25 October 2004
TREE BOX FILTERS. Tree box filters are in-ground containers typically
containing street trees in urban areas. These filters can be very effective at controlling
runoff water quality, especially when numerous units are distributed throughout a site.
Runoff is directed to the tree box, where it is filtered by vegetation and soil before
entering a catch basin.
Figure 8-11. Manufactured Tree Box Filter
Source: Virginia DCR Stormwater Management Program.
Most Appropriate Uses. Tree box filters can help meet a variety of
stormwater management goals, satisfy regulatory requirements for new development,
protect and restore streams, control combined sewer overflows (CSOs), retrofit existing
urban areas, and protect reservoir watersheds. The compact size of tree box filters
allows volume and water quality control to be tailored to specific site characteristics.
Tree box filters provide the added value of aesthetics while making efficient use of
available land for stormwater management. Typical landscape plants (e.g., shrubs,
ornamental grasses, trees and flowers) are an integral part of the bioretention system.
Ideally, plants should be selected that can withstand alternating inundation and drought
conditions, and that do not have invasive root systems which may reduce the soil's
Cost Data. A single-unit tree box filter costs approximately ,000 per unit
per 0.1 ha (1/4 acre) of impervious surface (total cost = ,000 per acre). This
estimate includes two years of operating maintenance and filter material and plants.
Additional costs include installation and annual maintenance. Installation varies with
each site, but is approximately 00 per unit. Annual maintenance is 0 per unit
when performed by the manufacturer and 0 per unit when performed by the owner.
(This sample cost estimate is based on a commercial tree box filter, the FilterraTM
Stormwater Bioretention Filtration System.)
Maintenance Issues. Tree box filters require little maintenance.
Maintenance includes annual routine inspection and the regular removal of trash and
debris. The first two years of maintenance are typically included with the purchase of