New greenhouse plastic covers were prepared by incorporation of fluorescent dyes into polyethylene sheets made by extrusion. These dyes convert light from the green part of the spectrum into red light. The use of these sheets as greenhouse covers increased the weight of the tomato fruit yields by 19.6% and the number of flowering branches on rose bushes by 26.7% as compared to sheets without the dye. The increased yield was primarily due to a morphogenetic response of the plants to the spectral change of the light, which enhanced the development of the desired organs. In addition, it is possible that there was an increased rate of photosynthesis.