Tree hazards are more easily prevented than repaired. Your efforts at prevention will be much less expensive and more successful than attempting to cure a damaged tree that is on its way to becoming a hazard.
Choose Which Trees To Protect
For fruitful damage prevention, you need to correctly identify which trees are worth saving. Many well-intentioned protection efforts fail because large old trees nearing the end of their lifespans were protected and younger trees weren't. Take time to look critically at your trees and decide what you want them to look like in 10-20 years. Some of your trees may be better off being removed; others may potentially become useful wildlife habitats. An ISA Certified Arborist can help you decide.
Trees don't exist independently of their environment. Trees in a group, known as a stand, grove, or patch, should be evaluated together as well as individually.
Evaluating Forest Remnant Stands
Stands, groves, or patches of native Pacific Northwest trees, such as Oregon white oak, Western red cedar, red alder, bigleaf maple, and Douglas fir, are often found in urban or urbanizing areas. These are remnants of the larger forests that previously covered the area. They may range from less than a quarter acre to several acres in size. Conservation of existing groves of native trees often provides greater economic and environmental benefit than preserving individual trees in the developing landscape. However, you should still evaluate the quality of the forest stand to determine if it is worth preserving.
Excellent Stand Protection Zone
A high-quality forest remnant has high tree densities and an undisturbed understory. Look for the following characteristics:
- Trees structurally support one another.
- Soil remains undisturbed.
- Wildlife uses are relatively unimpaired.
- Shady microclimate encourages natural woodland plants.
- Natural forest succession continues, and forest regeneration is ongoing.
- The stand is visually attractive.
- Ecological functions are relatively unimpaired.
Poor Stand Protection Zone
Scattered trees with a highly disturbed or missing understory may not be worth saving. A poor stand protection zone has the following characteristics:
- Trees blow over easily due to lack of support.
- Soil dries out and soil erosion occurs due to disturbed soils and lack of understory.
- Forest microclimate is disturbed.
- Sunlight and temperature are increased.
- Weeds and invasive species have taken over.
- Forest succession is interrupted, and little regeneration occurs.
- The stand is visually unattractive.
- Ecological functions are severely interrupted.
Protecting Forest Stands
The best way to preserve a forest stand is to leave it alone. Fence the entire stand, grove, or patch to protect understory vegetation and soil as well as trees. Healthy soils require little if any fertilization, pesticides, or irrigation to support tree health.
When evaluating the members of a stand individually to see if they should be kept, consider whether or not the tree is on the edge of the group. These trees provide support and protection to the interior of the stand. If the tree in question is large and providing wind cover, do not remove it. Avoid removing vigorous, healthy trees and vegetation from the stand, and do not retain isolated single, tall, spindly trees; such trees are more likely to become structurally unstable, bend or blow over in storms, or become diseased and infested with insects.
Protecting Individual Trees
Tree protection involves activities designed to preserve and protect tree health by avoiding damage to a tree's roots, trunk, or crown. The best way to do this is protect not only the tree itself but also the ground covering its most important roots, known as the critical root zone.
A critical step in retaining healthy trees is the protection of tree roots from disturbance. Each tree has a critical root zone (CRZ) that varies by species and site conditions. The International Society of Arboriculture defines CRZ as an area equal to a 1-foot radius from the base of the tree’s trunk for each 1 inch of the tree’s diameter at 4.5 feet above grade (referred to as diameter at breast height).
Critical root zone radius distances calculated by tree diameter at breast height
|Tree diameter||Critical root zone radius||Total protection zone diameter, including trunk|
|2 inches||2 feet||4+ feet|
|6 inches||6 feet||13.5 feet|
|20 inches||20 feet||42 feet|
|46 inches||46 feet||96 feet|
Another common rule of thumb is to use a tree’s drip line to estimate the CRZ (see figure). Evaluate both of these and choose whichever provides the larger CRZ.
Under certain circumstances, disturbing or cutting roots in a CRZ may be unavoidable. In such cases, the work should be done only under the on-site supervision of an ISA Certified Arborist.
Cutting or disturbing a large percentage of a tree’s roots increases the likelihood of the tree’s failure or death. Never cut tree roots that are more than four inches wide; roots that large are usually structural. Cutting them can destroy the stability of the tree, causing it to fall over!
