A stormwater treatment system including a substantially water impermeable treatment container for treating stormwater through a bioretention mechanism. The treatment container includes an overflow/bypass opening to receive stormwater that bypasses the treatment container filter media, or is only partially treated as a result of high stormwater flows in excess of the capacity of the container to treat the stormwater. The stormwater is directed into the container through an inlet, such as an inlet pipe, preferably connected to a roof drain system for processing and treating stormwater from the roof of a building. The overflow/bypass opening may be adjacent the top wall or bottom wall of the container.