How plastic mulches affect the thermal and radiative microclimate in an unheated low-cost greenhouse

Suboptimal regimes of air and soil temperature usually occur under unheated greenhouses during winter crop cycles. This work analyses the effects of three soil surface treatments (no plastic mulch, NM; transparent mulch, TM, and black mulch, BM) on the air-soil heat exchanges and the resulting soil and aerial microclimate. Experiments were conducted in unheated greenhouse compartments located in an area of mild winter climate (South-East Spain) during autumn and winter periods. In all treatments, the soil consisted of an artificial layer of 0.10 m gravel-sand material placed above a 0.3 m layer of imported loamy soil. When vents were closed, soil heat flux, ground net radiation and both air and root-zone temperature were higher in BM than in TM, while NM presented intermediate performances between BM and TM. When vents were open, heat storage and soil warming were substantially reduced with respect to unventilated conditions. This reduction was greater in BM, and so the advantages of BM with respect to the other treatments were only marginal under ventilated conditions. The main conclusions were: (i) The combination of black mulch + greenhouse appears to be a simple and low-cost passive heating system that can be recommended for the early stages of crop cycles starting at the end of autumn or in winter, when canopy leaf area index is small and most of the soil surface is free of vegetation; and (ii) ventilation had a negative effect on the benefits of mulching, implying that greenhouse ventilation management should reflect a compromise between maximizing greenhouse heat storage and fulfilling ventilation requirements for suitable crop growth.