Preserving Trees During Construction
Saving trees makes sense. It improves profit margins, builder reputation and sales. Careful planning, solid communication and a basic understanding of what keeps trees growing will make customers happy for a lifetime.
By Paul Fisette and Dennis Ryan – © 2002
Smart landscaping is the easiest way to increase value and speed the sale of a home. Bank America Mortgage found 84% of the real estate agents they surveyed think that naturally wooded lots are 20% more salable. NAHB researchers report 89% of the homeowners they polled want builders to leave as many trees as possible on their house lots. In another study, NAHB learned that 43% of the homeowners queried actually paid up to $3,000 more for the treed lots they built on. And 27% spent over $5,000 more for a naturally wooded site. Trees have market appeal and improve a home’s performance.
Trees can reduce a home’s energy bill. Strategically placed trees keep homes cooler during summer and warmer in winter. We measured the temperature of a brown-colored roof on a hot July afternoon. Its sunstruck surface was 140 degrees. The surface in the shade of a leafy oak was 50 degrees lower! Leaves give off water vapor, cooling the surrounding air as it evaporates. The combination of shading and evapotranspiration greatly improve a home’s natural ability to stay cool. The EPA figures it’s possible to reduce mechanical cooling by up to 50% with a thoughtful landscaping plan. It is important to shade the east and west sides of a home. The sun angle is lower and more direct on these sides. Shade the cooling equipment too. Air conditioners run more efficiently when they are cool. This single detail can save 10% on your cooling bill. As an extra bonus shading improves durability. Directs sun bleaches color from painted surfaces. It ages building materials like plastics, wood, and asphalt roof shingles.
A carefully positioned windbreak lowers the winter heating load by 20% in some cases. Early settlers apparently knew this. But the concept seems lost on a generation of builders who control the winter chill with blankets of insulation and the force of central heating. A screen of evergreens can passively reduce air infiltration and protect walls from heat-scrubbing winds. Don’t block south-facing windows that provide solar gain. Even deciduous trees that shed their leaves block 50% of the solar gain with their branches. Tree plantings provide an environmental bonus. Lowering a home’s energy requirement reduces the amount of pollutants exhausted up your chimney and utility smokestacks.
Preserving native plantings reflects an environmentally sensitive approach to development. Trees and the underlying vegetation intercept and absorb runoff from storm water, reducing erosion and siltation. They filter pollutants like lawn fertilizers, pesticides and other chemicals present in the landscape. Tree plantings buffer road noise and mask sounds from neighbors. They improve privacy and screen unsightly views. Builders that preserve trees are regarded as environmental stewards. Potential homeowners, regulators, municipal officials, and media recognize the effort. These projects sell faster because they are set apart from the competition as healthy and friendly. In a recent study of 1200 households, more than 70% of the respondents said, “Trees make you feel good!”
The evidence supporting tree preservation is overwhelming, yet builders continue to strip the sites they build on. Many builders like working with a clean slate. They want unrestricted access to all parts of the site and prefer to plant new tress later in the project. Sadly, the few builders who save trees, loose them to a slow but predictable death. Trees often look perfectly healthy 3 or 4 years after construction. However, unintentional construction damage has them marked for the chipper. When a tree finally looks sick it’s too late. Include tree preservation in your construction Master Plan. It pays huge dividends through improved curb appeal, enhanced reputation, and wider profit margins. Site development that preserves trees requires careful planning and thoughtful communication between all the members of the construction team. First, you must understand what a tree needs to remain healthy.
The most obvious injuries to trees are made to trunks and branches. Dangling branches are pruned and bark injury is trimmed with a limited degree of success. But damage to roots is lethal. The resulting death is unsuspected. Valuable trees often die several years after the project is complete. Homeowners mistakenly think their favorite tree has a disease. There is no association made between a thinning crown and the long-forgotten construction project.
There’s a stiff penalty for unintended damage. It costs10 times more to remove a tree near a house compared to the same tree on an open lot. A recent project I worked on is a perfect example. A builder in Denver was asked to preserve a 50 year-old Chinese elm. His clients loved the tree. The tree stood 25 feet from the new home’s southwest corner. Its wide-sweeping branches provided the homeowners with free air conditioning and a beautiful landscape environment – for four years! The tree died, became a hazard, and cost $2,500 to remove from the congested urban lot. The homeowners replanted a 15-foot tree at an installed cost of $500. The case was arbitrated and the parties split the cost. But the process was painful and damage completely avoidable. Tree preservation programs must be thoughtful, involve a certified arborist and be a central element in the original design process.
