Abstract: A greenhouse (1) is provided with an air humidifier (10) and a condenser ((35): the humidifier (10) has a semi-permeable membrane, of the type which permits the passage of water vapour between opposite sides of the membrane if there is a vapour pressure gradient between the said sides. A flow of sea water (13) and a flow of air (14) taken from the outside are supplied to the humidifier (10) and sent to opposite sides of the membrane, in such a way that the flow of air (14) is humidified before being introduced into the greenhouse; the humidified air (30) is then condensed in the condenser (10) to produce condensate (36) to be used for irrigating the plants.