Japanese Knotweed

Japanese knotweed foliage and flowers. Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Updated: February 6, 2026

Japanese knotweed (Reynoutria japonica, formerly named Fallopia japonica and Polygonum cuspidatum) is a flowering perennial native to Eastern Asia. It was introduced to the U.S. in the late 1800s for ornamental use and erosion control, and has since become invasive. Additional common names include fleeceflower and Mexican or Japanese bamboo. Cultivar 'Variegata' has been grown for its cream-marbled leaves.

Physical features of Japanese knotweed

Japanese knotweed is an herbaceous perennial which dies back to the ground in winter. Despite having "bamboo" as one of its common names due to similarities in stem structure and plant spread, Japanese knotweed is related to buckwheat, while true bamboo is a grass.

A mature Japanese knotweed plant has a shrub-like stature due to its large size and thick, sturdy stems. The plant blooms in mid- to late summer, and seeds ripen in autumn. This species is dioecious: it bears male and female flowers on separate plants. Only female plants produce seeds, which are dispersed by the wind.

Growth rate and habit: rapid growth rate; clumps wider than tall overall; spreads via rhizomes (underground runners)

Typical mature size: 4 to 10 feet tall; rhizomes may reach 60 feet long

Blooms: upright clusters of tiny creamy-white flowers; multiple clusters are produced along the length of the stem

Leaves: simple (not divided into leaflets) with smooth margins; broadly oval in shape, with a pointed tip; base of the petiole (leaf stalk) is surrounded by a membranous sheath (ocrea/ochrea) where it attaches to the stem; alternate arrangement on the stems

Seeds: in clusters, with each seed enclosed in a 3-winged papery husk

Stems: prominent nodes (where leaves attach to the stem) that can have zig-zag arrangement while new growth is expanding; green or spotted with red-purple; hollow interior except for solid junctions at the nodes

Tufts of tiny white flowers in dense upright clusters along a Japanese knotweed stem. — Dense upright clusters of tiny white flowers on Japanese knotweed stems.
Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Hanging clusters of light brown papery winged seeds on the end of a bare Japanese knotweed stem. — Clusters of papery, winged seeds hang from Japanese knotweed stems after the foliage has fallen.
Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Newly-emerged Japanese knotweed stem with expanding leaves and prominent pale sheaths around each joint. — Emerging Japanese knotweed stem with prominent ocrea surrounding each node where leaves are expanding.
Photo licensed by Adobe Stock.

Emerging Japanese knotweed stems with expanding leaves that are purplish-green in color. — New leaves on young stems may have a purplish blush.
Photo licensed by Adobe Stock.

Young Japanese knotweed rhizomes, white and comparatively slender, dug out and laid on the soil surface. — Excavated rhizomes on a young Japanese knotweed plant. Rhizomes can get thicker and more extensive as the plant matures.
Photo: John Cardina, The Ohio State University, Bugwood.org

Split Japanese knotweed stem exposing the bamboo-like hollow, segmented interior. — Split Japanese knotweed stems resemble bamboo with a hollow interior and divisions at the nodes.
Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Japanese knotweed seedling with young green leaves and a purplish petiole color. — Japanese knotweed seedling.
Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Broad growth habit of Japanese knotweed with arching branches that are lined with flowers when in bloom. — Broad, shrubby growth habit of mature Japanese knotweed; flowers are showy as they line arching stems.
Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Zig-zag shape of a Japanese knotweed stem, which also has many fine purple spots. — Japanese knotweed stems often have reddish-purple spots and a zig-zag shape as they grow.
Photo: Ansel Oommen, Bugwood.org

Dense cluster of emerging Japanese knotweed stems. — Japanese knotweed stems emerging in a dense cluster in spring before trees have leafed-out.
Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Conditions that favor growth

Japanese knotweed can be common in moist, well-drained soils near waterways, roadside ditches, power line cuts, and in clearings in disturbed habitat. It is drought- and salt-tolerant, preferring full sun but also tolerating shade. Sturdy stems and robust growth is capable of breaking through asphalt.

Large clumps of Japanese knotweed taking over a forest understory along a gravel road. — Shade tolerance allows Japanese knotweed to colonize the forest understory, especially along disturbed areas like roads and trails.
Photo: Miri Talabac, University of Maryland Extension

Expansive colonies of Japanese knotweed taking over a wide, flat riverbank. — Japanese knotweed taking over the shores of a river.
Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Large mass of Japanese knotweed in flower, growing on a sandy shoreline. — Japanese knotweed can take over sandy shoreline habitat.
Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Removing Japanese knotweed

Vigorous growth and resilience to damage and environmental stress (like flooding) make established Japanese knotweed very difficult to control. It spreads primarily with extensive rhizomes, though it can also spread from seed. Cutting a clump down can limit seed production, but will not eradicate the plant unless additional or follow-up measures are taken. Cut plants can easily regrow from rhizome pieces that are not removed. The most efficient and effective removal method is to use systemic herbicide to kill the roots.

Mechanical control

Japanese knotweed can be managed without the use of herbicide, but the process takes more effort and time. After a plant is cut down, any new sprouts must be removed promptly so they do not have time to make and store energy reserves for regrowth. Be vigilant for suckers or seedlings to remove as soon as they appear. Repeat the process until the roots are exhausted of energy and cannot generate any more sprouts. Successful removal of established Japanese knotweed colonies may take several years (possibly a decade) of cutting off all top growth several times per year.

Due to the substantial root system, digging out as many roots and rhizomes as possible could lead to erosion, but with precautions in place, this can speed-up the removal process in combination with continually removing above-ground growth.

Smothering a clump to deprive the plant of light, after first cutting down all stems around early summer, can work if the cover remains in place and intact for several years.

Chemical control

A systemic herbicide applied to the leaves or to fresh-cut stem stumps will be absorbed and kill the roots. This process takes time and may require repeat attempts to achieve full control. Commonly-used systemic ingredients include glyphosate and triclopyr. Follow all product label directions for use.

Applications of systemic herbicide can be more effective when applied to regrowth after first cutting all stems down in early summer to weaken the plant and then waiting about two months. The late summer application coincides with when deciduous perennial plants move carbohydrates down into their roots for winter; applied herbicide will more easily be transported by sap flow.

Additional resources

(PDF) Weeds Gone Wild: Alien Plant Invaders of Natural Areas

Japanese Knotweed | Penn State University

Weed Identification: Japanese Knotweed | Cornell University

Author: Miri Talabac, Lead Horticulture Coordinator, Jan. 2026.

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References

Kaufman, Sylvan Ramsey & Wallace Kaufman. 2007. Invasive Plants: Guide to Identification and the Impacts and Control of Common North American Species

Swearingen, J.M. and Fulton, J.P. 2022. Plant Invaders of Mid-Atlantic Natural Areas: Field Guide. Passiflora Press.