Post-construction damage to mature trees is common, whether it is from a new home, new addition, or smaller projects like patios, driveways and even irrigation systems. They all can impact the health of mature trees by disturbing or damaging their root systems, as a result of digging and compacting the soil around them.
The important thing to remember with mature trees is that rarely will the signs of damage show up immediately after it has happened, unless it is very severe. It can take years to surface, and when it does, people don’t often associate it with the real cause. I have seen old oaks that have been holding on for fifteen or twenty years after the damage was done.
The reason for this is that large trees have tremendous ability to store food for tough times such as drought or early frost damage to new leaves. When this occurs the tree will turn to its reserves for the food it needs. If a tree’s root system is damaged, the tree must turn to those same reserves to survive until it can repair and replace the damaged roots to get nutrient and water uptake back to normal. If growing conditions are good and environmental pressures are low, it should recover without many problems, depending on how severe the original damage was. If the tree was in bad shape to begin with or has too many environmental pressures like drought, insects or disease, the odds of long-term survival decrease after construction.
The species of tree also plays an important role in determining a plan of action to ensure success during and after construction. Some trees, like oaks, elms and locust are quite tolerant of disruption, but other trees such as beech and cherry can be very sensitive to even moderate change.
Too often an arborist is called in after construction to try and “fix the trees” after the damage was done, and the process of trying to get an accurate account of activity around the tree begins. What was the original grade level of the soil? Where were trenches dug for utilities? What time of year did this take place? Where were heavy pieces of equipment, bricks and other building supplies placed? And the list goes on.
Once all the information is collected and sorted out, a determination can be made as to the tree’s chances for long-term survival or if the owner will just be delaying the inevitable. If the only option is to temporarily extend the life of the tree, the owner needs to determine for how long and if the money spent to do so is worth the end results. Out of desperation, many homeowners will unfortunately spend more money after the damage has been done “to save the tree” than they would have if they contacted someone in advance qualified to help plan out the construction project before it started. Many homeowners have had greater success in preserving trees through the inclusion of incentives in the building contract for the protection of their trees. Educating and informing all of the building employees on the site and enforcement of the agreed upon rules will go a long way to help ensure tree survival.
You need to determine how to keep disruption to the root zone to a minimum. The 3 most important factors are soil compaction, trenching, and grade changes.
If you need to trench near a tree for a foundation, a minimum of 4 to 5 feet is required from the base of the tree and if it is possible, bridging the foundation with an I-beam near the tree is an option. When trenching for utilities, try to have all of them placed in the same area as far away from the base of the trees as possible. If you can’t avoid the placement near the trunk then boring under the root zone is an option that will greatly decrease the damage to the roots. This is important even during the installation of irrigation systems.
Grade changes are common but can be minimized to avoid burying the roots or scraping them away when removing soil. Removing soil is much more damaging than adding it. Most of a tree’s root system is found in the first 12 inches of soil, with the majority of the feeder roots in the top 6 inches. These are the roots that absorb water and nutrients. The myth that trees have these massive root systems that go down as deep as the tree is tall is false. They do have anchor roots and tap roots that can go relatively deep, but by far the majority of the roots are near the surface. Removing as little as 6 inches of soil over a large area of the root system can cause a lot of damage. Adding soil can be just as bad, but it tends to be less of a shock because the change takes longer to impact the tree. If more than 12 inches of soil must be placed over the root system, a tree well and ventilation system should be installed at the existing grade before adding soil. The deeper the soil, the more ventilation will be needed. Many people install the wells, but neglect the ventilation system that will buy time and aid in the development of new roots in the new soil. By allowing air and water to get down to the original grade and gases to get out, you give the tree a big advantage for long-term survival. To minimize stress, try to keep added soil to a depth of less than 6 inches if possible.
Soil compaction is still the most common construction problem around trees in our area. Our heavy clay soil can easily be compressed (especially when wet) so that most of the air pockets are eliminated. When it hardens, it is like cement, making root growth and water absorption next to impossible. It takes a long time for the soil to recover and a lot of trees will run out of reserves before the soil improves. There are some things that can be done to lessen the compaction after construction, but preventing it in the beginning is the key to keeping your plants healthy. Keep heavy equipment and supplies away from root zones and make the areas of activity around the site large enough to get the job done, but keep disruption to a minimum. Examples of this would be establishing a zone to store supplies, setting a path for equipment and trucks to enter and leave the property, placing fencing around root zones, and mulching the root zones heavily. If a root zone cannot be avoided, then using old tires, plywood and a foot or two of mulch to minimize the compacting in that area is a great solution.
The time of the year can be important also – winter is the ideal time for construction to proceed around sensitive plants, while spring and early summer are the worst times.
Some quick do’s and don’ts to remember: Do repair all damaged roots by making clean cuts that will heal quicker than jagged, torn roots. Don’t allow the cement out of trucks and equipment to be rinsed on site near the root zones of trees – cement is very alkaline and when leached into the soil, can cause pH shock in many plants. Don’t allow trucks to be parked under the trees. Do consult a certified tree care professional prior to construction. Don’t fertilize construction-damaged trees the first year.
There are many variables to these recommendations and there are always exceptions to the rules, such as time of year, health, species, and the size of the tree affected. For example, smaller trees adapt easier than large trees, and this becomes crucial in deciding which trees to try to preserve and which to remove before construction.
Hopefully, I have made you more aware of the limitations trees have when it comes to construction around them and, in turn, helped save a few trees from demise. Just because they are alive when the construction is finished does not mean they will stay that way in the long haul. Be wise and plan ahead!