Knotweed Replaces Native Vegetation
Knotweed infestations change the composition of riparian plant communities as they crowd out native trees and shrubs. Studies consistently demonstrate knotweed’s ability to displace resident species in riparian habitats (Urgenson 2009).
Knotweed Depletes Available Large Woody Debris
Replacement of native woody species by knotweed significantly reduces the amount of large woody debris available for recruitment into the system (Urgenson et al, 2009 page 4) as well as reducing shade and in-stream cover.
Knotweed Contributes to Slump Erosion of Riverbanks
Woody riparian plants such as willows and alders have a deep, strong root mat to bind the soil, whereas the rhizomes of knotweed are prone to slump erosion on the river banks (Tallmadge 2004).
Knotweeds Interrupt Nutrient Cycling
They are exceedingly efficient at stripping out available nitrogen in their leaves. They retain 75% of the available foliar nitrogen and store it in their woody rhizomes. Contrast this to the 5% stored by alder, 33% by the willow species that grow in riparian zones and make surplus nitrogen in their leaf litter available to the soil nutrient system (Urgenson 2009).
Knotweed Impacts Soil Health
It has also been discovered that various mycorrhizal fungi which normally partner with our native plants, are severely depleted or even absent in dense monocultures of knotweed. This phenomenon is currently under study, but implications are that the presence of knotweed alters the condition and chemistry of the soil, rendering it difficult to reestablish native plants. This phenomenon is already well documented with Scotch broom (Reichard 2009).