Impacts on biodiversity
Japanese knotweed can severely degrade the quality of wetland and riparian habitats where it becomes established. Dense thickets can reduce sunlight penetration by more than 90% and its thick mats of dead and decaying vegetation in fall/spring prevent other plant species from growing by shading them out.
As a result of the reduced native plant biodiversity and lowered invertebrate densities, established knotweed stands do not support the same levels of native amphibian, reptile, bird and mammal populations. For example, it has been shown that native green frog (Rana clamitans) presence is dramatically reduced in knotweed stands in riparian/wetland areas. Although further study is needed, it is believed that Japanese knotweed may have allelopathic properties. The roots contain unique compounds, which may alter soil chemistry or prohibit the growth of nearby native species (Japanese Knotweed BMP).
Impacts on infrastructure
This plant can significantly damage infrastructure. It is able to grow through concrete/asphalt up to 8 cm thick and building foundations. It is of particular concern in new housing developments. In the United Kingdom, developers must dispose of soil containing knotweed fragments at hazardous waste facilities. Japanese knotweed root systems, while strong, are not as dense as those of native plants, and do not hold soil as well. When Japanese knotweed establishes along stream banks, the bank can become unstable and more vulnerable to erosion and flooding. Reductions in available soil (because of erosion) and space (because of the larger root/rhizome biomass) affect the ability of the stream bank to hold water during heavy rains (Japanese Knotweed BMP).
Impacts on recreation
Japanese knotweed can block or interfere with access to water for activities such as canoeing, boating, angling, and swimming (Japanese Knotweed BMP).