Japanese knotweedJapanese knotweed

Name: Japanese knotweed

Latin name: Fallopia japonica (Houtt), (Reynoutria japonica, Polygonum cuspidatum)

Occurrence: Japanese knotweed is an invasive rhizomatous perennial introduced to the UK from Japan between 1825 and 1841 as an ornamental plant and as cattle fodder. In its native habitat, Japanese knotweed is a pioneer species found on volcanic larva, river gravels and managed pastures. In the UK it was reported as a garden escape in the late 19th century and naturalised populations were recorded in the early 20th century. Initially it was most prevalent in South Wales, perhaps due to the moist climate, but it is now widespread throughout the UK to the detriment of the natural plant communities. Japanese knotweed is established alongside railways, canals, rivers, streams and roadsides.

It forms tall thickets with a dense leaf canopy that exclude other plants even bracken. In the autumn the fallen leaves decompose slowly forming an impenetrable mulch that prevents anything else germinating. Japanese knotweed is tolerant of soil acidity, heavy metal contamination and air pollution. In spring gales, the leaves can suffer severe wind damage as they unfurl. The foliage is also sensitive to late spring and early autumn frost. Evidence from the east of England and the Mediterranean region suggest that summer drought restricts the spread of the weed in warmer climates.

In the UK, both F. japonica var. japonica and the dwarf variant var. compacta are naturalized along with several hybrids (Beerling et al., 1994). All of the plants of var. japonica found in the UK are functionally female and originate from the same clone. The closely related giant knotweed (F. sachalinensis) occurs in similar places to the Japanese knotweed and has the same growth habit but has heart-shaped leaves and is even more robust. A hybrid, (F. x bohemica), between the Japanese and the giant knotweed is known to occur.

In addition to being used for feeding to stock, young shoots of Japanese knotweed have been used for human consumption. The dried rhizomes are used in Chinese and Japanese medicines for treating a range of ailments. In September, the flowers are a valuable nectar source for beneficial insects and honey bees. There have been suggestions that Japanese knotweed could be grown as a renewable energy source if planted on derelict land or land of low agricultural value. However, in the UK, it is now an offence to cause Japanese knotweed to become established in the wild.


Biology: Flowering occurs from August to October but Japanese knotweed rarely sets seed. Some fruits were set in the hot summer of 1996 but it is not known if the seeds were viable. Almost all seed that is set is hybrid often with the Russian vine, F. baldschuanica, or the closely related giant knotweed as the pollen source.

The reddish shoots emerge in spring and grow rapidly up to 3 m tall. The roots can extend to a depth of 2 m. In the autumn, when the shoots are killed by frost, food reserves are translocated down to the stout rhizomes. These form a deep mat and can be more than 1 m deep and 15-20 m long. The rhizomes form pinkish nodules in early spring from which shoots develop in April. The previous stems may still be present as the new shoots emerge and can persist for 12 months or more.

Seedlings are susceptible to frost and are unlikely to survive in the open. There are no reports of any seedlings becoming established naturally in the UK.


Japanese knotweedPersistence and Spread: Established plants develop persistent woody stocks that increase in mass with age and continue to produce lateral creeping rhizomes. A significant proportion of the biomass of Japanese knotweed is below ground.

In the UK, the reproduction of Japanese knotweed is purely vegetative. Spread is by the encroaching growth of established clumps or the dispersal and regeneration of small pieces of plant material, especially rhizomes, but also of stem and crown material. Small fragments of stem of the size produced by a horticultural shredder will regrow and form new plants. The regenerating shoots from buried plant material have emerged through tarmac and concrete. The dumping of waste plant material and the cartage of soil containing plant fragments has been responsible for much of the spread of Japanese knotweed. It is common to see this weed on roadside verges where it has established from fly-tipped garden waste. Where Japanese knotweed is growing near water, plant fragments can be carried downstream for long distances before developing into new plants. The fragments can even survive a period in seawater.


Management: Plants should be removed as soon as they are seen, once established, eradication is much more difficult. Control by cutting alone is ineffective and may increase stem density and the lateral spread of clumps. Regrowth is very rapid. Pulling or digging out the weed has some effect if repeated regularly but all waste plant material must be burnt. Japanese knotweed can survive composting. Burning the plant in situ is ineffective. Sheep, goats, cattle and horses will graze the young shoots from February to July and keep the weed in check. Mature stems are not palatable and should be cut down to stimulate fresh young shoots. Root barrier fabrics made from reinforced polyethylene laminate have been successfully used to contain the spread of Japanese knotweed. Some on-going research is looking for biological control agents in the countries of origin.

Topsoil and other brought in soil should be checked for fragments of Japanese knotweed. If there is any doubt, the origin of the soil should be checked. Strict hygiene should be followed in dealing with living plant material of Japanese knotweed. All fragments should be destroyed by burning or by deep burial to at least 10 m deep. When control is limited to one problem area, re-infestation is likely from adjacent areas. It is necessary to deal with plants in the adjacent areas too and prevent the spread of plant fragments, especially near water or where loose soil is likely to become moved around.

Japanese knotweed would appear to be an appropriate candidate for biological control. However, few native insects or plant diseases are known to attack the weed in Britain. An extensive programme of research would be needed to evaluate and develop biological control measures introduced from elsewhere. A rust fungus Puccinia spp. from Japan, has shown some promise as a control agent. More recently, a pathogenic leafspot, Mycoshaerella spp. and a sap-sucking plant louse, Aphalara itadori have been found living on the weed in Japan. Both have given encouraging results and are under assessment as biocontrol agents for Japanese knotweed.

Updated November 2007

Further Information / Links:

Further information can be obtained from the weblinks below:
»UK farmers' case studies

« Back to Perennial Broad-leaved Weeds

Comments

  1. I have found a single stem of what I fear may be japanese knotweed in the garden of the house into which I have just moved. The stem is red, approcimately 20 inches high, and is growing off a small woody "trunk" as thick as my figer. The stem has side shoots.The leaves are less flat at the base than in your picture, and are yellowish green with tinges of red, and serrated. It is growing in heavy acid clay and is being eaten to a skeleton on the newest leaves by some pest I cannot find. Is this a knotweed hybrid?
    - melanie packwood 7---0-2004

  2. It doesn't sound quite right for Japanese knotweed which has red blotches on the stem and I would expect to be much taller. If you are still concerned that it may be I suggest you dig it out and pot it up, then if it turns out to be something more desirable you can replant it if you wish. At least this way if it doesn't regrow in the area you dug it from it you will know it is unlikely to be the knotweed.
    - Bill Bond 7---0-2004

  3. I was told that the weed can penetrate foundations of buildings if there is any routes left in the soil is this true
    Can you recommend the best way to remove the weed from a building site?
    Is it poisons?
    There are larg areas of this weed all over this site around one acre
    - John Connelly 1---1-2005

  4. Hi John

    I am about to but a groung floor apartment which leads onto a front garden where there seems to an infestation of Knotweed. Did you resolve your problem and if so, whoi did you employ?
    - Warren Mande 5---0-2006

  5. I have recently moved house only to discover i have japanese knotweed in the back garden, also growing in the neighbouring garden. At first, we attempted digging out as deep as 3ft and across, pulling out large rhizomes. A few weeks later new growth appeared since we have applied roundup, and several applications of root out, it dies back but still new shoots appear.
    - will 8---0-2006

  6. I have recently moved house only to discover i have japanese knotweed in the back garden, also growing in the neighbouring garden. At first, we attempted digging out as deep as 3ft and across, pulling out large rhizomes. A few weeks later new growth appeared since we have applied roundup, and several applications of root out, it dies back but still new shoots appear.
    - will 8---0-2006

  7. Could someone please tell me - does knotweed start of with just leaves then flower and then later produce small black berries. We have this huge tree like plant in our garden which has been there for the last year but has grown to at least 8 ft tall. Ive no idea where it came from.
    - Christine 22-08-2006 8---0-2006

  8. I keep finding little seedlig looking plants in my garden which i dig out. They seem to grow very quickly curling around the nearest thing it can get it's hands on....i had a look at the waste ground next door and there was a big patch of this so i zapped it with glyphosphate...but it keeps appearing...any ideas on how to get rid?
    - Ann 8---0-2006

  9. If you have a small patch I think persistant and meticulous digging is the only feasible way of erradicating this weed. On larger patches or wasteground you might need help from the council or a contractor. I am not sure where you are but we provide links to the Cornwall Knotweed forum above which gives comprehensive advice about controlling this weed.
    - Gareth Davies 9---0-2006

  10. Hi Does Japenese knotweed have a bamboo looking stem?
    I just moved into a new home with this 3 meter fence of bamboo with leaves. If so, how do I get rid of?

    - carol baldwin 0---1-2006

  11. Carol- it doesn't sound like Japanese Knotweed which normally has a reddish looking stem. Follow some of the links above for identification pictures.
    - Gareth Davies 0---1-2006

  12. Does this sound like Japanese Knotweed? About 2, maybe 3 years ago, 2 plants emerged in our front border and grew prolifically. We cut them down when in full leaf and suffered itchy rashes on our arms where the leaves had touched. The plants had woody stems and medium sized almond shaped leaves but no flowers and the stems were not reddish. I put systemic weedkiller on them for woody plants and they both died. Then the following year one grew back! So I cut it back and stuck more weedkiller on it. Still it grew...Now we have removed the raised bed where it was and have discovered thick roots (one as thick as my wrist, which are connected to 3 plants! Help! We are complete ignoramus's about gardening and don't know what to do.
    - Kate Kinson 5---0-2007

  13. I live in a fairly rural area and the common ground behind my house has prolific japanese knotweed which unfortunately has decided to descend into my garden. Our local council has plans to erradicate it on the common ground but will not be doing anything on domestic properties. However I did receive a visit from someone from the council who has advised that the only way to realistically get rid of the knot weed is to spray it, again and again and again with roundup. The advise was to spray but never cut when it is alive and kicking. Cut it back when it appears dead, i.e a hard twig you can snap off but be prepared to keep spraying for years to come. I still have it in my garden but even only after 2 months of spraying the amount growing has reduced dramatically and I no longer need a machete to get into my garden. The roots are very long which is the main problem so just keep spraying folks and hopefully it will die before you do
    - lindsay 7---0-2007

  14. how do i get rid of it please help me !!!!!!!!
    - mandy 7---0-2007

  15. I have Japanese Knotweed in my garden. I have just moved here can you please let me know the effective way of eradicating it.

    - Louise Wilcox 9---0-2007

  16. Lindsay, if the ground the knotweed was originally growing in is managed by the council, then I think they may have committed an offence under the Countryside and Wildlife Act by allowing it to spread onto your property! The most effective time to treat by spraying with Round-up (glyphosate) is just before the plant enters senescence (die-back) in late autumn. The chemical will be drawn into the crown and rhizome (underground stem) of the plant.
    - Richard 9---0-2007

  17. This weed is now spreading from some spare council ground into my very well kept garden . I would like to terminate it . I would also like to know if i can force the council to act as it's coming from there property. Where do I stand with this?
    - Eddie Kelly 9---0-2007

  18. My company has worked closely with a firm called Cityscape Design and Engineering who specialise in the removal of Japanese Knotweed. They are the sole importers of stem injection systems and work with a number of councils on this issue. You can find out some more info on them at http://www.cdae.co.uk - i hope this helps.
    - Joseph Sless 9---0-2007

  19. knotweed is an alien invader in our country

    - chang 0---1-2007

  20. japenese knotweed is an invasive species in alaska, juneau.
    - a student from jdhs 0---1-2007

  21. i have japanese knotweed growing in the middle of my 2 acre horse paddock. IT grew to about 3 ft tall and covered an area of about 4square metres. It is the most annoying, fastest growing, resilient, toughest weed I have ever tried to get rid off. The stems are the size of small carrots that seem to ascend from at least 10 metres below the surface. It reappears overnight and I am frightened of going on holiday! I have executed with military precision glyphosate every two weeks since start of august. Yes it is dying and i am checking the field every day for new shoots and blast them too but i also break of the leaves and squirt the herbicide down inside the stem! hopefully my perseverence will pay off in the end. When all looks dead and gone I will reseed the area in Mid september and hopefully will see some grass growing for a change.
    - maggie 0---1-2007

  22. how many treatments per year does knotweed require ?
    - sean 3---0-2008

  23. I have japanese knotweed in my horses field. Is there a safe chemical I can spray? i i do use round-up how long will my horses have to stay inside before it's safe for them to come out. i wouldn't want them to eat anything with round up.
    - jennifer 4---0-2008

  24. my family had japanese knotweed in our yard in cleveland, ohio, usa. my father faught with it year in and out on a patch of land behind our garage used intermittently as a veggie garden, loose brick/stone storage, etc. the plant was so strong that it would even push up through the asphalt. however, we noticed that it did not invade the rest of our yard that was covered in grass.

