Japanese Knotweed is another imported perennial ornamental run amuck. The stems are stout, erect to arching, growing to 10+ feet in height and well branched, forming a shrub-like clustered group. The stems are hollow, smooth, round, and resemble bamboo, green initially, becoming reddish-brown with age. Stems are jointed and the area above the joint is surrounded by a sheath which is usually brownish in color. Joints are swollen where leaves meet the stem.
The leaves are alternate, broadly oval, rounded to truncated base, the tip abruptly pointed (cuspidate). They are alternate on the stem, up to 6 inches long, smooth but wavy margins, the upper surface usually smooth, the lower surface usually with hairs along the veins. The vein pattern is conspicuous on both sides. The leaf stalk is short.
The inflorescence is a terminal and axillary erect to spreading cluster, sometimes branched as a panicle and sometimes spiked as a raceme. Several clusters can arise from the same leaf axil and by spreading in different directions give some heft to the clusters.
The flowers are either bisexual or pistillate. They are small with 5 white to greenish to pink sepals, (no petals) where the 3 outer sepals are winged and larger than the inner two sepals. There are 8 stamens with smooth filaments, white anthers and a single style with a 3-parted stigma. The base (perianth) of the flower is funnel-like and the sepals flare outward at the tips. The perianth is elongated when the flower is in fruit.
Seed: Fertile flowers produce a dry dark brown, smooth and shiny 3-angled achene.
Invasive: See notes at bottom of page.
Habitat: Japanese Knotweed grows from a rhizomatous root system with a deep central taproot. The spreading stolons are white, fine and thread-like, capable of spreading 20 to 60+ feet. It reproduces vegetatively quite quickly, allowing the plant to make large clonal colonies which is what makes it so invasive. One plant in England was found to cover 245 acres. The rhizomes are dark brown and knotty, up to 3 inches in diameter. It is found in waste places, disturbed sites, grasslands and woods. Full to partial sun, moist to moderate moisture. The plant dies back each winter in the northern climates.
Names: The common name is taken from the knotty rhizomes of the roots. In earlier times Japanese Knotweed was classified as Polygonum cuspidatum. The recent reconfiguration of the Polygonaceae family has resulted in this species being re-assigned to the Fallopia genus. The Minnesota authorities (The U of M and the DNR) have accepted the new genus designation as has Flora of North America. Other large organizations, such as USDA, have yet to change as of this date. The genus name, Fallopia, is an honorary for Gabriele Fallopi (1523-1562), Italian anatomist and professor of anatomy at Pisa and Padua after whom the Fallopian Tube was named. The species japonica simply means 'of Japan' as the plant is of far-eastern origin. As to the author names for the plant classification, the plant was originally described by ‘Houtt.’ which refers to Martinus Houttuyn (1720-1798), Dutch naturalist who published many books on natural history. His work was amended and updated by ‘Ronse Decraene’ which refers to Louis P. Ronse De Craene, botanist, author of over 80 papers on floral morphology, evolution of flowers and the use of floral characters in plant phylogeny. Currently (2013) Director of the MSc Course in the Biodiversity and Taxonomy of Plants, Royal Botanic Garden, Edinburgh. Other scientific names that have been used for this plant are Reynoutria japonica and Pleuropterus cuspidatus.
Comparisons: The plant is not difficult to recognize with its bamboo like stem, large leaves and whitish flower clusters held above the leaves. Another species, F. sachalinensis is similar but the leaves have a heart-shaped base, and that species is not found in Minnesota.
Above: 1st & 2nd photos - Japanese Knotweed grows to 10+ feet high on stems that are hollow, smooth, round, and resemble bamboo, green initially, becoming reddish-brown with age. 3rd photo - The large 6 inch long leaves have smooth, but wavy margins with a smooth upper surface.
Below: 1st photo - The flower panicle branches with each branch growing in a different direction. 2nd photo - The underside of the leaf is much paler in color with some hair along the main veins which have a conspicuous pattern under and above.
Below: Flowers do not have petals, but instead have just five colored sepals with the 3 outer sepals winged and larger than the inner two sepals. There are 8 stamens with smooth filaments, white anthers and a single style with a 3-parted stigma. Flowers are either bisexual or just pistillate
Below: Drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.
Notes: Japanese Knotweed has been present in and near the vicinity of Eloise Butler Wildflower Garden within Wirth Park. It first appeared in the Garden in 1952 when Martha Crone planted some plants she obtained via Hedby Jones from Highways 101 and 7 in what is now Minnetonka MN. It was listed on the 1986 and 2009 Garden census. Efforts by the Friends Invasive Plant Action Group have controlled it in the Garden and kept control of it in the buffer zone around the Garden. Five species of Fallopia are known in Minnesota per the DNR census. This species plus Fallopia x bohemica, Bohemian Knotweed; F. scandens, False Buckwheat; F. cilinodis, Fringed Black Bindweed; and F. convolvulus, Black Bindweed.
Invasive notes: Japanese Knotweed was introduced to North America in the late 19th century. It had been in Europe since the 1840s causing similar problems there and today it is one of only two plants in the United Kingdom that are restricted to plant anywhere. The plant overwhelms other vegetation, destroying plants that help with erosion control, cracks pavements with it strong stems, etc. In the winter when the plant dies back, the thick stems are unsightly. This article (pdf) from the University of Georgia may be of interest for further reading. It should be noted that in the plants native habitat in Japan it is not aggressive and the characteristics that make it so aggressive in the western hemisphere barely keep it in competition with other flora in its native habitat.
Control: The MN DNR recommends the following to remove the plant: For small groups, dig them out when in a sensitive area - this will have to repeated until you get all the rhizomes. Otherwise, cut the stems and treat the stems with glyphosate or triclopyr. Large populations - use a leaf spray. Biological control has not been tested.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"