If you must cut tree roots, do so cleanly with sharp tools. Never tear with a backhoe or other dull instrument. A clean cut encourages good wound closure and confines the spread of decay. If damage is severe, consider removing the tree because its stability may have been compromised.
Activities to Avoid in the Critical Root Zone
The CRZ that should be protected from negative interactions. Avoid the following activities:
- Stockpiling construction materials or demolition debris
- Parking vehicles or equipment
- Piling soil and/or mulch
- Trenching for utilities installation or repair, or for irrigation system installation
- Changing soil grade by cutting or filling
- Damaging roots by grading, tearing, or grubbing
- Compacting soil with equipment, vehicles, material storage, and/or foot traffic
- Contaminating soil from washing out equipment (especially concrete) and vehicle maintenance
- Installing impervious parking lots, driveways, and walkways
- Attaching anything to trees using nails, screws, or spikes
- Wounding or breaking tree trunks or branches through contact with vehicles and heavy equipment
- Wounding trunks with string weed trimmers and lawn mowers
- Causing injury by fire or excessive heat
Some tree species are more tolerant of damage and disturbance in the CRZ than others. A tree’s tolerance depends not only upon the species but also upon conditions present prior to and at the time of the damage. Tree health, age of the tree, soil aeration and moisture, the time of year the damage occurs, its severity, and the weather conditions prior to, during, and after the damage all contribute to the tree’s response. An ISA Certified Arborist can analyze these variables and make specific recommendations to retain or recover a tree’s health and safety during and after the construction process.
Protecting Trees From Construction
Tree protection during construction may be passive or active. Passive tree protection, most commonly used during the planning or post-development stages, simply means avoiding any disturbance or harmful activity near the tree. Active tree protection, by contrast, involves physical protective barriers and is generally required during any site disturbance that may impact your trees, such as grading, building or surface construction and maintenance, infrastructure and utility installation and maintenance, lawn renovation, and other landscape changes that may affect the structural integrity and stability of your trees.
While these practices are presented here as voluntary guidelines, some local jurisdictions have tree protection regulations that must be followed. Contact your local planning department for specific regulations for your area.
The goal of tree protection is to help trees remain as healthy after you work around them as they were before you began. Plan and budget for tree conservation and protection as part of the development process, before construction begins. Optimally, tree protection should begin at least one growing season prior to the beginning of construction activities.
Start by making an inventory of the trees you will be working around. Include not just your trees but also your neighbor's trees, if working close to the edge of your property. Evaluate soil health and past site damage; you will need to incorporate that information into tree protection measures. If you are just working around one or two trees, you can do that by making a simple map listing the size, species, and health of each tree. If you have a lot of trees that need to be protected, it may make sense to hire an ISA Certified Arborist to develop the plan for you.
Take your tree information and overlay it with your construction plans to determine how much the planned activity will impact the tree. If the planned construction will have such an impact on the tree that it won't survive, either make the decision to remove the tree or change the construction project to avoid the tree. Consider the tree's location, species, quality, health, and benefits such as energy savings by shade or wind protection in order to make your decision. Remove trees that:
- Are within ten feet of the proposed building or structure
- Cannot be adequately protected
- Have less than a quarter of their total height composed of tree crown (tall and spindly)
- Have trunks that are more than a third wounded
Once you have identified which trees are in the path of your planned construction activities, put that information down on paper and communicate it to anyone you hire to work on the project. Reinforce your tree protection intentions by writing tree damage and noncompliance with tree protection clauses into any service contract. This should provide financial penalties to any contractor who damages your trees. If your property is large, engage maintenance staff in early decision-making and education about care of retained trees.
Install strong fencing around the CRZ and require the fence to remain in place for the life of the development project. This barrier can be a chain link or other type of fencing. Fencing protects both the root system and the trunk from being damaged.
Clearly identify the perimeter of the protection zone with highly visible signs.
Protect high-value trees with stem, branch, and root padding or wraps in addition to CRZ barriers.
To minimize soil compression across the property, establish one access route into the site and one exit route out of the site.
Complete preconstruction tree maintenance, including mulch, fertilization, supplemental irrigation as necessary, and pruning to remove dead, structurally weak, and low-hanging branches.
