Japanese knotweed history and biology

History:

So where did Japanese knotweed come from? How did it get here? Why is it a problem? These are questions that many people have been asking for a long time.

Fallopia japonica (Japanese knotweed) was found in Japan by Phillipe von Siebold and brought back to Europe around 1829. The original plant was propagated in Holland and marketed around Europe by Von Siebold & Company of Leiden. It was sold (in common with many other new plant introductions from around the world) principally as an ornamental plant. We must be grateful for one thing at this point, and that is that Phillipe von Siebold only brought back one female Japanese knotweed plant. He did not bring back a matching male plant. Hence all Japanese knotweed in Europe is a female clone of the original plant.

Biology:

Japanese knotweed is a herbaceous perennial, this means that the plant dies down to ground level each winter and lasts for an indefinite number of years. The main way in which it spreads is by human activity. Essentially any part of the plant can, if broken off, grow into a new plant. The most usual way is for rhizomes (the underground root system, used to store food) to be broken off and spread onto new areas. The rhizomes are easily identified; normally they are brown, knobbly and hairy. They snap easily (like a carrot) and are orange coloured within.

Knotweed typically grows in dense colonies (Stands), and is all interlinked through the rhizome system. We have seen and treated Stands that range in size from small areas of 1 square metre, to very large Stands, perhaps covering 1 hectare or more.

In the Spring Japanese knotweed grows extremely vigorously; it uses stored energy in the rhizome system to grow at rates of 300mm per week – perhaps more in some cases. This energy can also be used to break through any solid barrier that may be in the way, such as roads, pavements, patios and even buildings. This is why Japanese knotweed is such a problem to the construction industry and also to the home owner.

The plant (in normal conditions) will reach a height of between 2 and 4m, its underground spread is even larger, rhizomes may be situated up to 3m deep and be situated 7m from the nearest visible growth. Although the plant may appear large above ground, below the surface it is bigger yet.

The sexual reproduction of Japanese knotweed in the British Isles is interesting. As it does not have a male Japanese knotweed partner it cannot set viable seeds fertilised by Japanese knotweed, but it can and does, cross breed (hybridise) with other related species, such as Giant knotweed (Fallopia sachaliensis), Russian Vine (Fallopia baldschuanica), Fallopia japonica compacta (a dwarf Japanese knotweed), Himalayan knotweed (Persicaria wallichii) and Persicaria campanulata. Although seeds may be set, they are not all viable in the wild; the main type of hybrid found is Fallopia x bohemica a hybrid of Japanese and Giant knotweed.

Japanese knotweed crown growing in a garden wall. As you can see the wall has been severely damaged by this growth.