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.
