The occurrence of increased consumption of fossil fuels is leading to usage of reserves, rising prices and environmental pollution. For those reasons the renewable energy sources are becoming increasingly more important. These sources also include biomass energy crops from which it is possible to produce liquid biofuels, biogas, electrical or thermal energy. There can be a problem with the distribution of areas for cultivation of crops for food and energy sources. One of the solutions for cultivation of energy crops is to grow them on less fertile, marginal soils. Switchrass is a perennial species that is capable of adapting on different climatic conditions and soil types so it can be grown on vide wide range of environments. Because of its well-developed root system, it tolerates dry soils well, as well as wet, skeletal, poor or sour. Though it achieves the best results on soils of neutral reaction. It's not too demanding during breeding and it can grow on the same area for 10 or more years. Because marginal soils are very often poor and sour, the ash of burned biomass can rise the pH value of the soil because of its alkaline reaction. Except as calcificator, it can also have a role as compost. It is important the ash which is being used in those purposes does not get contaminated with harmful substances. The goal of this work is to confirm the influence of different doses of ash of wooden biomass and mowing period on economical and morphological performances of switchgrass. Fertilization treatments of ash did not significantly affect the yields of dry matter, height of the plant or the content of dry matter in autumn and spring harvest season. It is recorded that the average yield of dry matter is 28.81 t/ha, average height of the plant 2.35 m, average content of dry matter 39.47 % in autumn and average yield of dry matter 23.35 t/ha, and average content of dry matter 86.52 % in spring harvesting season. Significant drop of yield of dry matter and growth percentage is noted in spring season.