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!

by Steve Turner

All of us who deal with plants – big or small, annual or perennial, flowers or foliage – have one thing in common. Working with the soil in which they grow. It is often the most overlooked aspect of the garden for new gardeners and the most cherished aspect for seasoned gardeners, who have grown to appreciate the value of good soil. It quite simply is the difference between success and failure for many gardens and can be the difference between the pleasure of gardening and the tediousness of work.

Many of you are familiar with terms like soil structure, pH, humus, etc. But what do these terms really mean and how do they effect our efforts to improve or maintain our soil’s fertility?

Let’s start by looking at pH. Most people understand it as whether the soil is acidic (less than 7.0), alkaline (more than 7.0), or neutral (7.0), and that different plants prefer different ranges, but why? In different pH’s, nutrients move at different rates through the soil. The more acidic the soil, the faster the rate. If a plant is acid-loving and is in alkaline soil, it will not get nutrients at a fast enough rate. Fertilizer in this situation would have no effect because it is not a matter of whether the nutrients are present, it is the rate at which they are available to the plant.

Now, if you need to raise or lower the soil pH for a particular plant, you will need to understand cations and how they affect your ability to change the soil. Cations are positive-charged nutrients in the soil. The most common, in order, are calcium, magnesium, potash, sodium and hydrogen. Calcium is the most important nutrient in your soil and its proper balance to all others is vital since it helps regulate the soil’s pH. Here is why. All nutrients either have a positive or negative charge, and in nature the normal flow of electrical charge is from positive to negative. All clay and humus particles carry a negative charge on their surface which attracts desirable and/or undesirable positive-charged nutrients or minerals to them. If in proper balance to calcium, the clay and humus will keep these nutrients loosely held in a soil solution, which means the nutrients are readily available to the plants’ negative-charged root tips. The measurement of this exchange is called the Cation Exchange Capacity or CEC of the soil, and is measured on most soil tests.

CEC is important in determining how easy it will or won’t be to change your soil pH. A high CEC will make it difficult to change. In a poor soil, for example, as soon as you obtain the proper pH, you then increase the CEC by adding organic matter or humus to the soil. This will increase the amount of negative-charged particles in the soil and help “lock in place” the pH by making it more difficult to change. So, if you are preparing a new soil area, your first concern is the pH. It is critical that you make all adjustments to the pH prior to adding organic matter.

While organic matter raises the CEC, salts in the soil are the enemy to proper CEC. All commercial or synthetic fertilizers contain salt; this is how they bind the nutrients together. Every time you add one of these to your soil, you are adding salt, which in turn lowers the CEC. They lower it by using up all the negative-charged particles in the soil and, as a result, the positive-charged nutrients just leach away. This increases the need for more frequent fertilizing, which in turn adds more salts to the soil, and thus begins a vicious circle that many gardeners fall into. This is why the use of organic fertilizers is beneficial. While it is true that plants cannot tell an organic nutrient from a synthetic one, the soil can. The additives in synthetic fertilizers cause problems for the soil as a whole and a lowered CEC is just one of them.

If you have a poor CEC and lots of salts in the soil, then odds are you have compaction also, which limits root growth and the flow of water and air through the soil. This decreases the activity of beneficial soil microbes and increases the activity of pathogens in the soil, which in turn causes diseases and so the vicious circle continues. 

Humus is one of the keys to breaking this cycle. Humus (or “finished compost”) is organic matter that has been broken down by soil microbes as much as possible. These microbes in turn produce ideal nutrient storage for our plants. Humus can hold 3 to 4 times more water and nutrients than clay alone. It helps store water in the soil for drought periods as well as nutrients that plants can readily absorb as they need them. This holding capacity will help reduce the leaching of nutrients out of the soil and lessen the need for additional fertilizers.

Humus also helps reduce soil erosion. For example, an acre of soil with only 1 percent humus can only hold less than 1/2 inch of water; any more rain than that will simply erode the soil. However, if the humus content is increased to 6 percent, that acre will easily hold over two inches of rain in the same soil. For those of you having drainage problems, this is something to consider.

Soil microbes are the keys to producing humus – if they are not present in sufficient numbers, then all organic matter in the soils stays in its original form or takes much longer to break down into humus. It takes many different types of microbes to accomplish this and they do so in a very systematic fashion. To get an idea of how many organisms there are in the soil, at a depth of 1 to 3 inches in good garden soil, there will be on average 9,750,000 types of bacteria, 2,080,000 types of Actinomycetes (organisms classified between bacteria and fungi), 119,000 fungi, and 25,000 algae in a single gram of soil (Microbiology: An Introduction, 4th edition). These numbers drop rapidly as you go deeper in the soil, but remain pretty high until about 18 inches in depth.

If you live in a newer subdivision, most of the beneficial microbes left with the topsoil that was probably scraped off the land before construction started, and the remaining ones were severely reduced by the soil compaction that followed during construction. This is a problem for many gardeners and up until recent times most of the research was focused on nutrients and soil structure alone. In the last 10 to 20 years, the focus has shifted to the beneficial organisms that make up a living soil.

By getting back to soil basics, we can be much more successful in our gardening pursuits by increasing our soil’s fertility. To correct problems we need to focus on the soil as a whole and stop looking for that one “miracle solution” that will supposedly give us healthier plants. The soil is a vastly complex environment. It can be, and often is, damaged in many ways that need different solutions to help correct each of the individual problems. The terms we discussed should help put you on the right track. Start by doing a soil test to determine your soil’s pH and CEC. Then adjust the pH as necessary, add humus to raise the CEC, reduce or eliminate the use of synthetic fertilizers, and fine tune from there. Good luck!

Steve Turner, Certified Arborist, is from Arboricultural Services in Fenton, MI.