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.
Bohemian knotweed is a vigorous and fertile hybrid that is the result of a cross between Japanese and giant knotweed.
Physical Description
Spectrum of leaf shapes of Japanese, giant, and hybrid bohemica knotweed species
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.
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.
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.
