Tree box filters are containers installed beneath trees that use rain garden and bioretention principles to clean and detain stormwater runoff. After runoff is directed to the tree box, it is filtered by vegetation and soil, before entering a catch basin or flowing off-site. The trees and vegetation are an integral part of the biofiltration system, and they need to be chosen for their ability to withstand varying levels of water inundation. Tree box filters are commonly used in urban areas with street trees, where they are a valuable method for efficiently using available land for stormwater management, as seen in figure 1.

 Figure 1: Manufactured Tree Box Filters For Stormwater Management
(Source: Virginia DCR Stormwater Management Program, http://www.dcr.state.va.us/sw/docs/tecbltn6.pdf)