Asian knotweeds are highly invasive plants found in riparian areas throughout the Pacific Northwest. These species pose a serious threat to our rivers, riparian forests, and floodplain areas. ODA classifies all 4 of these knotweed species as Class B noxious weeds, This means they high priority for containing their current populations and preventing their spread. Japanese knotweed is internationally recognized as 1 of the World's 100 Worst Invasive Alien Species by the Invasive Species Specialist Group, an international network of scientific and policy experts that work under the umbrella of the Species Survival Commission (SSC) and the International Union for Conservation of Nature (IUCN).
The introduction of Japanese knotweed into the Pacific Northwest may have been from many different sources or populations. Although in nature, Japanese knotweed produces both fertile male plants and sterile male plants, research suggests that many infestations in the Pacific Northwest are a clonal lineage of sterile male plants, halting any viable seed production between plants.
Regardless, the vegetative reproductive capacity of Japanese knotweed is no small threat. In the presence of Giant knotweed pollen, the resulting offspring produces a vigorous, fertile hybrid species called Bohemian knotweed (Fallopia x bohemica). There is speculation that a large percentage of the Japanese knotweed in the Pacific Northwest is actually the Bohemian hybrid.
The degree to which viable seed production contributes to the spread of knotweed is not yet clear.
Key Characteristics:
- Round to square-shaped leaf with a distinct pointed tip and flat leaf base (4 to 7 inches long)
- Hollow stems or canes have dense red to brown spotting and grow 5 to 10 feet tall
- Stems or canes are segmented like bamboo, with red papery sheaths between stem sections
- No true hairs on leaves
- Middle leaf vein square in cross section
- Cream colored, sweet-smelling, linear flower bunches at each leaf
- Typically, male sterile plants (don't produce pollen)
Where is it located in Lincoln County?
Japanese knotweed is the most abundant knotweed species in Lincoln County. Japanese knotweed is found in every watershed in Lincoln County, with the Yachats River being home to the largest historic infestation that was at 1 point greater than 90,000 square feet in size.
How does it spread?
Generally, Japanese knotweed spreads when root or stem fragments are moved during flood events or by the transportation of contaminated soil during construction projects. A fragment of Japanese knotweed as small as 10 grams in weight (1/3 of an ounce) is a sufficient amount of plant matter to take root and develop into a new plant. Disturbances, such as uprooting and cutting, promote rhizome growth in knotweeds.
Therefore, it is absolutely critical that any cutting or transport of knotweed be done by a natural resources professional. During professional control of knotweed, any excess plant material should be bagged and disposed of at a landfill. In England, Japanese knotweed is considered a controlled waste and legally must be completely excavated from the soil before a property may be developed, often at a very high cost.
Controlling Japanese Knotweed
Japanese knotweed infestations take several years of treatment to control, and the most effective method is repeated herbicide application. Contacting a natural resource professional as early as possible is integral to knotweed control. If you see Japanese knotweed, contact the Oregon Invasives Hotline as well as the Lincoln Soil and Water Conservation District (LSWCD) by phone at (541) 265-2653 or email Lincoln SWCD.
Physical Description
Giant knotweed regularly grows to heights greater than 10 feet, with an upper limit of about 18 feet. The large oval to elliptic-shaped leaves help to easily distinguish it from Japanese Knotweed. However, giant knotweed readily hybridizes with Japanese knotweed to produce the hard-to-distinguish Bohemian knotweed.
- Large (12 to 18 inches) oval to elliptic leaf shape with a heart-shaped base
- 6 to 18 feet tall
- Light green segmented stems that are hollow, similar to bamboo stems, with a red papery sheath located between stem segments (no red spots on stems like Japanese or some Bohemian knotweeds)
- Long flexible hairs on leaves
- The middle leaf vein is rounded in cross section
- Cream colored, sweet-smelling, linear flower bunches at each leaf
Where is it located in Lincoln County?
Giant knotweed is the second most abundant knotweed species in Lincoln County. It is predominantly found in the upper Salmon River basin, Middle Yaquina River (above and below Eddyville), and in Elkhorn Creek (tributary to Beaver Creek).
Control Efforts
Giant knotweed infestations take several years of treatment to control, and the most effective method is repeated herbicide application. Contacting a natural resource professional as early as possible is integral to knotweed control. If you see giant knotweed, contact the Oregon Invasives Hotline as well as the Lincoln Soil and Water Conservation District (LSWCD) by phone at (541) 265-2653 or email Lincoln SWCD.
Bohemian knotweed is a vigorous and fertile hybrid that is the result of a cross between Japanese and giant knotweed.