Monitor compliance with tree protection requirements and the impacts of construction activities on tree health regularly during construction. If there are incursions into the root zone, ensure roots have been severed cleanly, enforce penalties, and reestablish the protection zone. Confer with your contractors to make sure that construction offices, vehicular parking, worker break sites, concrete washout areas or other pollutants, and material storage will remain outside of protected areas. Diligence in maintaining barriers and in enforcing your protection plan will pay great dividends at the end of the project when the tree is still healthy.
Following the guidelines laid out above will serve in most situations, but occasionally construction plans will require impingement on the CRZ.
Trenching is a standard way to install utilities. It is best to entirely avoid trenching through the CRZ (see figure); such practice could severely destabilize a tree, as well as adversely affect its health through loss of roots. Workers performing such operations should understand that 85% of the mass of a tree’s root system is located within the CRZ and that most of a tree’s roots are within the top 18 inches of soil. Alter routes of underground infrastructure or use alternate methods such as pipe boring. Tunneling at least 18 inches beneath the root zone will prevent loss of critical root mass if underground utilities must unavoidably be placed within the CRZ.
A decision must be made as to where best to locate utility trenches. Planners and designers must be made aware that trenches may not cross a CRZ and design alternate alignments accordingly; such realignments are not the responsibility of the construction crew.
Best practices for trenching include the following:
- Protect the trunks of high-value trees from scraping and gouging to a height of at least eight feet.
- Keep equipment and excavated backfill on the side furthest from the tree, not against the trunk.
- Place excavated backfill on a plastic or canvas tarp outside the CRZ.
- Prune away jagged roots back to the trench wall closest to the tree. Use a handheld pruner or pruning saw to make sharp, clean cuts.
- Replace the backfill on the same day if at all possible. Cover exposed roots with wet burlap to prevent them from drying out; in hot dry conditions, small roots may be injured in as little as 30 minutes.
- Do not allow chemicals, trash, or other foreign debris to become mixed with the backfill.
- If earthwork specifications allow it, firm the backfill to the same compaction as the surrounding soil and no more.
- Water the backfill to prevent excessive root drying.
Grade or Ground Level Changes
Grade changes should be avoided in order to prevent serious damage or death to a tree. Fill that is added over existing soils can smother and kill roots, or invite disease if piled around the trunk. Even temporary fills such as stockpiling mulch or soil in the CRZ of a tree for as little as several days during the construction process can have severe, long-term negative effects, though symptoms may not appear for several years.
The extent of injury from adding soil around a tree varies with the kind, age, and condition of the tree; the depth and type of fill; drainage; and several other factors. Maple, oak and evergreens are most susceptible, while elm, ash, willow, sycamore, and locust are least affected.
Little can be done to save trees that have been suffering from soil added over an extended period of time. It is prudent to consider possible damage that may occur to a tree and take alternative action before the fill is made; prevention is less expensive and more effective than attempting to correct the situation after damage has been done.
Best practices for fill operations include the following:
- Never place any fill or organic materials directly against the tree.
- Never compact the soil within the CRZ.
- If using no more than two to four inches of fill around existing trees, significant damage may be avoided if the fill has a coarser texture than the existing soil.
Less damage to a tree’s roots is likely with a lowered grade than when it is raised, unless exposing or removing a great deal of the root mass. A general rule-of-thumb used by landscape architects is to remove no more than six inches of soil from the existing grade in the CRZ; however, this is dependent on the soils in which the tree is growing. A tree’s roots may all exist in the top foot of a shallow soil; removing the top six inches would have tremendous negative impact in that case.
Best practices for removing soil include the following:
- Consider removal and replacement if the tree is young, in poor condition, an undesirable species, or very susceptible to insects and disease.
- Plan grade changes well in advance of construction using the appropriate method to prevent injury to desirable trees.
- Use retaining walls or terraces to avoid excessive soil loss in the area of greatest root growth.
- Spread mulch over the exposed root area when possible to help prevent soil erosion, reduce moisture loss, and keep soil temperatures lower.
- Provide supplementary water when rainfall is less than one inch per week.
- Prune roots to prepare the tree for root loss due to grade lowering. Root pruning is best left to an ISA Certified Arborist, who can take into account the variables necessary to reduce the stress of the pruning to the tree.