Flow-through planters are self-contained systems that collect roof runoff and filter it as it percolates through the biotreatment soil media and permeable material below. Water not used by the plants enters the storm drain system through a perforated pipe at the bottom of the planter. Flow-through planters can be used to treat stormwater where infiltration is not desired, such as next to buildings, or where it is not possible due to poorly draining soils and steep slopes.