A stormwater treatment system and method for removing sediment, chemical pollutants, and debris from stormwater runoff by utilizing bioretention practices including physical, chemical and biological processes. Stormwater is directed into a primarily open-bottomed, multi-dimensional container whereby entrained sediment and other transportable materials are filtered and treated through a media filter layer consisting of inorganic and/or organic materials. A live plant (preferably a tree) situated within the container with roots resident in the media filter layer with the ability for expansion beyond the perimeter of the container through openings in one or more sidewalls. The treated water may be further conveyed beyond the perimeter of the container by additional openings and/or piping. A vertically positioned overflow/bypass/clean out piping apparatus may be included within the stormwater treatment system to provide additional water conveyance. Additional ancillary conveyance, filtration and storage facilities may be connected to the described stormwater treatment system as conditions warrant.