Abstract Background Aquatic plants are an important component of aquatic ecosystems. They are valuable for the oxygen and carbon dioxide household and generate habitats especially for small fish and other small organisms. However, problems for the maintenance of water bodies can result from mass occurrences of these plants. Invasive neophytes - such as members of the Elodea genus - are particularly problematic in this regard. Aquatic plants need to be harvested regularly to ensure that water bodies remain usable and to safeguard flood protection for flowing water bodies. Energy can be produced from the harvested material by anaerobic digestion in biogas plants. Therefore, it is beneficial to know the best time for harvesting in this context. Methods To identify the best time for harvesting, samples of the Elodea stock in the river Parthe in Leipzig-Schönefeld were taken each week over the course of the two hydrological years 2015 and 2016. The composition of these samples was analyzed in the laboratory. In the second hydrological year, three samples from surface areas of 1 m2 were also harvested once each month in order to determine the biomass yield per unit area. Results The best harvesting time for energy production from Elodea biomass in Germany is in the summer months (June to September). During this period, the specific yield of 0.5–0.7 kg VS/m2 is relatively high and the Elodea biomass contains the highest fractions of volatile solids (80.1 ± 2.3%), high contents of plant nutrients (N 35.9 ± 4.0 g/kg TS; P 6.1 ± 1.4 g/kg TS; and K 47.7 ± 8.0 g/kg TS), and low concentrations of heavy metals (Cr ≤8.9 mg/kg TS, Cd ≤0.9 mg/kg TS, Cu ≤120 mg/kg TS, Ni ≤30 mg/kg TS, Pb ≤8.6 mg/kg TS, and Zn ≤439 mg/kg TS). Conclusions Energy production from Elodea biomass is feasible. This biomass also provides the nutrients and trace elements necessary for the digestion in the anaerobic process.