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HOW to ... Keep Roots Healthy During Construction

From the January 2003 issue of The Forestry Source

Digging trenches through the critical root zones of trees will likely result in serious damage to root structures.

Credit for photo
Courtesy of USDA Forest Service

Construction damage is one of the leading causes of injury to trees in urban and suburban areas. While some of these injuries, such as damage to trunks and root collars, are rather obvious, less so are construction-related injuries to tree root systems.

Ed Macie, regional urban forester and project leader at the USDA Forest Service's Southern Center for Wildland Urban Interface Research and Information, recommends a four-tiered approach to prevent root damage during construction: organizing site activities, minimizing land disturbance, separating trees from construction activities, and maintaining trees.

Organizing Site Activities

Identifying potentially harmful construction activities, such as the parking of construction equipment and material storage areas, and locating them outside root zones can be noted on the site plans and executed before construction even begins.

Macie recommends that foresters begin by simply "knowing what trees are on site and organizing site activities to protect them."

Minimizing Land Disturbance

Macie also recommends looking for ways to minimize the affect of land disturbing activities, such as clearing, paving, or grading by making any necessary changes to the site plans, such as curving sidewalks around the trees instead of constructing them straight through critical root zones.

In addition to relocating items like paved areas, it is also beneficial to adopt techniques such as tunneling beneath root systems when construction near trees is unavoidable. Because the root systems of most trees live in the top 18 inches of soil, when engaged in projects such as the placement of underground pipes, tunneling at a depth of at least 2 feet deep will help avoid root system damage.

Separating Trees from Construction Activity

Third, control the amount of foot or vehicle traffic around trees to prevent soil compaction.

"This is where erecting fences at a tree's dripline or spreading mulch over its root zone come into play," says Macie.

A layer of wood chips or gravel mulch 6 to 12 inches deep on top of a geotextile landscape fabric placed over the root zone allows air spaces in the mulch to cushion the impact of machinery or foot traffic and disperse the weight over a larger area. The use of landscape fabric also facilitates cleanup.

Erecting fences can also help prevent damage from excavation, soil compaction, or the stockpiling of soil over roots. Because it is often easier to save groups of trees than individual ones, fences can be built around the dripline of the outermost trees to keep construction machinery away from the entire grove. Fences should come down only after all construction work is done, including final grading and smoothing of the site.

Tree Maintenance

Finally, Macie says there are several tree maintenance activities that can be done to minimize the impact of construction activities to root systems. Among these are fertilizing, pruning, watering, and aerating.

  • Fertilizing.
    Applying fertilizer before construction can enhance the ability of trees to withstand stress. It also helps trees resist the insects and diseases that often result from site disturbances. For construction-damaged trees, a slow-release organic form of nitrogen fertilizer is recommended because a quick-release fertilizer can create burst of vegetative growth that a damaged root system cannot support.

  • Pruning.
    Pruning of dead, diseased, or broken branches may be desirable before construction. Avoid pruning live plant tissue from a construction-damaged tree because this can accelerate the tree's decline. However, if the roots have been severed, pruning may be recommended to reduce the possibility of windthrow. Pruning deadwood is recommended only when the entire deadwood is evident, which may take from one to several years.
    Before, during, and after construction, monitor soil moisture conditions and water as necessary.

  • Aerating.
    Vertical mulching is recommended for trees in preparation for construction to improve their vigor. Holes 1 to 2 inches in diameter may be drilled in the compacted soil and filled with such porous material as sand, perilite, vermiculite, or other material. After construction, compacted soils and areas of fill or sedimentation in the critical root zones of trees should be aerated to allow for exchange of air between the soil and the atmosphere.

For information, contact Ed Macie, regional urban forester, Southern Center for Wildland Urban Interface Research and Information, 1720 Peachtree Road NW, Suite 850, Atlanta, GA 30367; (404) 347-1647; fax (404) 347-2776; www.urbanforestrysouth.org.

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