In different agro-climates the greenhouse horticulture are making use of various types of plastic cover where its influence on production in relation to crop, indoor climate and outside climate not considered in selection. In this study, major spectral properties of mainly used greenhouse cover for Ethiopian highland and their effect on yield were investigated. Cover spectral properties such as cover near-infrared (NIR) transmission, photosynthetic active radiation (PAR) transmission and cover longwave emission coefficient were major parameters that are considered in this study to determine their effect on dry matter harvest. To examine the influence of the cover parameters on biomass production, a generic tool or a model constructed in Matlab software was used. The model comprises energy balance equations, mass balance equations and growth model. The measured greenhouse indoor temperature data was considered for validation of the model. Simulations of indoor temperature, canopy temperature and vapor concentration were conducted for the existing greenhouse configuration.
The result of the simulation showed that in Ethiopian greenhouse for highland, high and low temperature, night time high humidity and low CO2 during day time are major problems which inhibit the growth and on the other hand create favorable condition for well know disease in the region that is botrytis. According to sensitivity analysis output of the model, 1% increase in PAR transmission resulted 0.009% increase dry matter harvest, whereas a percent increase in cover NIR transmission has caused 0.259% reduction in yield. Moreover, increment of cover long wave emission coefficient by 1 percent positively influences (0.044%) the rose production. The resulted depicted that as compared to low temperature problem during rainy season (2-3 months), high temperature problem during summer season (9-10 months) would become a foremost problem in Ethiopian highland greenhouse horticulture. Thus, plastic cover with lower NIR transmission and higher PAR transmission and long-wave emission coefficient is desirable for Ethiopian highland greenhouses.
However, the study revealed that night time low temperature particularly during rainy season should be studied to incorporate in respective growth model.