Effects of Diffused Cover Materials on Greenhouse Microclimate, Plant Growth, Fruit Yield and Quality, and Energy Use in Greenhouse Cucumber and Sweet Pepper Production

Greenhouse cover materials affect not only the light quantity and quality entering the greenhouse but also heat loss to the outside, which in turn have a large influence on greenhouse microclimate, plant growth, fruit yield and quality, and heating energy use. Diffused cover materials have the potential to improve the light distribution within crop canopy, and reduce high temperature stress and increase fruit yield and quality in the summer. In this study, two greenhouse trials (one on long English cucumbers, Oct. to Dec. 2013; and one on sweet peppers, Feb. to Aug. 2014) were conducted using six greenhouses covered with standard (STD) and newly developed diffused polyethylene film (DPE) and polycarbonate sheet (DPC), two greenhouses for each cover material. It was found that the diffused cover materials (DPE and DPC) improved the vertical temperature and light distribution within crop canopy, increased light use efficiency, and reduced fruit disorders in the summer. The improvement in light use efficiency could compensate for a small/few percentage of decrease in total light transmission. Therefore, DPE greenhouses had similar marketable fruit yield as STD in fall/winter production even if its light transmission was 3-5% lower than STD. DPE achieved higher marketable fruit yield than STD in summer production because of its lower incidence of fruit orders. DPE greenhouses used similar amount of energy as STD greenhouses in both trials. Therefore, DPE may be a better greenhouse cover material than STD for year-round greenhouse vegetable production. DPC greenhouses saved over 10% energy in comparison to STD and DPE greenhouses. However, its fruit yield was not as high as STD and DPE because of its much lower light transmission, especially at lower solar angles during fall/winter production. Therefore, when plastic cover material manufacturers/developers try to increase the proportion of diffused light with their cover materials, the total light transmission should not be compromised.