Japanese Knotweed is notorious for many things but perhaps its greatest claim to fame is its skill at colonizing new ground. Japanese Knotweed is native to Japan, Korea and China and was introduced to Britain in the 1850’s. It had an exotic appearance and grew easily making it an ideal ornamental species. Its rapidity of growth and solid wall of green vegetation made it an ideal candidate for screening purposes and its huge root system made it beneficial in loose soil situations where it was used to stabilize embankments on both rivers and railways.
Its decline in reputation was not sensationalized but was more of a gradual sink into notoriety. Its rapid spread and ability to withstand eradication attempts made it the centre of dire warnings issued by the Royal Horticultural Society in 1905 when they stated that it should only be planted in … ‘the most carefully managed situations’.
The first recorded ‘escape’ from cultivation was in 1886 (Storrie 1886) yet by 1996 it had been recorded in 1584 of the 2862 x 10km squares of the Biological Records Centre of the British Isles. It is regarded as … ‘the most pernicious alien weed in the British Isles’ and since 1981 under the Wildlife and Countryside Act it has been a criminal offence to knowingly introduce Japanese Knotweed into the wild.
What is most amazing about the rapid and aggressive spread of Japanese Knotweed is that it appears to have been done purely by vegetative propagation. Japanese Knotweed is gynodiocious – occurring as hermaphrodite and male sterile plants. Although both types occur in its natural range, in Britain only male sterile plants (female) are known (Bailey 1994)- this implies that the British populations of Japanese Knotweed have no capacity for perpetuation by seed which means the genetic base of these populations must be severely limited.
There are still questions unanswered about the genetic diversity of Japanese Knotweed in Britain not least of which is whether the plant is multi-clonal?
There are many (clones) of Japanese Knotweed Solutions Ltd… who are now trading in the business of eradicating this troublesome plant…. but how many of them bother to even identify which type of Japanese Knotweed it is that they are attempting to eradicate?
Studies are being carried out by the likes of Professor John Bailey at the Unversity of Leicester – (though John has now retired) – looking at the genetic variability between introduced populations and native populations. Preliminary results have shown a large amount of diversity within native populations – with none precisely matching the introduced plants in Europe and the USA.
Whilst this may all seem a bit overkill – and many would say let’s just eradicate anything that vaguely resembles Japanese knotweed it is vital that we understand the processes that are going on within these invading populations.
The future success and longevity of the Japanese Knotweed ‘invasion’ are difficult to predict. Whilst Japanese Knotweed may lack inter-individual variability it is polyploidal which gives it an evolutionary reprieve from the weaknesses inherent with asexual propagation. Note polyploidy is common to all 18 of the ‘worlds worst weeds’ (Holm 1977).
It should also be noted that the invasive capabilities of Japanese knotweed may increase and the current problems be exacerbated via hybridization. Hybrids between Japanese Knotweed and the related introduced species Giant Knotweed (Fallopia sachalinensis) are now common throughout the British Isles and share the invasive attributes of the former. These hybrids (Fallopia x bohemica) show some fertility – with backcrosses with Japanese Knotweed having been recently documented (Hollingsworth 1998).
Hybridization therefore seems to offer a potential escape from the lack of sexual variation caused by the absence of fertile males in Britain. The introduction of these sexually derived genotypes may thus serve to enhance and ensure the long term survival of this weed invasion.