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Knotweed

Japanese knotweed. Photo by Paul Rischmiller

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Common Name: Knotweed
Scientific Name: Reynoutria spp.
Origin: Asia

Description

Japanese knotweed, (Reynoutria japonica), is a fast-growing, herbaceous perennial with jointed, hollow stems and alternate, leathery leaves that are broadly ovate. A cascade of white flowers blooms in August, and dormant reddish stems are visible in winter. Two additional knotweed species are commonly found within western New York. Giant knotweed, (Reynoutria sachalinensis) leaves are much larger than Japanese knotweed. Bohemian knotweed, (Reynoutria x bohemica), is a hybrid of Japanese and giant knotweed, sharing characteristics of both. All three knotweed species are invasive.

Habitat

Knotweed is found along forest edges and stream banks, as well as in disturbed and open areas such as rights-of-way, roadsides and fallow fields.

Threat

Knotweed’s early spring emergence and dense growth give it a competitive advantage over native plants, enabling it to take over large areas. Its thick rhizomes can extend horizontally through soils for 60 ft. or more. Plants can emerge through concrete and asphalt, with potential impacts on infrastructure.

Management

Knotweed can be very difficult to control, depending on the extent (size) and location of the infestation. A stem injection or foliar treatment with systemic herbicide is the most effective management. Individual plants can be dug, if all pieces of the root are removed. However, keep in mind that knotweed roots extend up to 6 ft. down and very small pieces of roots are able to re-sprout. For the same reason, be mindful when mowing or otherwise cutting knotweeds, as plant fragments transported on mowers, etc., will spread the species. Whether using chemical or manual means of removal, 3-5 years of treatment should be expected.

Regional Distribution

Widespread

WNY PRISM Priority

Tier 4 – Local Control

Pathways

Knotweed was introduced as an ornamental plant and was used for erosion control. Knotweed’s primary method of propagation is vegetative, through rhizomes and fragments. Therefore, knotweed is primarily spread by humans through activities that move plant materials such as mowing, excavation and construction. Birds may also assist by spreading berries from bohemian and giant knotweed, however Japanese knotweed is not believed to produce viable seed.

 

All three knotweed species are prohibited species New York State – for more information on Prohibited and Regulated Species, visit http://www.dec.ny.gov/animals/99141.html.

Additional Resources

WNY PRISM Best Management Practices Fact Sheet:  BMP Knotweed