Invasive knotweed (Reynoutria japonica Houtt. var. japonica) is a perennial herb with bamboo-like stems. It typically grows in thickets 3-6 feet tall but can reach as high as 15 feet. Native to Asia, Japanese knotweed was introduced to North America in the late 19th century and now can be found in most states and Canadian provinces, including Alaska.
The plant is an aggressive riparian invader but can also thrive in wetlands, and along roadsides and other disturbed areas. Once established, the plant’s long rhizomes allow it spread rapidly and it can easily create a monoculture. Its extreme tolerance to deep shade, high temperatures, saline soils, and drought conditions aids in its adaptability.
Invasive knotweed spreads through the dispersal of rhizomes and broken plant fragments. Care should be taken when fishing or enjoying other activities along waterways with invasive knotweed present as plant stems can easily break off and land in the water and be spread to new areas.
Invasive knotweed has a long history of serious invasive impacts in Europe and it is now regarded as the most pernicious weed in the United Kingdom. Impacts in North America will likely mirror those of the UK.
Knotweed spreads quickly to form dense thickets that exclude native species, reducing species diversity and diminishing an area’s value to wildlife, especially phytophagous (plant-eating) insects and species that rely on them.
All of the dead stems and leaves can create a fire hazard during the dormant season. It is particularly problematic in riparian areas because it can survive floods and rapidly colonize scoured shores and islands.
Invasive knotweed is an upright, shrubby, herbaceous perennial that spreads primarily by seed, stem fragments, and by vegetative means with long, stout rhizomes. Seeds can be transported by water, wind, on people’s shoes, by animals, and as a contaminant in soil.
Height: 10 to 15 feet in height.
Stems: smooth, stout, swollen where the leaf meets the stem. Membranous sheath surrounds joints of the stem.
Leaves: normally about 6 inches long by 3 to 4 inches wide, alternating on the stem, broadly oval to somewhat triangular or heart-shaped, pointed at the tip.
Flowers & fruits: small, greenish-white flowers in branched sprays in summer, followed by small winged fruits.
Seeds: triangular, shiny, very small, about 1/10 inch long.
Mechanical control: Manual removal of established knotweed plants is usually ineffective due to the easily fragmented rhizomes. Methods such as cutting, mowing, and pulling are most effective for small or environmentally sensitive areas where herbicides cannot be used.
It’s best to wait for flowering, which occurs in the summer, to cut knotweed to better deplete the stored energy in the rhizomes. Care should be taken as even a small piece of stem will regrow wherever it touches the soil, all plant material must be removed from the site and properly disposed of to prevent re-establishment or spread to other sites.
Cut stem treatment: In early fall, stems should be cut about 2 inches above the ground and followed immediately by the application of glyphosate or triclopyr to the cut stem. This treatment is best when the knotweed is growing mixed with or near other species.
Stem injection: Using an injection gun herbicide can be directly injected into the knotweed stem. This method can be more time-consuming but allows the herbicide to be specifically calibrated for the size and spread of the plant, and doesn’t require the removal of plant fragments as with the cut-stem approach. Furthermore, given that knotweed stems are hollow, herbicide injected into the plant will reach the roots and rhizome of the plant very quickly.
Foliar treatment: A foliar application of glyphosate can be used to control large populations, but multiple treatments may be required. It is most effective to spray in late summer or early fall after cutting the stems in late spring or early summer.
Note: Permits may be needed and care must be taken when using chemical treatment near water.
Photo Credits: Title photo: Emma Erler bugwood.org. Leaves photo: Jan Samanek, State Phytosanitary Administration, Bugwood.org. Flower photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org. Seeds photo: USDA PLANTS Database, USDA NRCS PLANTS Database, Bugwood.org.