Knotweeds are large perennial plants with a massive root system that makes them difficult to control. They spread out by their long, creeping rhizomes, forming dense thickets. Bohemian knotweed, found mostly in Washington State, is the most common type of knotweed. The shortest species is Japanese knotweed, which is common not only in Japan, but also in Europe and the United States. Giant knotweed, also called elephant ear bamboo, is the largest species.
The Problem of Invasiveness
Japanese knotweeds, which are one of the most invasive organisms known in the world, thrive on disturbance. A tiny piece is able to re-grow and then spread by both natural and human activity. According to Authorstream.com, latent buds can res-sprout on rhizomes and crowns with roots spreading 20 feet from a parent plan, going down seven feet into the soil. It's not long before this plant starts to overtake railways, riverbanks, gardens, road edges and hedgerows.
The invasiveness of knotweeds threatens the survival of other plants and animals. For example, Japanese knotweeds rapidly spread, forming dense thickets that exclude native species. The plant is not even valuable to wildlife, so it's considered an environmental weed causing ecological harm. Because of their extensive rhizome systems knotweeds grow long enough to cover large areas, thus shading out other vegetation. This reduces the diversity of native species depending on plants the weeds have destroyed for food.
Dead stems that remain washed into stream channels and rivers continue to hinder native plants from regeneration, leaving river banks vulnerable to erosion and flooding. Flooding helps propagules (plant materials) spread downstream in fragmented stems and rhizomes that quickly colonize banks and islands.
Besides destroying an environment, knotweed also causes serious structural harm. Once it takes hold near buildings, it soon damages them, in addition to paved areas including patios and footpaths. This destructive weed has even been found growing through floorboards of living rooms. Knotweeds can spread over an area as large as a few hundred miles within a year.
Problem of Expense
Getting rid of knotweed is not only difficult, but also expensive. Although small stands of the weed can be killed by cutting it, this treatment needs to be repeated six times or more times each season for many years. Besides the expense of trying to remove the weed, there's also the tremendous cost of repairing damage caused by the plant.
Knotweed restricts access to riverbanks for fishermen and bank inspections. Because of the damage done by the plant, land value depreciates. Knotweed increases the threat of bank instability and soil erosion. Litter accumulation is another problem, besides causing an area to be aesthetically displeasing. Knotweed also damages archaeological sites.