Serious construction damage is almost always root damage. People don’t understand where root systems are located or how sensitive they are to construction activity. Root networks are shallow, limited to the top 18 inches of soil. The one deep root, the taproot, only provides stability. All other roots provide the nutrients and moisture required by the plant. Roots extend well beyond the drip line of a tree. They extend in a radius that equals 2 times the height of the tree. So feeder roots for a 20-foot tree extend 40 feet from the tree stem. The very fine feeder roots grow like branches. They extend further every year. As the crown of a tree grows and expands, the roots must grow to supply the extra food and water required to support new growth.
There are 3 construction activities that kill most trees. Builders cut roots when they trench or dig near the dripline of a tree. Digging cellar holes, septic systems or even grading a lot will cut roots and kill trees. When roots are eliminated, the tree is not able to draw the water and nutrients it needs. The tree becomes dehydrated and starts to die at the top of the tree crown.
Soil is compacted when trucks and heavy equipment drive over the root zone. Stockpiling lumber, building materials, loam, or excavation soil over the root zone also compacts the soil, smothering the roots. Everyone thinks trees breathe in carbon dioxide and give off oxygen. The only part of the tree that does that is the green leaves during photosynthesis. Other living cells take in oxygen for respiration to convert stored sugars and starches into energy (food). The byproduct is carbon dioxide. So if you compact the soil or lay a driveway over the roots, you cut the supply of oxygen and carbon dioxide can’t escape. It’s just like putting a plastic bag over your head.
When you change the grade around a tree you cut into the root system, add soil, remove soil, or undermine part of a tree’s anchoring system. Excavation fill is often spread over the site. This raises the grade and smothers the roots. If you lower the grade, you expose roots. A good guide is to maintain the root flare at the bottom of the tree. Do not bury the trunk flare. Some professionals claim you can add 2 or 3 inches of well-draining topsoil to an older, well-established tree and get away with it. My experience shows it’s best not to add any soil around trees.
|Alter grade||Loss of structural support, dehydration and suffocation|
Unfortunately, removing all the roots from one side of a tree is a common practice. Trenching, excavating, and driveway grading create “wind-throw” problems. Wooded lots are frequently cleared to leave little islands of trees in the median of a 2-sided driveway. These trees are doomed. Soil compaction, root smothering and wind-thrown trees are inevitable. Unstable trees are lawsuits waiting to happen.
As trees die, people notice they are infested with disease or insects and think this is the cause of death. This is a secondary problem. The real problem is root damage. Cut enough roots and this will effect food production. The tree will weaken and the secondary organisms finish it off.
Effective tree preservation must be integrated with the project design and land development process. Hire a certified arborist that works with residential construction projects and knows what builders are up against. A construction project is no place for an idealistic theorist. The arborist must be familiar with the roles played by members of the project team and become a central member of the team. They must understand the design concept and walk the site before any plans are drawn. The arborist will help lay out the site and communicate appropriate information at critical times during the project.
A professional arborist knows:
- which trees are healthy, need pruning or need removal
- which trees will survive proposed changes in landscape
- how to accomplish development goals, minimizing injury
- which trees pose a hazard due to weak root systems
- which trees have invasive roots that threaten pipes, utilities and foundations
- which trees are pest and disease resistant
- which trees will provide the most aesthetic benefit.
- how to protect the trees that are valued.
- where to plant new trees; and how and where to transplant existing trees.
- which trees can be sold for lumber or firewood.
Successful development requires careful planning. Tree preservation is an important part of a project’s master plan. It should be contemplated at the very first stage of the process, before any work is done on the site. There are several key elements that guide an effective tree preservation plan:
o Identify trees suitable for preservation
This step provides the most critical information. Here, the arborist creates a Tree Stand Delineation, which describes the quantity and quality of existing trees on the site. Key team members walk over the property during this phase. Brainstorming and visualization is encouraged to stir the imagination and build enthusiasm in the project. Valuable trees are tagged, numbered and referenced on the site plan. This should be done when there is a general understanding of the project goals, but before a conceptual plan is final. This allows the delineation to influence the placement of roads, driveways, buildings, drainage, scenic vistas, wildlife corridors, and guide the very ambiance of the development. Only trees that have a strong potential for sustained long-term growth are selected. A rating chart can be developed to indicate the characteristics of tress listed in the inventory. Remember, the site these trees are used to growing on will be drastically changed, so a projected view of the landscape must be considered. Characteristics such as species, size and health of the trees are noted. Any work that must be done before the lot is cleared and graded is planned at this time.
o Define tree protection zones, recognizing impacts of planned development
Protection zones are the areas located directly around the trees you want to save. Root zones are critical areas. Root zones are depicted as little circles around each tree shown on the landscape overlay or site map. These areas are off limits! No construction activity can occur in these zones. That means grading, digging, storing of materials and all traffic is prohibited in these areas. The size of the zone depends on the health, age and species of the trees you are trying to protect. A healthy tree will probably survive if at least 60% of its roots remain unaffected by construction. The rule of thumb is to hold all work outside a tree’s dripline. However, some trees need more protection. Trees that lean have roots that extend far in a direction opposite of the lean. Narrow trees like Lombardy poplars, some cedars and tightly grouped trees (like those on a forested lot) have roots that run far beyond the dripline. A better rule is to allow 1 foot of protective radius for every 1 inch of trunk diameter.
