Pretty, isn't it? The creamy-white flowers are plentiful in late summer, festooning river banks and roadsides across the US and the UK.
In the mid-1800's, a Bavarian physician in the Dutch East Indies Army took an interest in the local flora whilst stationed in Japan. Phillip von Siebold brought to Europe a variety of specimens, including knotweed. Looking like bamboo, it won an award in the U.K. for Most Exotic New Ornamental and was hailed as an excellent stabilizer of soils. Within fifty years, knotweed was being sold through nursery catalogs to the United States.
Knotweed's lovely sprays of delicate creamy-white flowers are attractive to the eye in late summer, and the nectar fills a seasonal gap in the honey-maker's trade. It is easy to see the attraction that some folks have for this rugged plant! The young leaves, often used in teas and in vitamin supplements, hold an antioxidant called resveratrol. In late April or early May, tender young shoots (bearing a striking resemblance to asparagus) emerge before most other vegetation. They can be harvested and eaten in a variety of ways; check out this website from Steve "Wildman" Brill for recipes: http://www.econetwork.net/~wildmansteve/Plants.Folder/Knotweed.html
Now known for its rapid and persistent growth, knotweed has recently become a nuisance and has potential for serious habitat alteration. The U.K. treats the plant as a hazardous material, excavating infestations some nine feet deep and sixty feet around. Fragments of root material can easily re-sprout a whole new colony, a common event when high water scours river banks, taking chunks of stem and root material downstream. While the main root system is very shallow, the finger-thick rhizomes can extend up to nine feet deep! It is the shallow nature of the roots that can cause a streambank to more easily give way to rushing water, sending sometimes massive quantities of soil downstream to alter the aquatic habitat for all manner of life-forms. While some sediment can be not only natural but also healthy for a stream or river, too much can blanket the bottom, suffocate aquatic insects and fish, slow or alter the course of the stream-flow, raise the temperature of the water (which lowers how much oxygen it can hold, which is bad news for fish and insects), and can even be a breeding-ground for harmful bacteria like Giardia.
As the first plant to burst forth from the soil in early spring, knotweed unfurls large, alternating leaves and crowds out most other vegetation attempting to take root and reach the sunlight. It is this behavior, along with its rapid spreading capabilities and lack of predators that qualify it as an invasive species. After the plant flowers in late summer, the root (or rhizome) draws back the available nitrogen from the leaves to store for next spring. The leaves fall, and the dead canes persist for up to two years without disintegrating. While not strong enough to build anything with, dead knotweed canes can be rigid enough to cut skin or poke through landscaping tarp. They are often washed downstream in high water, causing miniature long-jams and exacerbating the damage already being done by lots of water with nowhere to go.
Knotweed is doing what plants do; it is simply doing it extremely well and has no natural controls here in the U.S. or in the U.K. In Japan, a rust fungus and a small beetle are among knotweed's predators; some of these are being tested rigorously as potential controllers of knotweed here and in the U.S. Travel the country and there will be stands of knotweed in almost every state; there are streams and rivers with literally nothing other than knotweed lining both banks. Rarely does knotweed extend far enough over the water to provide a cooling shade, nor does it add to aquatic habitat. Tree canopy is vital for this. In addition, when a tree falls into a stream, it stays there long enough to create a great little hiding spot for fish as well as food and habitat for other creatures. When knotweed falls into a stream, it decomposes too rapidly to be of value for the river creatures, and tends to float downstream until it finds a new spot to root.
Complete eradication of this plant in the U.S. or the U.K. is probably never going to be a reality. Managing it, controlling its spread and preventing new colonies is our best shot at keeping our backyards from being overrun and our streams and rivers healthy and thriving.