•The possibilities of energetic use of willow biomass form constructed wetland system (CWS) were investigated.•The annual yield of willow biomass was averagely 1.01 kg d·m· m−2.•High and low heating value were typical for wood biomass and it was respectively 18.13 and 17.38 MJ·kg−1.•Young parts of the shoots contained more ash and elements than the old ones, and had lower energy parameters.•The rational way to use biomass from CWS is to use it for energy purposes. The paper presents the results of a study of selected parameters of common osier (Salix viminalis) biomass sampled annually, over a 12-year period, from a constructed wetland wastewater treatment plant. The focus of the study was on parameters that were relevant to biomass energy management. The study was conducted in a constructed wetland wastewater treatment plant consisting of one horizontal flow bed with a total area of 186 m2, planted with osier willows. The following biomass parameters were analysed: biomass yield, volume, humidity, ash content, higher and lower heating values, and contents of N, P, K in different parts of plant stems. The study showed that osier biomass harvested from the investigated constructed wetland system could be used as an energy source in thermochemical processes. The higher and lower heating values were similar to those obtained by other authors (18.13 and 17.38 MJ·kg−1, respectively). The energy yield (17.49 MJ·m−2) and energy density (3486–5411 MJ·m−3) were also calculated. Young shoot parts had higher ash and micronutrient contents and lower energy parameters than old parts.