April 19, 2001

Giving street trees a better chance

  • Creative design underground means longer life .
    Hough Beck & Baird

    cross section

    Street trees are an investment that pays on many levels. They raise the value of real estate and enhance the attractiveness of commercial areas. By providing shade, purifying the air, reducing wind, glare, reflection and noise, and helping with the infiltration of stormwater, street trees moderate the environment of urban areas.

    People develop strong ties with trees — evident when residents resist the removal of large or historic trees in their community.

    Despite these benefits, Seattle has many urban street trees in declining health or in danger of being removed due to sidewalk or utilities damage.

    With adequate space to meet growing requirements below and above ground, some of Seattle’s street trees successfully grow into healthy, mature trees. Roots find room to search out moisture, nutrients and air; and the tree canopy maximizes its growing space for air and light. Many of these street trees grow and develop into mature specimens, defining the sense of place in their respective communities.

    However, many street trees exist in conditions hazardous to their survival. Street trees in industrial areas struggle to grow in poor soils with inadequate drainage, made worse through constant damage by heavy truck traffic and sometimes vandalism. Often, street trees are planted in spaces left over during the design process and simply lack the space they need to grow.

    What's new under the street?
    Here are three ways to relieve crowded street trees:

    Structural soil. The innovative solution involves the use of structural soil to replace subgrade material under sidewalks. Structural soil consists of a mixture of gravel and planting soil, designed to meet standard compaction requirements and yet permeable enough to allow for healthy root growth.
    According to city of Seattle senior landscape architect Shane DeWald, “Interest in structural soil is high because often, space for the standard planting strip is simply not available; Seattle Transportation operates under the assumption that even though the planting strip can be a hostile place for trees, it is also one of the most important places to provide trees, both for pedestrian safety and as a public amenity.”
    However, the cost to remove existing soil and backfill with structural soil within a typical project budget is simply not affordable unless it does more than support tree growth, he said. That cost can be as much as four and a half or five times more than planting soil.

    Continuous trenches. A continuous trench of soil between street trees creates a greater area for root growth and the sharing of resources between individual street trees. SeaTran has already used this technique in several projects to date.
    A variety of engineered soils have been placed in continuous planting strips as a cost effective and value adding alternative to standard infrastructure, according to DeWald.
    “The cost of soil trenching is offset since standard drainage trenches are no longer needed and we avoid the increased cost to upgrade our stormwater infrastructure,” he said.

    Root paths. Root paths provide an alternate planting option. Through the use of root paths, tree pits can be manipulated into less geometric configurations than today’s current square or rectangular pits.
    According to arborist James Barborinas of Urban Forestry Services, root paths provide a spider web network that directs roots under paving and into more favorable areas for air, water and nutrients. This technique is most effective when large areas of lawn or planting beds exist on the opposite side of the sidewalk.

    Competition for underground growing space is a concern in dense urban areas where pavement and underground utilities take increasingly large amounts of room. Correct street tree planting methods offer the best solution to street tree survival. With proper planning, street trees thrive in urban areas.

    The current edition of the Architectural Graphic Standards includes a new section on urban tree planting. New planting options include continuous soil trenches, structural soil and root path trenches. Also, both SeaTran and Seattle Public Utilities are conducting experiments and gathering information from sources like the University of Washington and the Public Works Department in Vancouver, B.C.

    Reaping all the benefits that trees provide incurs some initial cost and investment. The biggest cost is site preparation, plant purchasing and initial care. With the correct installation of street trees, the long-term cost is minimal.

    “The cost of planting a tree incorrectly and then having to return, modify the planting, repair or replace the sidewalk, and replace the plant entirely, is higher than if the soil had been prepared correctly to begin with and a proper tree chosen for the site,” said arborist James Barborinas of Urban Forestry Services.

    Urban forest professionals in Seattle have and continue to work on enhancing urban street tree growing conditions for tomorrow.

    “Seattle Transportation standards have changed in very simple but important ways,” said city of Seattle senior landscape architect Shane DeWald. “SeaTran has increased the size requirement for unpaved areas, many tree pits are now converted to planting strips with minimal root damage, and mulching retains soil moisture and minimizes soil compaction.

    “As we continue to learn more, projects combining continuous soil trenches with root paths under the walkway could be seen as an intermediate step toward the use of an engineered structural soil mix under the entire sidewalk.”

    Effective street tree planting standards and specifications do exist and will continue to evolve. However, consistent application of planting and preparation standards would be of additional value in the development of our urban forest. Under reasonable urban conditions, street trees given proper attention should not only survive, but also mature with healthy, vigorous growth.

    Continued improvement of the health of Seattle’s street trees requires a coordinated effort.

    This effort can be seen in the Tree Selection Symposium held last month in Seattle. The symposium united city planners, city foresters, landscape architects, tree suppliers and tree maintenance personnel in a discussion on design and maintenance concerns, as well as street tree lists. Increased communications between professions would further education efforts in new methods and requirements and improve the current condition of our street trees.

    Dean W. Koonts is a landscape architect at Hough Beck & Baird. Jim Howard is a senior associate.

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