    one summer, when new neighbors had moved in behind us (and their back garden along our property line had it as well) he convinced them to join him in laying down fresh sod along the whole of our property line that year. they cut down the weed, prepped and turned the earth, removing the plants they could see, and put down fresh sod (my dad used bluegrass, not sure what the neighbor's used). as far as i know, they used no chemical herbicides. any new shoots on the periphery were pulled out. the knotweed has since not returned.

    they decided on using sod instead of planting seed because the knotweed was such a fast growing plant. they wanted the mat that grass eventually forms to suffocate the knotweed immediately instead of waiting for the grass to grow thick & possibly lose out to the knotweed.

    hope that story helps some people - i know it's not a solution for all. (p.s., i've also read online that young shoots can be eaten & taste similar to rhubarb. eat your enemy! i have not tried that, though.)
    - julia 5---0-2008

  25. I've read a lot of internet information in a short time on JKW (due to a sudden infestation from a neighbouring property, which had apparnetly been dormant until heavy plant disturbance by another neighbour earlier this year) but nowhere can I find an answer to the quetion of what does the underground rhizome system mean for my shrubs while stems are allowed to grow big enough to be cut and treated individually after flowering; and what does the underground stem system do to neighbouring plants after above-ground growth is apparently relatively controlled by herbicide treatment?
    The underground system must deprive other plants of nutrients, but does it also choke them mechanically? what is the mechanism for the JKW to eradicate existing plants, does anyone know?
    Our infestation is a spinoff of a this huge apparently previously dormant one, so pulling up and applying herbicide in our small part of it will just encourage spread, as pruning does. But leaving this to romp away centimetres away from established plants I am fond of and that have sentimental connections is difficult!
    Incidentally, the information on the Environemtn Agency (development site approaches) and National Trust (conservation approach, as in gardens) is well worth reading and would leave noone in any doubt about many of the question earlier in this chain.
    - kate 7---0-2008

  26. We are totally overcome with jk in a plot of land that we found out belonged to us after buying our property. Last year we cleared the area and laid some tarpolin after spraying with roundup glysophate, this year it has come back just as thick if not worse, we tackled the council on this as the house was bought of the council by the previous owner, she only purchesed the house two years before we bought it but the council are saying that its now our responsibility to clear it as it now belongs to us, the lady we bought the house off claims she didnt know what was growing in their but as she had lived their for thirty years we couldnt understand why she had never spoke to the council about the ground, can anyone offer us any advice, it is also spreading into the pathway along the side of the house which separates the land. Also could julia explain what sod is as this sounds like a good solution because funny enough, fingers crossed it hasnt invaded our good garden which contains a lawn.
    - Cara 7---0-2008

  27. check out the Environment Agency web site as they have produced extensive guidance on controlling Knotweed - not very organic which is why I was looking at this site
    - Henry C 7---0-2008

  28. when we built our house 15 years ago there was jk established on the plot of ground. Building work and perseverance finally erradicated the deadly weed. However our neighbours also have jk and new growth was getting perilously close to our boundary. Not wanting it back on our patch I spoke to them telling them the facts and suggested they sought help with getting rid of it. They have done most of what I have read other sufferers have done including using a reliable source to dispose of cuttings. To no avail!! Their house is now up for sale and they are desperately trying once again to erradicate jk. My concern is that all the roots stems and foliage have been bagged and taken to the local tip where I suspect it has been dumped without the knowledge of the tip, which I now know to be an offence. These are well educated people who I think are well aware of what should be done--- has jk driven them to use these anti-social habits?
    - rosie 22-08-08 9---0-2008

  29. Try UNEX (Natriumchlorat), Inject Roundup and the knotweed will gone after one or two sesons
    - Jens W 9---0-2008

  30. If anyone needs help and advice on dealing with Japanese Knotweed feel free to contact me at: www.ecocontrol.co.uk.
    - Chris Johnston 2---0-2009

  31. Why didn't you eat your knotweed, being touted as equal to resverstrol or the grape skin wonder for everybody. Its supposed to taste pretty good.
    - Terri Nicely 2---0-2009

  32. If anyone needs help and advice on dealing with Japanese Knotweed feel free to contact me at: www.ecocontrol.co.uk.
    - Chris Johnston 2---0-2009

  33. Cara,
    When Julia refers to 'sod', I believe that this is what Americans (rather amusingly for us Brits) call turf. http://en.wikipedia.org/wiki/Sod. I'm amazed that turf would prevent JKW recurrence though.
    - Andrew 1---1-2009

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