Biogenic carbon accounting in LCA is challenging. In our letter to the editor we discuss the problematic of incomplete C balances by taking into account only the C assimilation during photosynthesis without considering the short-term C storage in biomass and subsequent emissions of CO2. This can lead to biased results and misleading conclusions. We summarise some key principles of biogenic C accounting: * Only net and persistent changes of CO2 concentrations in the atmosphere should be taken into account. * For temporary storage of C, we recommend considering only storage for a minimum of 10 years. * In case of temporary storage, the duration of storage needs to be considered. * If no net and persistent changes occur, the biogenic flows can be ignored to simplify the calculation. * If biogenic flows are taken into account, the balance must be complete. * Biogenic C flows and their impacts on the climate should be represented separately from fossil emissions. * If biogenic C flows are calculated based on generic LCI databases or greenhouse gas accounting tools, the completeness and correctness of the calculations need to be checked. We also discuss the challenge to represent biogenic C flows in generic LCI databases.