Environmental Damage
Although native to Asian countries (Japan, China, Taiwan, Korea, primarily), Japanese knotweed (Fallopia japonica) was introduced as an ornamental plant in the United Kingdom in 1825 and arrived in North America in the late 19th century. Today, Japanese knotweed is present in 43 U.S. states. Infestations are worst throughout New England, including Connecticut, as well as the Northeast, certain Midwestern states and the state of Washington.
Knotweed thrives in disturbed areas, such as roadsides and construction sites, as well as stream and riverbanks, and railways. Once established, it can spread alarmingly fast, dominating the landscape and creating monoculture stands that threaten native plant communities and destabilize riverbanks.
Not only does it tolerate deep shade, high temperatures, high soil salinity and drought, but it is extremely persistent and difficult to eradicate due to its underground root system, which is made up of rhizomes that can regenerate from the smallest of pieces. It’s important to remember that if the stem is cut, both the stem and the trimmed portions are still capable of becoming new plants if they make contact with moist soil.
Effects on native plant communities
Studies focusing on knotweed’s effect on native plant communities have been done in Europe and North America, but most have been local. Nevertheless, knotweed’s impact on biodiversity and ecological processes is clear: knotweed is a major threat to the biodiversity and ecological integrity of ecosystems:
• Knotweed lowers species richness and biomass of native plant communities.
• Knotweed suppresses the growth of neighboring herbaceous flowering plants with allelopathic compounds, or chemicals, that inhibit the germination, growth (photosynthesis) and reproduction of other plants.
• The acidity of tough knotweed leaves and reduced presence of other herbaceous food sources harms herbivorous invertebrates like snails, slugs and worms.
• Knotweed’s abundant leaf litter and deep rhizomes alter soil chemistry for its own benefit. Knotweed leaves drop at different times and more quickly when compared to native species, altering nutrient availability and nutrient cycling.
According to one study reported by the Global Invasive Species Database, plots adjacent to knotweed stands had 1.6-10 times as many species, demonstrating the dramatic reduction of species diversity where knotweed is present. Researchers counted 63 species outside knotweed stands (78% native), but only 13 species (58% native) within the knotweed stands.
Aquatic habitat, water quality and flood risk
Knotweed readily invades flood plains, wetlands, stream corridors, forest edges and drainage ditches. Because the ground under knotweed thickets contains very little other growth, this bare soil is prone to erosion, presenting a particular threat to riparian areas.
Japanese knotweed can shade small streams and divert rainfall elsewhere. In winter months, the canes can increase erosion of river banks. When the foliage dies back, it can cause siltation and form blockages and dams of river channels, causing floods and blocking flood infrastructure like sluices, drains and ditches.
Knotweed represents a serious threat to streambank stability. Researchers have found that storms and hurricanes like tropical storm Irene, and subsequent dredging and cleanup work afterwards, is responsible for dispersing substantial numbers of new knotweed infestations due to erosion along waterways and spread of its underground rhizomes.
In 2018, local officials in Litchfield County identified knotweed as a serious problem after noting thick patches of it growing along the banks of the Housatonic River, a designated National Wild and Scenic River. Knotweed has a secure footing throughout the Connecticut River watershed. It’s present on the Farmington River, the Naugatuck River Greenway and elsewhere on the state’s rivers.
Knotweed significantly lowers the biomass of aquatic macroinvertebrates - insects in their nymph and larval stages that spend at least part of their lives in water. Macroinvertebrates are an important part of the aquatic food chain because they recycle nutrients and are a vital food source for fish, birds and amphibians.
Roadside incursions
Knotweed infestations are often visible along roadways. That’s because a significant source of its spread is through the accidental movement of rhizome fragments or cut stems. Roadside maintenance is a primary way it is spread with mowing equipment that transfers root fragments caught in the mower’s tires, or by using fill with soils containing root and stem fragments, or cut stems. Snowplows can also spread it along roads.
Knotweed can reach heights of 6 to 10 feet, and by the end of May, it is at nearly full height. It reduces sight lines, crosses guard rails, encroaches onto travel lanes, blocks road signs and endangers motorists. Knotweed damages pavement and overruns low maintenance areas.
