Japanese Knotweed – May 2016 Plant of the Month (Knot!)

Japanese Knotweed

PLANT of the MONTH (Knot ! ) Japanese Knotweed, Polygonum cuspidate


by Ann Farnham, LLA

It is unlikely that today you will find this plant for sale at your local nursery, but that was indeed the case for many years. Originally introduced from Japan to England around 1825 and then to the United States, it was found in Philadelphia, New York and New Jersey in 1894. This plant was praised for its ornamental qualities, easy care, landscape screening potential, and erosion control. Today Japanese Knotweed thrives in 36 of our 48 States. It is extremely difficult to eradicate and its dense thickets exclude all other plants.

Japanese Knotweed along HighwayThis plant, also known as “Japanese Bamboo”, “Mexican Bamboo” and “Fleece Flower”, can be identified by its scale-covered hollow stems, smooth- edged pointed leaves, 4-6” long and 3-4” wide, arranged oppositely on the stems. It can reach 3 to 10’ high and blooms in August and September with white to creamy-white flower groups (panicles) 3 to 6” long. It prefers full sun and wet sites, but will tolerate dry soil, salt, shade, and high temperatures, especially in disturbed sites, road-sides, ditches and vacant lots. Deciduous and dioecious (the male and female flowers are not on the same plant), it is very susceptible to frost and turns brown, dying back immediately, becoming at the same time a dormant season fire hazard. Being a perennial plant, the roots and rhizomes (underground stems) remain very much alive.

Japanese Knotweed Sprouts

Japanese Knotweed is very difficult to control or to get rid of. If you find it in your garden or elsewhere, do not pull it out without learning more about it. Any small particle of root or stem left behind (and the rhizomes can extend 30’ beyond the plant) in a removal will sprout into a new plant which then forms a new thicket; the dense, thick colony suppresses all other plants.

Cutting, mowing, and digging stimulate new growth. Eradication requires, un- fortunately, the use of an herbicide, glyphosate (Roundup) or triclopyr as a foliar spray when the plant is fully and recently leafed out. Do not spray to the point where the herbicide drips off.  One of these products can also be applied to a stem cut about 2” above the soil surface, but within a half hour of its cutting, as the cut surface will heal over after a half hour. Extreme care must be taken not to use these herbicides if there is a wind, breeze, or rain, and the application must be very precisely applied. Check the recommended solution carefully.

Bag the dead or cut remnants of the plant and dispose of them carefully in the garbage where they will not sprout and start anew. Do not compost or add to a municipal waste site where the brush, leaves, and garden trash will be composted.

Several years ago a team of Ewing Environmental Commission members attempted to rid the Johnson Trolley Trail of Japanese Knotweed; the challenge continues to this day.

The medicinal qualities of Japanese Knotweed were and are today enumerated in detail to cure or lessen bone loss, cancer, tumors, fever, constipation, burns, cardiac problems, infections, and more. It is touted as a health supplement, “antioxidant” and “cardio protectant”. Too good to be true? Buyer beware!

The Ewing Environmental Commission welcomes suggestions for the Plant of the Month from all Ewing residents.

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