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Talkback: Japanese knotweed

I've got my work cut out this year then. One hedgerow is riddled with the stuff. It's going to be a tough fight to get the lot to die. Carefully digging between the roots of the hedge is a big job alone.
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  • We have two large sections 4x4 metres of Japanese Knotweed in our paddock, as we graze this area a friend has advised that we cut the stems to the last leaf shoot and pour salt and oil in each stem then cover with black sheeting - is this good advice?
  • I note from http://www.phlorum.com/identify-japanese-knotweed.html there's a cross between JK and its Giant brother that CAN set seed....? I'm a bit concerned about this:my current problem has what look like seeds on the dead flower heads?

    Is it really a good idea to dig it out if a fragment of the rhizome can persist in the soil and regenerate even after 20 years? See . http://www.environment-agency.gov.uk/homeandleisure/wildlife/130079.aspx#How_to_control_Japanese_knotweed
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  • Sorry hit the wrong button!

    In my experience digging it out while it is still alive is a decidedly bad idea as one seventh of a gram of root can result in re-growth.



    Having spent wasted time trying to spray the stuff we came up with a more radical approach.In Spring, wait until it have reached at least 75cm high, it needs to be making strong growth.Cut the top off so that you can see down the stem, which is a bit Bamboo like in that it grows in section. Straighten a metal coathanger, and poke this down through the sections as far as you can. Make up a solution of Glyphosate, from the concentrate, then using a hypodermic syringe or similar (you don't need a needle) inject the solution carefully into the stem and keep doing this until the stem is almost full. Then proceed to the next plant. Repeat this in Autumn, when the sap is retreating into the roots and again the following Spring, by which time the plant will really be struggling. Once you are certain it is dead and you may need to wait a further season, then it is safe to dig up and dispose of legally.

    We have used this successfully on Knotweed which was over 1.8m high and it didn't endanger any of the surrounding trees or shrubs.
  • WintersongWintersong Posts: 2,436

    Yes that's exactly how to do it properly, and you can't throw the stuff in rubbish bins either in case it regrows. Burn burn burn it.

    You know we introduced this thug as a garden plant back in the day? Jeeese.

  • Excitable BoyExcitable Boy Posts: 165

    We had this in our last house. Glyphosate, and lots of it, is the only answer! I found spraying with a stronger than normal solution to be fine, but it took about three years to kill it all. Do not dig it up as you have to dispose of at a licensed site as it is notifiable.

    Be aware that it can damage house foundations, so if you're thinking of selling you need to make sure you don't have any within two metres of your property or your buyer may not be able to get a mortgage according to a suyveyor I know!

  • If anyone ever needs help removing Japanese Knotweed or needs advice, just call JKSL. Where always happy to help with these tricky things.
  • Hi Folks, 

    Just to clarify:

    "Once you are certain it is dead and you may need to wait a further season, then it is safe to dig up and dispose of legally."

    This is not legal - all material should be disposed of in licenced landfill. This is for two prinicipal reasons: 1) rhizome may remain viable for decades and 2) herbicides may induce dormancy, not kill. If you are satisfied that your herbicide treatment has worked, leave the knotweed alone (disturbance is likley to promote regrowth), or as another poster suggested, burn to ash in situ, following excavation.

  • Additionally, dont use diesel or salt to control knotweed - neither is effective and diesel (in particular) is toxic to plants and animals. Just a point, but treating a large patch of knotweed with salt would be very expensive...!

  • Alan4711Alan4711 Posts: 1,657

    Hi some time ago we were thinking of purchasing a house in Pembrokeshire and in the suryey notweed was found on the land,let me just say that after talking to the surveyer ,the council ,the solicitor i was amazed at the effect Jap Knot can have on the garden AND the surrounding areas,i was warned that it would be my job to get rid of it and if it could be proved  that it had spread to anyone elses property i would be to blame and suffer the cost of removal,my solicitor gave me a phone number of a specialist in Jap Knot who i spoke to,he explained all the do,s and donts about this plant and how to get rid of it,most people who try the job themselves fail, becouse of other problems as well we decided not to buy,i have the phone number of this specialist if anyone wants to talk to him ,he is a very nice chap and knows exactly what he,s talking about ,if i came across Jap Knot again i would not even try to get rid myself i,d call this man for advise, if anyone wants this phone no Email me and ill send it to you , please dont think im scare mongering ,im not as this weed has caused many a tear  for many people  [email protected]

    Good luck

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