Japanese Knotweed

Polygonum japonicum, syn. P. cuspidatum, Fallopia japonica, Reynoutria japonica
Other names: Donkey rhubarb, Gypsy rhubarb, Japanese bamboo, Mexican bamboo, Sally rhubarb Family: Polygonaceae

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 volcanos 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.

It is actually quite an attractive plant, 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.

It is very aggressive damaging paving and tarmac as it spreads. At Growing concern Scotland we have encountered it pushing its way through floorboards and driveways. 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.

Land that is infested with the weed will have a lower value as the cost of removal would have to be taken into account on purchase. 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.

There are some good qualities to this invasive weed though. 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, it is a commercial source of the antioxidant resveratrol which has been shown to extend the lifespan of some non-mammalian labratory 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.

The roots 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.

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.

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