abstract: The reed canary grass (Phalaris arundinacea L.) features high content of bioactive compounds from the group of alkaloids with allelopathic activity. Habitat factors and the harvest time determine the synthesis of chemical compounds required for plant survival in adverse conditions. The aim of the research was to reveal the inhibitory effect of extracts from the reed canary grass on some weed species. The biotypes of the reed canary grass originated from two floristically different habitats. The material was collected in two periods: in spring and in late autumn before the winter dormancy. In each period, aboveground parts and runners of each biotype were collected as raw material for preparation of the extracts. Several series of the tests showed strong inhibitory activity towards Amaranthus retroflexus L., Papaver rhoeas L.,and Stellaria media (L.) Vill of the extract prepared from the runners collected on the bank of a midfield watercourse in Pisarzowice. The strongest inhibitory effect was exhibited by the extracts from runners collected before the winter dormancy period. Extracts derived from both aboveground parts and runners collected from the habitat in Jarnołtów had a considerably weaker phytotoxic and inhibitory effect regardless of the harvest date.