roots and canopy can be a double impact.

5) When replacing the infrastructure, be careful to limit compaction of the soil as much as possible to help maintain a good growing environment for the remaining roots.

In general, the root pruning will buy you several years without displacement. However, this is not a permanent solution and may need to be repeated periodically (every 10-20 years, depending on the tree, soil, irrigation, etc.). As to the foundation, these are typically 2-3 feet deep and constructed of concrete (depending on the age of the house). So, it will take a long time for roots to get to the foundation and beneath it, although it is certainly possible. If you see evidence of damage (cracks in the foundation or in the bathroom sheet rock), then I would be more concerned. I would use the same approach as described above but would excavate the trench along the foundation to the depth of the foundation. Any roots exposed during this process should be pruned 6" away from the foundation. Once the roots have been pruned, you can install a linear root barrier along the length of the foundation (or in the area adjacent to the tree). Ideally, the barrier would extend 6 inches below the foundation to help prevent any future intrusion.

Roots are opportunistic; they do not "search out" water. The only way they can get into the plumbing is if there is a crack/leak/weak seam in the plumbing. Once they encounter this, they will proliferate with the benefit of the water. So, maintaining the plumbing in good condition is key.


Responder: John Leffingwell, RCA #442, Pleasanton, CA