Japanese Knotweed 

Polygonum cuspidatum Sieb. & Zucc. 
Buckwheat Family (Polygonaceae)

 

Status: Increasing and invasive in Connecticut, primarily at disturbed sites. 

Description: Japanese knotweed is a fast-growing herbaceous perennial that grows in large clumps three to six feet in height. It has hollow stems similar to bamboo, with swollen joints along the stem. Leaves are broadly egg-shaped, with pointed tips and squarish bases, The flowers are greenish white and profuse, growing in slender fingerlike clusters where the leaves meet the branches.

Preferred habitat: This plant occurs in a wide variety of habitats, in many soil types and a range of moisture conditions. It appears to be found primarily in disturbed open areas with plenty of sun; shade depresses its growth. Edges of roadways and stream banks are common locations at which to find Japanese knotweed. 

Seasonal cycle: In Connecticut, leaves appear on Japanese knotweed in April. Flowers, which develop in August and September, are pollinated by bees and other insects. The seeds mature about two weeks after the plant flowers and are dispersed by wind. Once established, the species reproduces primarily through its extensive rhizomes, which may reach 45-60 feet in length.

Distribution: Native to Japan, Japanese knotweed was introduced into the United States in the late 1800s as an ornamental. The species has been widely cultivated and has escaped and naturalized.  It is now widely distributed throughout the eastern United States and is found as far north as Nova Scotia and Newfoundland and as far south as North Carolina. It is also found in much of the midwest and in the coastal areas of Washington and Oregon. 

 

Other points of interest: The early emergence of Japanese knotweed leaves in the spring and its stand-forming habit produce a dense canopy beneath which few other plant species can survive. In addition, the persistent accumulation of stem litter within established stands also reduces species diversity and damages wildlife habitat. 

Japanese knotweed is a wild edible. Young shoots up to one foot in height can be harvested early in the spring, then steamed or boiled for four to five minutes and served like asparagus. Slightly older stems can be used to make a rhubarb-like jam by peeling and boiling the sour rind with sugar and pectin.

This plant was classified as Reynoutria japonica by Houttuyn in 1777 and as Polygonum cuspidatuin by Siebold in 1846. Recently, it has been suggested that it should be reclassified as Fallopia japonica Japanese knotweed is also known by the common names Mexican bamboo and Japanese fleece flower. A similar species, Polyganum sachalineuse, is much less common and appears to escape only infrequently, if at all. It can be distinguished from P. cuspidatum primarily by its larger size, greenish flowers, and heart-shaped leaves which gradually taper to the tip. 

Control: It is extremely difficult, if not impossible, to eradicate large established stands of Japanese knotweed. However, establishment can be prevented fairly easily by removing plants before they become firmly entrenched. Current control methods include both mechanical and herbicidal treatments. Mechanical control includes cutting with persistence, at least three cuts in one growing season. Herbicides such as glyphosate (active ingredient in Roundup) may be more effective when applied to the regrowth of cut stems. Glyphosate is a non-selective herbicide and great care should be taken in its use.

Additional information sources: Gray's Manual of Botany. Eighth edition, corrected printing. M. Fernald. D. Van Nostrand Company, New York, 1970. 

Element Stewardship Abstract for Polygonum cuspidatum, Japanese knotweed. Leslie Seiger. The Nature Conservancy, 1992. Unpublished document.

Fallopia japonica (Houtt.) Ronse Decraene (Reynoutria japonica Houtt., Polygonum cuspidatum Si~b, & Zucc.). D. Beerling, J. Bailey, and A. Conolly. Journal of Ecology, 1994, 82, 959-979

A Field Guide to Edible Wild Plants (Eastern and Central North America). L. A. Peterson. Hougliton Mifflin Company, New York, 1977,

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Diagnostic information: Leaves: Petioled, round-ovate, truncate to slightly cuneate at base, abruptly cuspidate, becoming firm, and 2"-10" long. Flowers: Greenish-white, dioecious, in forking axillary panicles, Fruit: Calyx wing-angled, 1/3" long; achene shining, trigonous, about 1/5" long. Stems: Erect, glaucous, often mottled, widely bushy-branched, 3' - 8' high; ocreae membranous, tubular; rhizomes are stout, subterranean (up to 45 to 60 feet in length).

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This fact sheet has been prepared by The Nature Conservancy Connecticut Chapter