In a greenhouse (1) air is mixed around plants (2) by means of a ventilation and/or circulation system (5), which comprises an air duct (6A). The air duct comprises a variable passage structure (14, 24) through an outer circumferential wall structure (12, 22) of the air duct. Said mixing is performed in that via said variable passage structure air is exhausted out of the interior of the air duct and in the direction of a row of plants. Each individual plant in the row of plants experiences intermittent spurts of air (8), which spurts of air are caused by exhaust of air from time-dependently varying different exhaust locations (9) of the variable passage structure, while said time-dependently varying different exhaust locations are traveling (30) in the longitudinal air duct direction (7) for exposing all individual plants along the longitudinal row direction (4) to said spurts of air (8).
