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The Nature Conservancy in Connecticut Connecticut Home Places We Protect Contact News Room Ways of Giving Volunteer Government Relations Corporate Council CLCC Invasive Species |
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Description
Japanese knotweed (Polygonum cuspidatum Sieb. & Zucc.) 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 streambanks 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 cuspidatum 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 usage.
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, but has been measured as tall as 11'; ocreae membranous, tubular; rhizomes are stout, subterranean (up to 45 to 60 feet in length).
Nature picture credits (top to bottom, left to right): Photo © Leslie J. Mehrhoff, University of Connecticut, Bugwood.org (Japanese knotweed); Photo © Philip Rusted, Thurlow Countryside Management (r&d), Bugwood.org (Japanese knotweed).
