Save the Trees! Avoiding and Alleviating Construction Damage

Last week we talked about the kinds of damage a tree can sustain during construction. This week we'll move on to discuss how to avoid injury, what to do if damage has already occurred, and when to call an expert for help.

Avoiding Damage

A healthy, mature oak like this would certainly be worth saving.  By Daniel Dumais - Own work, CC BY-SA 4.0

A healthy, mature oak like this would certainly be worth saving.
By Daniel Dumais - Own work, CC BY-SA 4.0

By now we've established that trees are surprisingly sensitive to changes in their environment. Fortunately, there are steps you can take to prevent or minimize damage. For the best results, start this process while you are still early in the planning phase of your project. First you need to pick which trees you want to save, and it's rarely practical to try to keep every single one. Pick the ones that are most important design-wise, for practical purposes (shade and wind protection, for example), or to you personally. There can be different levels of priority as well. You may have a tree planted in memory of a relative that you are willing to take very little risk with, while there may be other lovely trees that you would like to keep, but that you are willing to expose to greater risk to make the rest of the project run smoother. Another factor to consider is the cost of removing a tree if it does die. A small dogwood is much easier to take down than a mature oak. The trees you pick should be healthy and strong going into the project to ensure that they have the best chance of survival. If you aren't sure which ones to protect, an arborist can help you evaluate your trees and decide which ones to keep.

Before we go into the details of planning for tree survival, you need to know how to calculate the critical root zone of a tree. Protecting all of a tree's roots is impractical given how far they can extend. In most cases you can simply protect the area directly under the tree's canopy. However, for more sensitive trees and trees with a narrow canopy, you need to protect more space. The University of Minnesota resource at the bottom of the post lists trees according to their sensitivity to different kinds of damage. A tree's trunk diameter is the most reliable predictor of how far a tree's critical root zone extends. The rule of thumb is that for every inch of trunk DBH (Diameter at Breast Height, or 4.5' off the ground), you need to protect a 1.5 feet of root space. So if your tree has a DBH of 10", the critical root zone is a 15' diameter circle centered on the trunk. Now we can move forward to talk about how to protect the trees.

This boulder wall was designed with the goal of saving the tree, but without the knowledge that it was doing the opposite. Photo by Maria Gulley

This boulder wall was designed with the goal of saving the tree, but without the knowledge that it was doing the opposite.
Photo by Maria Gulley

Once you have selected and prioritized the trees you want to save, it is absolutely essential that you work with the designer or architect to make sure that the plan itself doesn't put the trees in danger. Driveways or patios should not be placed on top of the root zones of protected trees. Houses, pergolas, and other significant structures should be far enough away from trees to avoid future conflicts (make sure you understand how large the tree in question will grow instead of just basing this off of the tree's size at the time of the project). Some planting and building under a tree is okay, but you don't want to disturb more than a third of the root zone. If you do cut roots, be sure to make clean cuts. Cover the cut roots with soil and water them soon as possible to minimize damage from drying. If there will be elevation changes, tree wells need to be built an appropriate distance from the trunk to maintain the same soil level within the critical root zone. For sensitive trees that you absolutely want to keep, you may decide that any activity at all within the critical root zone is off limits. An arborist can help you make that call.

Now you have a design that takes trees into consideration. Fast forward almost to the time when construction is scheduled to begin. There needs to be a job site plan that protects the trees from damage during the project. Your site manager needs to be made aware of the restrictions you have put in place to protect your trees. Measure out and mark critical root zones of protected trees (or ask your contractor to). If possible, put up temporary fencing or roping to make these areas totally off-limits for all construction activity, especially for sensitive tree species. For less sensitive trees, you can decide what level of traffic or work to allow, but your safest bet is to allow foot traffic and light equipment traffic only. Placing stakes around the perimeter will allow foot traffic while reminding larger equipment to keep out.

Designate a single route on and off the site that minimizes impact to root zones. Ideally, this will be the driveway or the future location of the driveway. To further reduce compaction in this area, put down pieces of plywood or a thick layer of mulch that will later be removed. On-site parking should be restricted to designated areas outside of the critical root zone(s). Materials such as soil, gravel, mulch, and stone should be piled away from trees and future planting areas (soil compaction is bad for all plants, not just trees!). These routes and protected areas need to be clearly marked and discussed with all parties ahead of time. Before the project begins, you should also have any low-hanging limbs pruned back from driveways or other places where taller vehicles might drive.

During the project, you will have to keep an eye on things to make sure your expectations are being met. Stakes and fencing around critical root zones must stay in place, subcontractors need to be filled in on the rules, and workers must remember that no chemicals are to be dumped in root zones under any circumstances. A good foreman or site manager will keep track of this for you, but it's good to check in with them. If you see evidence of a worker violating your tree protection requirements, speak up right away.

Alleviating Damage

An air spade uses pressurized air to loosen soil without hurting roots.

An air spade uses pressurized air to loosen soil without hurting roots.

Once damage has been done, your options for remediation are limited. The easiest fix is for broken branches. Pruning the broken end off with a proper cut at the next branch union is your best solution, and the sooner it is done the better. Compaction can be alleviated by tree care professionals using an air spade to gently remove the soil around the roots in a radial pattern, and then the newly loosened soil is put back around the roots. Chemical spills on root areas should be swept or soaked up immediately if possible, and if not they should be diluted with water. Damage to a tree's bark or trunk is trickier to fix. There is a process called bark tracing that can clean up the edges of the wound. It's a task best reserved for professionals, so ask for help if you have serious wounds a the trunk of a tree. People used to recommend using a special paint to seal off the wound, but recent research shows that this doesn't truly help speed up healing, and it may actually hurt the tree in some cases. The one injury you can't fix is root loss. If you cut through too much of a tree's root system, there is no going back. That's why it is so important to have a design that doesn't require extensive digging or trenching in root zones. Any trees that have been under stress because of construction (or any reason, really) will benefit from consistent watering and appropriate fertilizing in the first few years after the project the help them overcome and damage they may have experienced.

Calling in the Experts

Image source:

Image source:

If you are dealing with a lot of trees - or a few particularly high value trees - on an extensive project, you may want to hire an ISA certified arborist to help you throughout this process. Some landscaping companies have arborists on staff, and they can oversee the planning and construction process from within the company. If not, a consultant can be indispensable for many steps. They can help you decide which trees to preserve, they can make a map of appropriate site usage during the project, and they can ensure that specifications protecting trees are written into your contract with the builder or landscaper (with fines and consequences for violating the terms if necessary). To find a certified arborist in your area, visit the International Society of Arboriculture's arborist search tool.

We wish you the best of luck in protecting your trees! For additional information, check out the links below. They are ordered from least to most extensive depending on the amount of information you're interested in. As always, never hesitate to ask a professional for advice.

Additional Resources (ordered from least to most extensive)
"Avoiding Tree Damage During Construction" - International Society of Arboriculture
"Construction and Trees: Guidelines for Protection" - Purdue University
"Protecting Trees from Construction Damage: A Homeowner's Guide" - University of Minnesota

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