System for vegetation on a roof, in particular a flat roof, more in particular a flat roof placed at a slope, comprising one or more first accommodation spaces for holding a growth medium and vegetation placed in or on it, a number of lower trays placed on the roof which trays have a bottom and on which trays the first accommodation spaces are supported, wherein second accommodation spaces have been formed underneath the growth medium and on top of the bottom of the lower trays, which second accommodation spaces are in water transporting connection with the first accommodation spaces, for holding a storage of atmospheric water for the benefit of the growth medium and the vegetation and/or for buffering atmospheric water, characterized in that the lower trays have one or more outlets for letting out the water out of the lower trays, wherein the system further comprises one or more water discharge conduits extending from one or more of the outlets to a location of discharge, wherein the system further comprises one or more closing devices of which the position can be controlled for influencing the quantity of water flowing out of the lower trays to the discharge and means for controlling the position of the closing devices, which control means are operative in dependency on an external parameter