It is the arborists’ job to minimize damage to valuable trees. All construction activity is referenced on the working drawings and specifications. The trees that could be affected are included in the construction documents and discussed at project meetings. Details regarding the impact should be included in each section of the design plan.
o Outline protective measures and develop specifications
With the relevant information delineated on the site map; designers can now locate the building, driveway, utilities, and develop the grading plan. If an overlay sheet is used, it can be colored to show where construction will impact protected zones. If destructive site development can not be modified, these overlay warnings may indicate which trees must be removed. Better now than after the building is constructed. A good site plan will show permitted parking, storage, washouts and other areas critical to the preservation plan.
Materials and methods required to control damage must be clearly described in the construction documents. Include an enforcement or penalty clause in the specifications. Complicated details should be illustrated on the working drawings. Prescriptions are part of the construction documents that are forwarded to the conservation commission, building department and subcontractors who will bid on part of the project. During this stage, hold a meeting with the owner, foremen, subcontractors, and others who will work on the site. Make it clear that preservation is important on this job, requiring everyone to work together.
Typical Protective Measures
- Erect protective fencing around root zone prior to clearing.
- Do not change the grade around trees.
- Use pavement materials that allow air and water to pass.
- Run utilities in a single raceway or trench.
- Place irrigation on the surface (don’t bury) and cover with mulch.
- Eliminate or minimize traffic in the protected areas. Build boardwalks.
- Prohibit the storage of building materials and soil in protected areas.
- Keep heavy equipment out of the protected zones.
- Control competition among plants in sensitive areas.
- Control storm water runoff.
o Field inspection and administration
Critical decisions are made during the design phase, but follow-through makes or breaks a project. You must verify that field workers are following the preservation plan. Tree preservation is unusual for many workers. Some may think the extra care required is a bunch of baloney. Keep a watchful eye. Surveyors, well diggers, excavators and truck drivers are usually first to arrive on site. Meet them as they pull in. Instruct them not to wash down equipment near desired trees. Trees are sensitive to chemicals and washing out a concrete truck affects the pH of the soil. Petroleum washed from equipment also hurts. Calcium chloride is often used to keep down dust. Be careful. Salt is toxic. It draws water from plants and seals the soil’s surface, smothering roots.
Verify that all workers understand their role. And be sure the required protective measures are implemented at the appropriate time during the work schedule. The arborist can be hired to oversee field implementation, but the most effective policy is to have the arborist advise a fully invested site supervisor.
The site supervisor should clearly mark the location of each tree being saved on the site. Erect signs that mark storage and clean-out areas. Install protective fencing before any work begins. It should be rugged, like an anchor fence or one built using 2x4s. The fence must be conspicuous. It must be high enough to be seen by operators of heavy equipment, so those workers won’t run over it. Snow fencing is not good enough. Hay bales should be used to protect wooded areas and individual root zones from silt and run off. And a professional should be hired to perform some important tree-care work before building begins.
- Remove unwanted trees.
- Prune and improve saved trees.
- Reduce crown to minimize impact on root zone reduction
- Fertilize, water and aerate where needed
- Root prune outside of protected root zone
- Mulch where needed
Many handbooks recommend tree wells as a system used to change the grade around an existing tree. I don’t like them. You can build a stone wall and hold an elevated level of soil back away from the tree trunk, but the rest of the root zone is buried and suffocated. To do it right you must construct a radiating network that provides air and water to the entire root system. Proper tree well construction is incredibly expensive and impractical in most cases. You are better off working with the existing contour of the land if at all possible.
The cost of hiring the arborist depends on the house and scope of project. The service can run from a couple hundred dollars for a plan review and site visit to a couple thousand for a full consulting service. Given the numerous benefits afforded by professional tree preservation, hiring a certified arborist is a sound investment.
National Arborist Association, 3 Perimeter Rd., Unit 1, Manchester, NH 03103
International Society of Arboriculture, P.O. Box 3129, Champaign, IL 61826
American Society of Consulting Arborists, 15245 Shady Grove Rd., Suite 130, Rockville, MD 301-947-0483 www.asca/consultants.org
Trees and Development: A Technical Guide to Preservation of Trees During Land Development by Nelda Matheny & James Clark, International Society of Aboriculture.
Trees & Building Sites, by Gary Watson and Dan Neely, International Society of Arboriculture.
Building Greener Neighborhoods: Trees as Part of the Plan 2nd Edition, by Jack Petit, Debra L. Bassert, and Cheryl Kollin, National Association of Home Builders Press and American Forests.
Last updated: November 28, 2007