The air conditioned room has a ribbed floor (52) with the sides of the ribs having air holes (42) which are not covered by the plant holders. By sliding the ribs relative to each other the air holes are opened and closed to control the ventilation. The sides of the space have air flow control through the walls.Air is extracted through the roof and is ducted into the space through control units (58, 60). By controlling the transmission of the walls and floor the optimum climatic conditions can be set up for the particular plants.