In North Stamford, knotweed has been described as a safety problem and a “scourge” that grows into roadways, pushing pedestrians and bicyclists into traffic lanes.
Knotweed thrives in disturbed areas, such as roadsides and construction sites, as well as stream and riverbanks, and railways. Once established, it can spread alarmingly fast, dominating the landscape and creating monoculture stands that threaten native plant communities and destabilize riverbanks.
Not only does it tolerate deep shade, high temperatures, high soil salinity and drought, but it is extremely persistent and difficult to eradicate due to its underground root system, which is made up of rhizomes that can regenerate from the smallest of pieces. It’s important to remember that if the stem is cut, both the stem and the trimmed portions are still capable of becoming new plants if they make contact with moist soil.
Effects on native plant communities
Studies focusing on knotweed’s effect on native plant communities have been done in Europe and North America, but most have been local. Nevertheless, knotweed’s impact on biodiversity and ecological processes is clear: knotweed is a major threat to the biodiversity and ecological integrity of ecosystems:
• Knotweed lowers species richness and biomass of native plant communities.
• Knotweed suppresses the growth of neighboring herbaceous flowering plants with allelopathic compounds, or chemicals, that inhibit the germination, growth (photosynthesis) and reproduction of other plants.
• The acidity of tough knotweed leaves and reduced presence of other herbaceous food sources harms herbivorous invertebrates like snails, slugs and worms.
• Knotweed’s abundant leaf litter and deep rhizomes alter soil chemistry for its own benefit. Knotweed leaves drop at different times and more quickly when compared to native species, altering nutrient availability and nutrient cycling.
According to one study reported by the Global Invasive Species Database, plots adjacent to knotweed stands had 1.6-10 times as many species, demonstrating the dramatic reduction of species diversity where knotweed is present. Researchers counted 63 species outside knotweed stands (78% native), but only 13 species (58% native) within the knotweed stands.
Aquatic habitat, water quality and flood risk
Knotweed readily invades flood plains, wetlands, stream corridors, forest edges and drainage ditches. Because the ground under knotweed thickets contains very little other growth, this bare soil is prone to erosion, presenting a particular threat to riparian areas.
Japanese knotweed can shade small streams and divert rainfall elsewhere. In winter months, the canes can increase erosion of river banks. When the foliage dies back, it can cause siltation and form blockages and dams of river channels, causing floods and blocking flood infrastructure like sluices, drains and ditches.
Knotweed represents a serious threat to streambank stability. Researchers have found that storms and hurricanes like tropical storm Irene, and subsequent dredging and cleanup work afterwards, is responsible for dispersing substantial numbers of new knotweed infestations due to erosion along waterways and spread of its underground rhizomes.
In 2018, local officials in Litchfield County identified knotweed as a serious problem after noting thick patches of it growing along the banks of the Housatonic River, a designated National Wild and Scenic River. Knotweed has a secure footing throughout the Connecticut River watershed. It’s present on the Farmington River, the Naugatuck River Greenway and elsewhere on the state’s rivers.
Knotweed significantly lowers the biomass of aquatic macroinvertebrates - insects in their nymph and larval stages that spend at least part of their lives in water. Macroinvertebrates are an important part of the aquatic food chain because they recycle nutrients and are a vital food source for fish, birds and amphibians.
Roadside incursions
Knotweed infestations are often visible along roadways. That’s because a significant source of its spread is through the accidental movement of rhizome fragments or cut stems. Roadside maintenance is a primary way it is spread with mowing equipment that transfers root fragments caught in the mower’s tires, or by using fill with soils containing root and stem fragments, or cut stems. Snowplows can also spread it along roads.
Knotweed can reach heights of 6 to 10 feet, and by the end of May, it is at nearly full height. It reduces sight lines, crosses guard rails, encroaches onto travel lanes, blocks road signs and endangers motorists. Knotweed damages pavement and overruns low maintenance areas.
In North Stamford, knotweed has been described as a safety problem and a “scourge” that grows into roadways, pushing pedestrians and bicyclists into traffic lanes.