Physical Description
There is a high degree of morphological variability in Bohemian knotweed. Physical characteristics of Bohemian knotweed range from being indistinguishable from Japanese knotweed to being indistinguishable from giant knotweed, with most individuals exhibiting intermittent characteristics between the 2 species.
- Broadly ovate to narrowly ovate leaves, 7 to 14 inches long, with a flat to heart-shaped base
- 6 to 11 feet tall
- Green segmented stems, with or without red to brown spotting, that are variable in width
- Stout leaf hairs up to 1/2 mm long
- The cross section of the middle leaf vein is square to slightly rounded in shape
- Papery sheaths between stem sections
- Hollow stems
- Cream colored, sweet-smelling, linear flower bunches at each leaf. That often appears to have a more upright appearance than Japanese or giant knotweed
Control Efforts
Bohemian knotweed infestations take several years of treatment to control, and the most effective method is repeated herbicide application. Contacting a natural resource professional as early as possible is integral to knotweed control. If you see giant knotweed, contact the Oregon Invasives Hotline as well as the Lincoln Soil and Water Conservation District (LSWCD) by phone at (541) 265-2653 or email Lincoln SWCD.
Physical Description
Himalayan knotweed has several distinct identifying characteristics that help with distinguishing it from other knotweeds. The leaves are much narrower than those of Japanese and giant knotweed and have long, pointed tips. The clumping growth habit and comparatively narrow stems cause Himalayan knotweed to form very dense patches that are quite difficult to travel through.
- 5 to 12-inch lanceolate or spear-shaped leaves with a pointed tip
- 4 to 8 feet tall
- Thin, light green, segmented stems
- Green to red colored papery sheaths between stem segments
- High density of stems
- Narrowly hollow stems
- Cream colored, sweet-smelling flower bunches are seen mostly at the top of stems
Where Is It Located in Lincoln County?
Himalayan knotweed infestations in Lincoln County are centralized in the urban riparian corridors of Lincoln City and Yachats.
Control Efforts
Himalayan knotweed infestations take several years of treatment to control, and the most effective method is repeated herbicide application. Contacting a natural resource professional as early as possible is integral to knotweed control. If you see Himalayan knotweed, contact the Oregon Invasives Hotline as well as the Lincoln Soil and Water Conservation District (LSWCD)by phone at (541) 265-2653 or email Lincoln SWCD.
What is knotweed?
Knotweeds are shrubby herbaceous perennial plants native to Asia that were brought to the U.S. in the late 1800s as garden ornamentals. They have dispersed in watersheds throughout the country due to their ready ability to spread vegetatively and by seed. Knotweed plant tissue and seeds often contaminate fill material used in construction projects, which facilitates their spread into new riparian corridors. Other common names for knotweed include Mexican or Japanese bamboo, elephant ear, Godzilla weed, and fleece flower.
Why is knotweed a problem?
Knotweed rapidly spreads by seed dispersal and from the spread of broken pieces of plant tissue. Knotweeds grow rapidly, averaging 2 to 4 feet a month in late spring through the summer. In riparian corridors and flood plains, uncontrolled knotweed populations will completely displace native vegetation. This increases soil erosion potential due to the above-ground portions of the plant completely dying back during winter dormancy, creating an absence of buffering vegetation during winter storms and high water events. Uncontrolled knotweed populations also prevent the natural establishment of native trees and shrubs, which is critical for maintaining the watershed structure and regulating the water temperatures necessary for fish and wildlife habitat. Large patches are very difficult to eradicate, even with herbicides!
How does it spread?
Generally, knotweeds spread when root or stem fragments are moved during flood events or by the transportation of contaminated soil during construction projects. A fragment of knotweed as small as 10 grams in weight (1/3 of an ounce) is a sufficient amount of plant matter to take root and develop into a new plant. Disturbances, such as uprooting and cutting, promote rhizome growth in knotweeds. So, it is absolutely critical that any cutting or transport of knotweed be done by a natural resources professional. During professional control of knotweed, any excess plant material should be bagged and disposed of at a landfill.
Controlling Knotweeds
Knotweed infestations take several years of treatment to control, and the most effective method of treatment is repeated herbicide application. Contacting a natural resource professional as early as possible is integral to knotweed control. If you see knotweed, contact the Oregon Invasives Hotline as well as the Lincoln Soil and Water Conservation District (LSWCD) by phone at (541) 265-2653 or email Lincoln SWCD.
Salmon River Watershed
Lower Siletz Watershed
Yaquina River Watershed
Alsea River Watershed
Yachats River Watershed
