Known as Itadori in its native Japan, it is a herbaceous perennial of the family Polygonacae - the knotweeds ("Poly " - many, "gony " - knee or joint)found on waste ground, riverbanks and on the sides of roads and railways. It is dioecious which means there are male and female flowers on separate plants. In fact the majority if not all of the plants outside their native Japan are female. So all must have been reproduced vegetatively either intentionally, when it was introduced originally in the mid-19th century as an ornamental, or accidently when it was dumped in transported soil or washed away in storms.
As a plant it is quite attractive with stems which are red-brown at the base and mottled green toward the tip, with bright green, heart-shaped, hairless leaves arranged alternately on the stem which zig-zags where they are present. When introduced as a garden plant, it was granted awards for being an excellent specimen, but soon lost favour when its thuggish behaviour was realised. The stems become woody as they mature, growing at a rate of 100 to 120mm per day and reaching up to 3 metres in height. There are nodes at intervals up their length, leading to comparisons with bamboo.
Large stands are very dense excluding all other plants, so it is a threat to native plants. The extensive underground rhizome system can be to a depth of 3m, giving it great resistance to eradication. It is capable of spreading to an area the size of a tennis court in one year and the underground parts may be extending up to 7 metres beyond edges of the topgrowth - so any soil within this area could contain rhizomes and should not be spread elsewhere or taken off site. Estimates put the cost of removal at about £9 per square metre, including herbicides and landscaping, this can cause a decrease in the value of infested land. It is estimated that about £1.6 billion is spent annually in the UK on removing it. The site of the 2012 Olympics in London had quite a large infestation which had to be cleared along with many other contamination problems before construction began - at an estimated cost of £70 million.
There are some redeeming qualities. The young, Spring shoots can be cooked and eaten, after peeling to reveal the white core. They have a mild rhubarb flavour, due to the presence of oxalic acid (hence some of the common names). Medicinally, is is a commercial source of the antioxidant Resveratrol which has been shown to extend the lifespan of some non-mammalian experimental animals and is sold as a supplement. The roots contain emodin which
has a mild laxative effect and it is part of traditional Chinese and Japanese herbal medicines for this effect. Other uses are as a diruetic, and it can be applied externally to sooth burns and skin lesions.
Some progress has been made in isolating some of the natural controlling pests and diseases in its native environment where it is usual to find lots of damage to the foliage, stems and roots inflicted by invertebrates. There are over 180 species of plant-eating invertebrates which feed on it and about 40 fungi. Of these, two insects have been isolated which have so far shown no interest in other plants which grow here, a weevil which damages the stems and a 2mm psyllid or plant louse (Aphalara itadori), which sucks the sap, thus weakening the plant. A fungus (Mycosphaerella sp.) which causes leafspot is another possiblity. After thorough testing to ensure that they will attack only the Knotweed, they may be released here as a biological control. This will not kill it outright, but the damaged plants will not grow so big and spread so fast - native flora may have a better chance to compete and keep it in check or overwhelm it completely.
Removal takes much persistence, but if it is an isolated patch and not coming from adjoining property, then constant pulling or cutting of the stems should work by exhausting the rhizomes, but this may take several years. Any cut material should be gathered up and placed on plastic sheeting to dry out until all signs of life have gone. Digging out the rhizome is not recommended on a large scale as it produces too many fragments; also the depth and toughness of the root would make it very difficult. After the initial knockdown with herbicide the dead topgrowth can be cleared and left to decay or burnt, then the crowns can be broken up to further weaken any regrowth. This is a tough job and can be done with a digger on a larger site. Any equipment used must be carefully cleaned so that nothing is carried off site afterwards.
It is resistant to most weedkillers, but Glyphosate or 2,4-D should work after repeated treatment over a 3 year period. Foliar spraying is the best method, but for large areas and where important plants are growing nearby, injecting Glyphosate into the hollow stems after cutting is a method being employed in conservation areas. The best time to inject is late summer when growth has finished and the plant is returning sap to the rhizomes for storage. The stems are cut just below the first node (at 8 to 10cm) and about 10mls of glyphosate made up 10 times stronger than if it were to be used for spraying, is injected into the remaining growth. Any regrowth is sprayed with Glyphosate as it tends to be distorted and unsuitable for injecting. There are injector systems designed to treat the stems without cutting so this would reduce the work and the risk of regrowth from the cuttings.
The latest weapon in this category is Picloram, sold under the trade name Tordon22k for professional use. It is a selective weedkiller which is best applied to fresh growth as it mimics the natural growth hormones of the plant, so it is best if applied early in the season as the stems are developing. It also has some residual action, so remains to affect emerging shoots. Dense stands later in the season can be cut down and the new shoots which emerge are sprayed; any cut material must be disposed of under license or treated on site by drying out completely before composting or burning.As a novelty I had a small plant growing in a container, but it has just been killed over the winter by an attack of Vine Weevils. Which is worst?
The National Trust have carried out a number of trials to find the best way to tackle the problem.
Under the 1981 Wildlife and Countryside Act, it is an offence in the mainland to allow it to grow in the wild - in Northern Ireland the offending property owner would have to be taken to the civil court. Waste material when digging should be disposed of according to the Environmental Protection Act 1990 (Duty of Care) Regulations, and a license is required. It should not be placed in amenity skips.
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