In 2010-12 research related to irrigation of fallows was carried out. Fallows were spontaneously settled by plants which mainly consisted of goldenrods (Solidago sp.) – a species which can be successfully used in agro-energy. The objective of the conducted research was to verify if sewage systematically delivered to a fallow fundamentally differentiates the floristic composition and biomass of the dominant species – goldenrod. Two irrigation rates: 10mm and 20mm on the patches of 1m2 each were applied on the fawn soil formed by clay sand. The effects of irrigation with initially purified sewage were compared to the object without irrigation (control). In the vegetative season, from May to September, 21 irrigation rates of sewage in total were applied with 7 days intervals. The irrigation contributed to the increase of the goldenrod shoots number (plants) on the area unit. The rest of species covering the fallow changed. The biggest group with the following species: Agropyron repens, Rumex crispus, Taraxacum officinale, Artemisia vulgaris, Trifolium repens, significantly increased its plants number due to irrigation. However, the second group with such species as: Poa pratensis, Calamagrostis epigejos, Tanacetum vulgare, Achillea millefolium, decreased its plants number. For the irrigation doses of 10mm, the increase of the number of plants of species reacting positively to irrigation was lower than the decrease of plants which decreased its number of plants. The situation connected with doses of 20mm irrigation was different. Hence, the total number of plants irrigated with doses of 10mm diminished comparing to the control object, however, it increased on the patches irrigated with 20mm doses. The irrigation caused significant increase of the goldenrod biomass as a dominant species in relation to the biomass of other taxons.