City Trees

May/June 2011

City Trees is a premier publication focused on urban + community forestry. In each issue, you’ll learn how to best manage the trees in your community and more!

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R O U N D T ABL E Bioswales and Raingardens in the Urban Forest Street-side bioretention wells in Upper Arlington, Ohio capture stormwater from the street before the water enters the storm sewer system. Carefully selected shrubs and perennials fill the well adjacent to sawtooth oak (Quercus acutissima). Photo by Steve Cothrel Bioswales and raingardens offer improved stormwater management over the conventional method of funneling water on impervious surfaces into a storm drain. The benefit for trees in bioswales or raingardens is additional soil space for root growth. In parking lot designs, most bioswales offer more soil volume to col- lect water runoff than the typical curbed planting spot or tree cutout. This increased volume of soil allows for larger trees and less adjacent infrastructure damage by tree roots. The use of structural soils under hardscape and pavement near bioswales creates additional stor- age reservoirs for stormwater collection. In larger bioswale designs, trees can be arranged in a non-linear pattern, offering the opportunity for size and species diversity, which may reduce disease spread and increase the longevity of the planting. Species selection must consider the wide variation in soil moisture from very wet to dry, and irrigation is usually needed during dry periods. In some climates, rain only occurs in the winter and in this case, evergreen trees will be the most 30 effective at stormwater interception. Depending on cli- mate and availability, these could include Douglas fir (Pseudotsuga menziesii), swamp gum (Eucalyptus ovata), pond or marsh pine (Pinus serotina), and southern mag- nolia (Magnolia grandiflora). In many communities, bioswales and raingardens fit well with the push for sustainable infrastructure and climate protection. They can offer reduced capital costs compared to traditional storm drain system construc- tion. In addition to rain interception, trees in bioswales provide benefits such as shade, improved air quality, and connection with nature. If designed properly, the bioswale or raingarden will not cause a reduction in parking spaces or site use while still meeting the stormwater needs of the site. During an exceptional storm event the system may be over- loaded; however, this can also occur with traditional storm drain systems. As with any landscape design, the proper trees should City Trees

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