A treatment device for stormwater is provided that comprises a container which in use contains a growing medium and a hydrodynamic separator located within the container. The hydrodynamic separator comprises a separation chamber, an inlet duct extending from the outside of the container to the separation chamber, a solids outlet; and a liquid outlet that opens into the container. In use, liquid flow containing solids enters the separation chamber through the inlet duct and is caused to swirl within the separation chamber, thereby causing solids to exit the separation chamber through the solids outlet and liquid to exit the separation chamber through the liquid outlet and to flow into the growing medium.