| Picture © 2002, |
| Known Knotweed locations |
Japanese Knotweed - Reynoutria japonica
(Donkey or Gypsy rhubarb, Mexican bamboo, Japanese bamboo, Japanese Fleece Flower.)
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Perennial of the family Polygonacae ("Poly " - many, "gony " - knee or joint), commonly called 'itadori' in Japan, a name meaning 'strong plant'. Invasive fast growing perennial reaching 2m (6ft) on thick bamboo like stems, commonly seen near roadsides and ditches. Large triangular leaves with curved and pointed edges on red - green stems with a zig-zag pattern, greenish white flowers in slender clusters appearing from the leaf bases during August to October. New shoots appear March to April growing rapidly to full height, at roadsides, ditches and streams where broken parts of the root system can be transported to new sites, establishing a new plant system extremely easily. It will regenerate from pieces as small as 2cm (0.8in) in soil or in water, it's growth is very aggressive, with an extensive and deep underground rhizome system giving it great resistance to eradication. Bamboo like stand-forming habit produces a dense canopy beneath which few other plant species can survive and an accumulation of growth debris within established growths reduces other species diversity. Knotweed reproduces primarily through it's root system, which may reach 18 meters (60 feet) in length, and quite capable of growing through tarmac, concrete and paving, doing extensive damage as it spreads.
A dioecious plant so there are male ♂ and female ♀ flowers on separate plants, the majority of the plants outside their native habitat in Japan are female and are of a single clone, presumably from when it was introduced originally in the mid-19th century as an ornamental garden plant. This weed is one of a number listed under Schedule 9 of the 1981 Wildlife and Countryside Act, it being an offence in the UK to plant it or otherwise cause it to grow. Waste material when digging should be disposed of according to the Environmental Protection Act 1990 (Duty of Care) Regulations. The Environment Agency has published a free guidance booklet, 'The Control of Invasive Plants near Watercourses'. Copies are available from Environment Agency offices.
Knotweed can be resistant to most weedkillers but can controlled either by cutting and burning and then digging up the root structure or by spraying with Glyphosate (the active ingredient in Roundup) or 2,4-D. This should be done on established growth and then at repeated intervals with late season sprayings being most effective when the plant "shuts down" for the winter peeriod. Use of such chemicals requires careful use and knowledge of herbicides and their effects especially along water courses. It may be more effective when applied to the regrowth of cut stems, but may need repeated treatment over a 3 year period. Injecting Glyphosate into the hollow stems after cutting is another method of treatment which is said to work. Glyphosate is non-selective and extreme care should be taken in its use.
It can be effectively eradicated by repeated cutting (which exhausts the rhizomes), after clearing and burning, if possible further digging up or rotavation and then collecting as much of the root system as possible which should again be be burnt. Burning should be done on site where possible so the plant is not being transported thereby avoiding possible repercussions under the 1981 Wildlife and Countryside Act. Cut bi-weekly (or sooner if growth rates are high) in the first year until growth stops in the autumn and then monthly in year two and as required in year three. This regime requires little effort after the first clearance as each cut is a quick run over with a strimmer, mower or bill hook. Digging as an alternative to chemical control is difficult as the plant has an extensive root structure and will root from the smallest portion left behind, but may be the only alternative where the plant is growing next to streams and watercourses. Brickfields Country Park have successfully eradicated Japanese Knotweed using this regime of digging and then repeated cutting. 
Recent observations of a local stand subjected to weekly mowing suggests such a cutting regime will kill the plant in 2 years.
Japanese knotweed is listed as a wild edible plant, and young shoots up to 30cm (1 ft), can be cut early in the spring, steamed or boiled 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. FBCP do not advise or recommend that Knotweed is eaten. Historically classified as Reynoutria japonica by Houttuyn in 1777, and Polygonum cuspidatum by Siebold in 1846, recent suggestions are that it should be reclassified as Fallopia japonica.
Other web pages have have further information on this plant and links to more information.
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