Knotweed (Reynoutria sp.)

Knotweed stem. Photo Credit: Ansel Oommen, Bugwood.org
Knotweed leaves. Photo Credit: David J. Moorhead, University of Georgia, Bugwood.org
Knotweed flowers. Photo Credit: Jan Samanek, Phytosanitary Administration, Bugwood.org
Knotweed infestation. Photo Credit: Chris Evans, University of Illinois, Bugwood.org
Knotweed infestation. Photo Credit: Barbara Tokarska-Guzik, University of Silesia, Bugwood.org
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Key Identification Features

  • Bamboo-like woody, hollow stems

  • Triangular or heart-shaped leaves

  • ‘Fountains’ of white flowers when in bloom

Description

The are two species of invasive knotweed, Japanese knotweed (R. japonica) and giant knotweed (R. sachalinensis) in NY. Additionally, the two species regularly hybridize to create bohemian knotweed (Fallopia x bohemica). Telling the three varieties apart can be difficult but they share many traits in common that make them easy to distinguish from native plants.

Although unrelated, knotweed resembles bamboo with its woody, hollow stem. Individual plants can reach 15’ and quickly form dense stands that shade out native plants and can be difficult for humans and other animals to traverse. The leaves are broadly ovate (rounded at the base and tapering towards the end) and are triangular or heart-shaped. The leaves are between 3 to 6 inches (8 to 15 cm) long and 2 to 4 inches (5 to 10 cm) wide and alternate along the stem. Where the leaves are attached to the stem there is a membranous sheath surrounding the joints, further adding to knotweeds’ resemblance to bamboo. Knotweed flowers from August to September with cascades or sprays of green to white flowers.

When young and just sprouting knotweed somewhat resembles asparagus with red spotting. The shoots are tender and can be harvested (make sure the area has not previously been treated with herbicide).

Native Range

Eastern Asia

Habitat and Dispersion

Knotweed is a generalist and thrives in a variety of conditions that other plants would struggle in. Disturbed areas and roadsides are common places to find knotweed. Knotweed can thrive in areas of shade, high salinity (like roadside ditches), high temperatures, and drought.

It can also easily be found along riparian corridors and thrives in wet habitats.

Best Management Practices

Once established knotweed can be difficult to control given its hardy rhizome and ease of spread.

BMPs below based on NYISRI Japanese Knotweed BMPs

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Management Goals Management TechniqueDescription of TechniqueDrawbacks to Technique
Eradication/ContainmentDigging/PullingDig 4ft down and hand pull smaller plants. Remove all rhizomes and runnersFollow-up monitoring required and multiple attempts. Expensive and high disturbance levels.
EradicationStem InjectionCut stems 2in above ground, inject 25% glyphosate or triclopyr into the stem between nodes, in early fall. Treat regrowth with 4% glyphosate sprayFollow-up monitoring required and multiple applications. Ineffective for small stems. Laborious and expensive.
EradicationFoliar SprayCut early in the season and wet all foliage with a 2% solution of glyphosate or triclophyr. Treat regrowth when height reaches 1m and leaves are green.Applications may be difficult due to plant height, may require multiple applications. Air temperature should be at least 65F. Laborious and expensive.
WeakeningCutting/MowingTrim stems to the ground every three weeks from early spring through late summer. Recommended to follow this treatment with herbicides.Multiple seasons may be required to see effects. Care must be taken with equipment/biomass to prevent further spread. Very labor intensive.
Knotweed PDF