Japanese Knotweed
Fallopia japonica
Physical Description
The introduction of Japanese knotweed into the Pacific Northwest may have been from many different sources or populations. Although in nature Japanese knotweed produces male sterile an male fertile plants, research suggests that many infestations in the Pacific Northwest are a clonal lineage which have male sterile flowers halting any viable seed production between plants. Regardless, the vegetative reproductive capacity of Japanese knotweed is no small threat and poses the greatest problem. In the presence of Giant knotweed pollen the resulting offspring produces a vigorous, fertile hybrid species called bohemian knotweed (Fallopia x boehemica). There are suggestions that a large percentage of the Japanese knotweed in the Pacific Northwest is actually the bohemian hybrid. The level of viable seed production as a major source of spread is not clear.
- Round to square shaped leaf with a distinct pointy tip and flat leaf base (4-7 inches long)
- Hollow stems or canes have dense red to brown spotting, 5-10 ft 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 one point was greater than 90,000 square feet in size (photos above).
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. 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. 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 Lincoln SWCD by phone at 541.265.2653 or email.
Click the link below for an in-depth summary of current knotweed control practices:
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 one point was greater than 90,000 square feet in size (photos above).
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. 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. 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 Lincoln SWCD by phone at 541.265.2653 or email.
Click the link below for an in-depth summary of current knotweed control practices:
