Knotweed Polygonum cuspidatum s.l. is a large, suffrutescent, perennial forb native to northeastern Asia that was imported as an ornamental and has become a widespread invasive species in urban and rural environments of North America and Europe. Studies have demonstrated knotweed allelopathy to the germination and growth of many tracheophytes, but we have found no studies of knotweed toxicity to bryophytes. Therefore, the aim of this study was to determine if knotweed extracts affected the growth of the gametophytes of two mosses: Atrichum angustatum and Thuidium delicatulum. Both moss species were exposed to aqueous rhizome extracts of knotweed in concentrations of 0 (control), 10, 25, 50 and 75% for a total of nine days in the laboratory. All non-zero concentrations resulted in significant losses of green biomass, with the greatest losses occurring at the highest concentrations. Samples exposed to the three highest concentrations lost 80% of green mass after nine days. These results help explain the scarcity of moss growth on or near live knotweed crowns.