Ohio Perennial and Biennial Weed Guide

Family: Smartweed (Polygonaceae) family

Other Names: Fallopia japonica, Pleuropterus zuccarinii, Polygonum japonicum, P. zuccarinii, P. sieboldii de Vriese, Reynoutria japonica, Japanese bamboo, Mexican bamboo, Japanese polygonum, Japanese fleeceflower, false bamboo, Kontiki bamboo, bombascus.

Origin and Distribution: Japanese knotweed was introduced from eastern Asia in the late 1800s as an ornamental, but soon escaped from gardens to colonize disturbed areas. By the 1960s it had spread to local infestations from Maine south to Virginia and west to Indiana. Today it is found from Nova Scotia to the Carolinas and west to Minnesota and Iowa, as well as in Colorado, Utah, and coastal areas from Washington to Northern California. It is found throughout much of Ohio, especially in urban and suburban landscapes, roadsides, gullies, and waste areas. It is particularly troublesome along riverbanks, edges of ponds, and other wet areas. It is often associated with moist but well-drained sites with nutrient-rich soil, and it tolerates semi-shaded environments. It has also been planted in sandy sea-shore areas where it stabilizes soil and withstands salt and low nutrients.

Plant Description: Japanese knotweed is an erect, broad-leaved, semi-woody perennial that spreads by long rhizomes and occasionally by seeds. The plant forms dense clumps that exclude other plants, and radiates rapidly to form patches that can be as large as 1 to 3 acres. It is one of the most persistent, and hardy of weeds, and it tolerates many control measures. Japanese knotweed is an invasive plant that can overrun natural areas, gardens, yards, roadsides, and utility and railroad rights-of-way.

Root system - The root system is fibrous, but rhizomes are white when young, becoming brown, thick, and woody with age. Rhizomes have prominent nodes with dark papery sheaths. They may be shallow or deep, and are responsible for the spread and persistence of this weed.
Seedlings & Shoots - Young shoots are reddish, with mostly heart-shaped leaves. Because seed production is uncommon, true seedlings are uncommon. Most young shoots arise from rhizomes.
Stems - Stems are erect, tall (up to 10 or more feet for mature stands), and hollow except at the prominent, swollen, knot-like nodes. The stems are thought to resemble those of bamboo. Attached to each node and surrounding the stem is a light green to brown, hairless, papery sheath. Stems are round and smooth, red-brown at the base, and mottled green toward the tip. Stems die back to the ground during winter, but semi-woody stem bases persist.
Leaves - Leaves are alternate (one per node), broad, flat to round at the base, tapering to a pointed tip, and attached by long petioles. The upper leaf surface is dark green and the lower surface is pale green.
Flowers - Small, greenish white flowers are clustered along branching panicles arising from upper leaf axils. Plants are unisexual, with male and female flowers on separate plants. Male flower stalks are mostly erect and female flower stalks are drooping. When blooming (July to September), the plant puts on an attractive floral display befitting the common name 'fleece flower'.
Fruits & Seeds - Rarely produced fruits have three triangular papery wings surrounding a single dry, brown, triangular seed. Fruits are rare because colonies seldom contain a mixture of male and female plants.

Similar Species: Japanese knotweed resembles bamboo because of the robust hollow stems with distinct nodes and internodes; however, true bamboo is a grass. The broad and pointed Japanese knotweed leaves can be mistaken for Broadleaf dock (Rumex obtusifolia), but docks lack rhizomes and the tall, spreading habit of Japanese knotweed. Other less invasive relatives (such as P. virginianum) grow from similar rhizomes and are difficult to eradicate.

Biology: Japanese knotweed shoots resume growth in early spring, reaching a fast pace (reportedly 2 to 4 inches in a single day) and attaining heights over 10 ft by late summer. Plants form dense colonies, spreading by rhizomes that can extend up to 65 feet. New colonies can regenerate from as little as a 1-inch piece of rhizome, which can easily be transported wherever soil is moved. Rhizomes send out shoots from April to August, even from a depth of over 3 feet. Shoots can even be initiated from internode tissue. Japanese knotweed exhibits great tolerance to most herbicides. It is reported to be a poor invader into grass cover and can be crowded out by taller trees. It does not survive frequent mowing.

Toxicity: Japanese knotweed has been used as a folk medicine in eastern Asia; however, this species contains tannins that were found to be carcinogenic. Large quantities of tannins were found to inhibit digestive enzymes in rats. Some chemicals isolated from Japanese knotweed have antimicrobial properties, others have been used as antioxidants and antimutagens in cancer research. Other chemicals isolated from this plant have been used to promote healing of burns, and still others to enhance the immune system and cardiac functions.

Facts and Folklore:

Dense stands of Japanese knotweed exclude native and other desirable vegetation and reduce wildlife habit. It decreases water flow through rivers and streams and thereby contributes to flooding. It is a long-term threat because it occupies edges of woods and waterways that are valued for biological and visual diversity. It is one of the most troublesome weeds along railway rights-of-way, and is said to create a fire hazard in the dormant season.

Japanese knotweed is highly regarded for its attractive flowers and has been planted by beekeepers for its nectar. It is also prized for its tolerance of harsh conditions like rocky soils with limited nitrogen and low pH. It has been planted along highways to control soil erosion and has been used for revegetation of strip-mine spoil and to stabilize land affected by volcanoes.

Japanese knotweed has caused damage to sidewalks and parking lots where shoots have been able to grow up through concrete.

Fast-growing branch tips picked in spring are said to be have a unique almond-like flavor when prepared in the manner of rhubarb pie.

Japanese knotweed was used in folk medicine as a laxative, but contains tannins that are carcinogenic and inhibitors of digestive enzymes.

The dense, hedge-like growth of Japanese knotweed was commonly used as a screen around out-houses.