Tree filters are compact bioretention systems consisting of an open-bottomed chamber with one or more trees and filled with engineered soil media. Tree filters collect, temporarily store, and filter stormwater runoff through the engineered soil media, and the tree provides pollutant uptake. Tree filters are particularly well suited to urban or built-out areas where they can easily fit into small footprints and/or work as retrofits. Tree filters often work in tandem with existing stormwater networks allowing less frequent, high-intensity storm events to bypass the system.

Tree filters consist of three main parts: the tree, soil media, and chamber. The chamber is typically filled with engineered soil media that is designed for rapid infiltration. The system is planted with non-invasive trees or shrubs. The top of the chamber typically has a tree grate to protect the base of the tree, soil, and root system, as well as for pedestrian safety. The grate also serves to keep trash and debris from entering the top of the chamber. Most of the stormwater enters the system through a curb cut under the grate. Within the chamber there is typically storage for ponded stormwater runoff above the soil media. The engineered soil media filters the stormwater runoff as it flows downward through the system. The filtered runoff is collected in an underdrain and returned to the storm drainage system or infiltrates into the underlying soil. Tree filters provide pollutant removal via filtration, infiltration, pollutant uptake, and adsorption.