Abstract: An at least partially contained, reduced weight (PCRW) plant growth medium, providing a sand/soil matrix including a sand/soil mixture in which plants can grow. The sand/soil matrix preferably includes sand/soil displacement structures proximate a plurality of isolated locations where the uninterrupted sand/soil depth in that zone is less than in a first set of isolated locations. The sand/soil displacement structures having a weight per unit area that is less than the weight per unit area of the sand/soil mixture, thereby reducing the weight of the growth medium, while still providing a suitable sand/soil profile for plant growth. A building structure, a spectator pavilion and a modular plant growth system including such a PCRW plant growth medium, or a plurality thereof, are also disclosed as are methods of building a reduced weight green space on a roof of a building structure.