Japanese knotweed is an herbaceous, perennial, weed in the buckwheat family (Polygonaceae). It is native to eastern Asia, and it is an invasive species in North Carolina. It quickly grows 10 to 13 feet tall and forms dense stands 3 to 20 feet wide that crowd out native plants. The erect, hollow stems of Japanese knotweed have swollen nodes that give it the look of bamboo, but Japanese knotweed is not related to bamboo. It dies to the ground in winter and new growth is reddish-brown. Flowering occurs on erect panicles in late summer to fall. Older stems become woody near the ground. It spreads by strong underground stems (rhizomes) that can damage foundations, roads, walls and other structures. Cut or broken stems will root if left on moist soil or put directly into water. Japanese knotweed dominates an ecosystem and can be particularly problematic in riparian areas where it lowers the quality of habitat for fish and wildlife. Also, dead stems can accumulate and cause flooding or it can contribute to streambank erosion. In parts of the UK where this weed is a problem, its presence on a property lowers the property value. Avoid adding this plant to a landscape. Do not grow or propagate this plant. Quick ID Hints: Upright stems are hollow and resemble bamboo. Leaves are heart to triangular shaped and alternate. Insects, Diseases and Other Plant Problems: Weedy and listed as invasive by the NC Invasive Plant Council. Do not plant . See native alternatives to the left.
Curated pairwise relationships between this plant and others in the library — who feeds
whom, who competes, who confuses the pests. Green reads as a good pairing, red as one to avoid.
No curated interactions recorded for this species yet.
Also noted by NC State
Related species the NC State Plant Toolbox flags for this plant — look-alikes to tell
apart, and plants that play a similar role in a planting. Ones we carry in the library are linked.
Full sun (6 or more hours of direct sunlight a day)
Soil drainage
Moist | Occasionally Wet
Available space
6-feet-12 feet; 12-24 feet
Mature size
Height: 10 ft. 0 in. - 13 ft. 0 in. | Width: 3 ft. 0 in. - 20 ft. 0 in.
Growth rate
Rapid
Maintenance
High
Canopy layer
4 - Herbaceous (3-15 ft)
Form & Appearance
Habit / form
Arching
Leaf color
Green; Red/Burgundy
Leaf type
Sheath; Simple
Leaf arrangement
Alternate
Leaf shape
Deltoid; Elliptical; Subcordate
Leaf margin
Entire
Leaf size
3-6 inches long, 1-3 inches wide
Hairs present
No
Leaf detail
3 to 7 inches long and 2-4 inches wide broadly oval, to triangular, sharply pointed, alternate leaves with truncated bases and entire margins. Leaves are dark green on top and light green below with reddish veins. Where they meet the stem there is a sheath-like structure called an ocrea. New growth is red.
Flower color
Cream/Tan; Green; White
Flower form
Panicle; Spike
Flower size
< 1 inch
Flower detail
Small creamy-white flowers with 5 tepals in 3-6 inch long erect panicles from the leaf axils in later summer to fall. Male and female flowers are on separate plants with the male flowers being more erect and the female flowers drooping.
Bloom time
Fall; Summer
Fruit color
Cream/Tan
Fruit type
Achene
Fruit size
< 1 inch long, < 1 inch wide
Fruit detail
Small winged triangular achene with small shiny dark brown to black triangular shaped seeds.
Stem color
Brown/Copper; Green; Red/Burgundy
Stem detail
Stems are green to reddish-brown large, hollow, slightly rideged lengthwise, jointed and mottled. They resemble bamboo and remain after the plant has died back to the ground. Older stems become woody near the ground.
Wildlife & Ecology
Native status
Non-native
Origin
Eastern Asia
Food & Medicine
Edible parts
Stems
Fruit type
Achene
Edibility
Young stems are edible with a sour rhubarb flavor.