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Protecting trees from construction damage: a homeowner's guide

Gary R. Johnson

Are you planning to build or remodel a home? Are your city's streets, curbs, sidewalks, and buried utilities about to be widened, modernized, or replaced? Before you start, consider the impact of construction on plants.

Trees and shrubs contribute to property values by enhancing appearance, reducing noise, cutting energy costs, screening unsightly views, and attracting songbirds and other wildlife. Unfortunately, plants meant to be part of a home's permanent landscape often are needlessly damaged or killed during construction. Careful planning and coordination with a tree-care specialist and your builder can reduce damage and save you the trouble and expense of treating or removing injured plants.

This publication explains some things that landowners can do to minimize the impact of construction on trees. It describes landscape protection plans, special construction techniques, symptoms of damage, and treatment strategies. Although the information presented focuses on trees, it also can be applied to protecting shrubs.

Hiring a tree care specialist

Each construction site has its own unique set of soil, tree species, and building process conditions. For this reason we recommend that you get advice from a professional urban forester or arborist with experience in protecting trees from construction damage. This person will be familiar with the growth characteristics and common problems faced by tree species in your area. He or she can help you evaluate plant health and the likely impacts of construction activities.

For your own protection:

Membership in the National Arborist Association, Minnesota Society of Arboriculture, or International Society of Arboriculture or certification from the International Society of Arboriculture are good indicators of reputable businesses.

Check with your local Extension office, or contact the local chapter of the International Society of Arboriculture (217-355-9411) for a directory of tree-care companies with certified arborists.

The root of the matter...

Trees can be damaged or killed by a wide variety of construction activities. Some practices lead to obvious injuries such as broken branches or torn bark. Open wounds of this type deplete a plant's energy resources and provide entry points for insects, or for diseases such as oak wilt.

The worst damage, however, often remains hidden underground. Roots are one of the most vital parts of a tree. They are responsible for nutrient and water uptake, store energy, and anchor the plant. Because they are so important, it is critical that you protect roots that lie in the path of construction.

Finding a tree's protected root zone

Figure 1. A tree's protected root zone (PRZ) is often considered to be the part of the roots that lie directly below its branches within an area known as the dripline.

Calculating protected root zone diagram

Figure 2. Approximate a tree's PRZ by calculating the critical root radius (CRR). First, measure the tree diameter in inches at breast height (DBH). Then multiply that number by 1.5 or 1.0. Express the result in feet.

Trees are never the same shape below ground as they are above, so it is difficult to predict the length or location of their roots. Typically, however, approximately 90-95 percent of a tree's root system is in the top three feet of soil, and more than half is in the top one foot. The part of this root system in which construction damage should be avoided is called the Protected Root Zone (PRZ).

One common method used to identify the PRZ is to define it as the "dripline"—the area directly below the branches of the tree (Fig. 1). However, many roots extend beyond the longest branches a distance equal to two or more times the height of the tree. For this reason you should protect as much of the area beyond the dripline as possible.

Unfortunately, on most sites space is limited and this rule must be bent. Just how close an activity can come without seriously threatening the survival of a tree depends on the species, the extent of damage, and the plant's health.

Some healthy trees can survive after losing 50 percent of their roots. However, other species are extremely sensitive to root cutting, even outside the dripline.

Table 1 shows the relative sensitivity of various tree species to root disturbance. If possible, disturb no more than 25 percent of the roots within the dripline for any tree, protect intermediate species to the dripline, and allow extra space beyond the dripline for sensitive species. For all trees, avoid needless or excessive damage. A qualified tree-care specialist can help you determine how much root interference a particular tree can tolerate.

When dealing with trees that have been growing in the forest or that naturally have a narrow growth habit, an approach called the "critical root radius" is more accurate than the dripline method for determining the PRZ. This is particularly true for columnar trees and for those where competition has reduced the canopy spread.

To calculate critical root radius, begin by measuring the diameter at breast height (DBH). This is done by measuring the tree's trunk diameter (thickness) at a point 4.5 feet above the ground. The measurement should be done in inches. For each inch of DBH, allow for 1.5 feet of critical root radius for sensitive, older, or unhealthy trees, or 1.0 feet for tolerant, younger, healthy trees. For example, if an older tree's DBH is 10 inches, then its critical root radius is 15 feet (10 x 1.5 = 15). The PRZ is then the area around the tree with a diameter of 30 feet (2x critical root radius), and is the area in which a critical amount of the tree's roots may be found. Whenever possible, isolate this area from construction disturbance (Fig. 2).

