A herbaceous perennial of the family Polygonacae, the knotweeds ("Poly " - many, "gony " - knee or joint). 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 on storms. Long rhizomes up to 20m in length have a reddish hue when broken and are the main method by which it spreads. If chopped up, it can regenerate from as little as a 2 cm piece of rhizome in soil or in water. Small pieces of fresh stem are also able to take root so flailing and chipping can produce lots of new plants.
It prefers moist but well-drained, nutrient-rich soil, and tolerates semi-shade - but its ability to grow in a wide range of conditions, acid or alkaline and even with heavy metals present, adds to the problem. The native environment of the plant is on the slopes of volcanoes and there are many species of insects and fungi which attack it there; away from these it is unhindered and the better conditions in our gardens must be paradise.

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 leaves. 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. Flowering occurs between July and September; small, greenish white flowers clustered along branching panicles (similar to Russian Vine flowers), arising from the upper leaf axils.
Recently a 'superweed' hybrid, Fallopia x bohemica, has become a possibility as it has been found that the female plants growing here can be pollinated by Sakhalin or Giant Knotweeed (Polygonum sachalinensis ). If the hybrid was to pollinate the female Japanese Knotweed plants the resultant plant could be capable of seeding as well as sending out long rhizomes, but fortunately P sachalinense is not widely grown here.
There are some redeeming qualities. The young, Spring shoots can be cooked and eaten, they have a mild rhubarb flavour, due to the presence of oxalic acid (hence some of the common names). Medicinally, as with rhubarb there is a mild laxative effect, also it is a diruetic and it can be used externally to sooth burns and other wounds. The roots will produce a yellow dye and there is a possibility that it could be used as a source of biomass for conversion to fuel, which would be carbon neutral.

It is very aggressive damaging paving and tarmac as it spreads - on one occasion it found its way into a livingroom, coming up through the floorboards. Large areas are very dense excluding all other 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. 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. The site of the 2012 Olympics in London has quite a large infestation which will have to be cleared along with many other contamination problems before any new construction can be done.

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 two insects, a weevil which damages the stems and a psyllid which sucks the sap, thus weakening the plant. A fungus which causes leafspot is another possiblity and 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 persistance, 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.
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.
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 UK to allow it to grow in the wild. Waste material when digging should be disposed of according to the Environmental Protection Act 1990 (Duty of Care) Regulations, and a licence is required. It should not be placed in amenity skips.
This site gives information and guidance on disposal of Japanese Knotweed.