Japanese knotweed (Polygonum cuspidatum) 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 of deep shade, high temperatures, saline soils, and drought conditions aids in its adaptability.
Japanese 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.
Japanese 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 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 methods such as cutting, mowing and pulling can be effective over a long time scale, but they need to be done consistently. It is most effective for small or environmentally sensitive areas where herbicides cannot be used. Because 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.
While biological means of control are being examined, none are viable on a large scale.
Cut stem treatment: In early fall, stems should be cut about 2 in. above the ground and followed immediately by application of glyphosate or triclopyr to the cut stem. This treatment is best when Japanese knotweed is growing mixed with or near other species.
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: Care must be taken when using chemical treatment near water courses.