Table 1. Tree characteristics

Species Root Severance6 Soil Compaction & Flooding6 Soil pH Preference8 Mature Tree Height (ft.)8 Mature Crown Spread (ft.)8 Hazard Tree Rating*7 Damage- Causing Roots Landscape Value**1
Northern white cedar Tolerant Tolerant 6.0-8.0 40-50 10-20 Low . High
Balsam fir Tolerant Tolerant 4.0-6.0 40-60 20-35 Medium . Medium
White fir Tolerant Sensitive 4.0-6.5 50-75 10-20 Medium . High
Tamarack Tolerant Tolerant 4.0-7.5 50-75 15-25 Medium . High
White pine Tolerant Sensitive 4.5-6.5 80-100 50-80 Medium . High
Jack pine Tolerant Sensitive 4.5-6.5 30-80 20-30 High . Low
Red pine Tolerant Sensitive 4.5-6.0 50-80 20-40 (Medium) . Medium
Scotch pine (Tolerant) (Sensitive) 4.0-6.5 60-100 30-50 Medium . Medium
Eastern red cedar Tolerant Sensitive 4.7-7.8 40-50 10-20 Low . Low
Black spruce Tolerant Tolerant 3.5-7.0 30-70 15-30 (Medium) . Low
Colorado spruce Intermediate Tolerant 4.6-6.5 50-100 20-30 Medium . High
White spruce Tolerant Intermediate 4.5-7.5 40-80 20-30 Medium . Medium
Black ash Tolerant Tolerant 4.1-6.5 40-70 30-60 (Medium) . Medium
Green ash Tolerant Tolerant 6.0-7.5 30-60 30-50 Medium . Low
White ash Tolerant Intermediate 5.0-7.5 70-80 50+ (Medium) . Medium
Bigtooth aspen Tolerant Sensitive 4.8-6.3 50-75 20-35 Medium Yes Low
Quaking aspen Tolerant Sensitive 4.8-6.5 40-60 20-35 Medium Yes Low
Blue beech Sensitive Sensitive 6.5-7.5 20-30 15-20 Low . High
Paper birch Intermediate Sensitive 5.0-8.0 50-70 30-50 Medium . Medium
River birch Tolerant Tolerant 4.0-6.5 40-70 30-50 Low . High
Yellow birch Intermediate Sensitive 4.5-8.0 50-70 25-50 Medium . Medium
Boxelder Tolerant Tolerant 6.5-7.5 40-60 35-50 High Yes Low
Ohio buckeye Intermediate Intermediate 6.1-6.5 30-50 30-40 Medium Yes Medium
Butternut Sensitive Intermediate 6.6-8.0 40-60 50-60 (Medium) . Medium
Catalpa Intermediate Tolerant 6.1-8.0 50-80 30-50 Medium . Medium
Black cherry Intermediate Sensitive 6.0-7.5 50-70 40-50 Low . Low
Kentucky coffeetree Intermediate Intermediate 6.5-7.5 50-80 40-50 Low . High
Eastern cottonwood Tolerant Tolerant 5.5-8.0 80-100 80-100 High Yes Low
Red-osier dogwood Tolerant Intermediate 6.1-8.5 8-10 10-12 (Low) . Medium
American elm Tolerant Intermediate 5.5-8.0 70-100 70-150 Medium Yes Low
Slippery elm (Tolerant) (Intermediate) 6.6-8.0 60-70 40-60 Medium Yes Low
Hackberry Tolerant Intermediate 6.6-8.0 30-130 50+ Low . High
Hawthorn Intermediate Intermediate 6.0-7.5 20-40 20-30 Low . High
Bitternut hickory Intermediate Intermediate 6.0-6.5 40-75 30+ (Medium) . Medium
Honeylocust Tolerant Intermediate 6.0-8.0 50-75 50-75 Medium Yes Medium
Ironwood Sensitive Sensitive 6.1-8.0 25-50 20-30 (Low) . High
Basswood (Intermediate) Sensitive 5.5-7.3 70-100 50-75 (High) . Medium
Black locust Tolerant Sensitive 4.6-8.2 30-60 20-50 (Medium) . Low
Red maple Tolerant Tolerant 4.5-7.5 50-70 40-60 Medium Yes High
Silver maple Tolerant Tolerant 5.5-6.5 60-90 75-100 High Yes Low
Sugar maple (Intermediate) Sensitive 5.5-7.3 60-80 60-80 Medium Yes High
Mountain ash Tolerant Intermediate 4.0-7.0 15-25 15-25 Medium . High
Black oak Sensitive Sensitive 6.0-6.5 50-80 50-70 (Medium) . High
Bur oak (Tolerant) Intermediate 4.0-8.0 70-80 40-80 Low . High
Northern pin oak Sensitive Sensitive 5.5-7.5 50-75 30-50 (Medium) . Medium
Red oak Tolerant Sensitive 4.5-7.0 60-80 40-50 (Medium) . High
Bicolor oak (Intermediate) Tolerant 6.0-6.5 60-70 40-50 Low . High
White oak Sensitive Sensitive 6.5-7.5 60-100 50-90 Low . High
Wild plum Tolerant Sensitive 6.5-6.6 20-25 15-25 Low . Medium
Serviceberry Intermediate >Sensitive 6.1-8.5 6-35 6-15 >(Low) . >High
Black walnut >Sensitive Intermediate 6.6-8.0 70-100 60-100+ Medium . Medium
Black willow Tolerant Tolerant 6.5-8.0 30-60 20-40 High Yes >Low

1: Hightshoe, 1988; 2: Minnesota Association of Soil and Water Conservation Districts Forestry Committee, 1986; 3: Matheny and Clark, 1991; 4: Minnesota Society of Arboriculture, 1996.

Values in parentheses reflect the authors' or technical advisors' opinions.

*Hazard Tree Rating refers to the relative potential for a tree to become hazardous. For a tree to be considered hazardous, a potential "target" (e.g., a house, a sidewalk, or other trees) must be present. A high hazard tree rating does not imply that the tree will always fail.

**Landscape Value refers to the relative value of each species in Minnesota based on hardiness, form, color, growth habits, flowering and fruiting characteristics, structural strength, longevity, insect and disease resistance, maintenance requirements, and general desirability.

Plan ahead!

House with overhanging trees

Figure 3. Careful planning may avoid the creation of hazardous tree situations such as damaged trees located too close to the house or dangerous overhanging limbs.

You'll save time and money if you develop a landscape protection plan before construction begins. Careful planning will help you avoid the expense and heartache of later repairing or removing trees located too close to construction activities.

These steps will help you create a successful landscape protection plan:

Diagram of house locations

Figure 4. You may be able to save some trees by siting the new construction away from the center of the lot.

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