Japanese knotweed is an herbaceous perenniala plant whose growth dies down annually but whose roots or other underground parts survive that appears woody, and reaches 3-10 feet in height. The round stems are hollow and covered with scales. The shoots can reach 65 feet in length. The leaves are broadly oblong-ovate or ovate-lanceolate, 3-6 inches long and 2-4.75 inches wide. The tips of the leaves are abruptly acuminate, while the bases of the leaves are truncate. The numerous, greenish-white flowers are borne in panicles from the upper axils. These panicles measure 3-6 inches long. The flowers are functionally unisexual: each of the male and female flowers still have the complementary organs, but they are vestigial. The inflorescences of the male flowers tend to be upright, while those of the female flowers tend to be drooping. Flowers appear from August to September. The fruit are papery and winged, and are 0.25-0.4 inches long. These fruits contain black, smooth, shiny, 3-angled achenes that are 0.2 inches long.
Japanese knotweed can be found in a variety of habitats. It thrives in riparian areasthe area immediately adjacent to running fresh water and wetlands, but can be found along roadsides and other disturbed areas. It prefers full sunlight, but can tolerate moderate shade. It is tolerant of high temperatures, dry soil and salt. It is extremely intolerant of frost, and after the first frost, it turns brown and dies back for the season.
History and Introduction
Fallopia japonica is native to China, Japan and Korea. This plant has been reported from all the states of New England.
This species was introduced from Japan to the United Kingdom probably sometime after 1830. It was first distributed around 1855 by the Royal Botanical Gardens at Kew. From here it was popularized through garden catalogs. By the early 1900's, the rampant nature of its growth was observed, and the plant began to decline in popularity. This plant was most likely brought into the United States from Britain close to the turn of the century for use as a horticultural plantplants grown in gardens to produce food and medicinal ingredients, or for comfort and ornamental purposes. By 1894, it was reported as naturalized near Philadelphia, Pennsylvania; Schenectady, New York; and Atlantic Highlands, New Jersey. Regardless of these early observations of naturalization, in 1910, it was being described in garden catalogs in this country. By 1938, information was already being published on how to get rid of the plant in gardens. As in other parts of the country, Fallopia japonica was brought into New England for ornamental gardens. From the gardens it likely spread via rhizomes as well as some of the few viable seeds.
The threat of Japanese Knotweed was first recognized in Britain, where it has been present for a longer period of time. It appears to behave similarly here, forming dense, persistent thickets that exclude other vegetation. Its vegetative reproduction has proved quite successful. Established populations are difficult to eradicate.
Though this plant produces winged fruits that can move some distance, it spreads mostly through long rhizomes. Rhizome fragments can sprout new plants, and the plant often disperses via natural or human-aided movement of such fragments.
Japanese knotweed is an extremely difficult plant to control due to its ability to re-grow from vegetative pieces and from seeds. Mechanical and chemical methods are most commonly used to eliminate it. Single young plants can be pulled by hand when soil is moist and roots are small. Roots and runners must be removed to prevent re-sprouting. Glyphosatea widely used herbicide that can kill certain weeds and grasses, it works by blocking an enzyme essential for plant growth and triclopyrherbicide used to control both broadleaf and woody plants herbicides have been used effectively, applied to freshly cut stems or foliage.
Sequential mowing or cutting coupled with sequential herbicide treatments will provide the highest rate of control of knotweed. Plants should be mowed or cut prior to foliar applications, and two applications may be necessary during the first year of control. As an alternative, managers may choose to use the stem injection method, eliminating the need for cutting. For very small infestations, cutting and spot spraying or stem injections will eventually achieve eradication.
Japanese knotweed is very difficult to kill or control, requiring consistent attention over a period of years for success. The method recommended for homeowners with small infestations is continuous cutting.
Pulling / Digging Up: Pulling by hand is an ineffective method of control and will stimulate increased growth due to rhizome fragmentation.
Mowing: In small populations mowing or hand cutting at least three times during the growing season appears to diminish bohemian knotweed’s ability to re-sprout. Although eradication via this method of management is unlikely, persistent cutting will shrink the infestation.
Prescribed Grazing: Although knotweed is palatable to livestock, grazing will not suppress its growth, nor eradicate it.
Soil Tilling: Not advisable. Tiling will fragment roots and encourage re-sprouting and vigorous growth.
Glyphosate and triclopyr herbicides have been used effectively, applied to freshly cut stems or foliage.
Fallopia japonica (Also: Polygonum cuspidatum & Reynoutria japonica)
Mexican Bamboo, Fleeceflower, Huzhang, Himalayan Fleece Vine, Donkey Rhubarb, Pea Shooters, Asian Knotweed
Japan, China, and parts of Korea and Taiwan
Giant Knotweed (Polygonum sachalinense)
These species can look very similar to each other. The most reliable character for distinguishing them is the type of hair on the veins of the leaf undersides, which can be seen with the aid of a strong hand lens.