This spraying assembly comprises at least one nozzle (1) for liquid that is to be sprayed, and at least one active substance injector (21a-21d) which is situated directly upstream from at least one nozzle (1) and to which liquid to be sprayed is supplied by means of a pump (33a-33d) or of a pressurized reservoir (25a-25d), with interposition of an obturator (24a-24d) on a conduit (23a-23d) connecting the pump or the pressurized reservoir to the injector. This assembly additionally comprises, for each injector (21a-21d), an outlet valve (22a-22d) designed to close the or each outlet orifice (26a-26d). The invention avoids overdosing or underdosing of the product that is to be sprayed as active substance.
