Japanese knotweed

Discussion in 'HortForum' started by Len Beaty, Mar 9, 2003.

  1. Len Beaty

    Len Beaty Member

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    Have a problem with infestation on property near Bowen Bay on
    Bowen Island, immediately above the high-tide line and seek
    best control/eradication techniques.
    Last edited: Mar 9, 2003
  2. Douglas Justice

    Douglas Justice Well-Known Member UBC Botanical Garden Forums Administrator Forums Moderator VCBF Cherry Scout Maple Society 10 Years

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    Vancouver, Canada
    Japanese knotweed is a difficult weed to eradicate. We (UBC Botanical Garden) recently used a back-hoe to remove a rather large patch. The area will be seeded to grass, so that remaining roots (it seems one never gets all of it in the first attempt) will be killed by repeated mowing.

    Exclusion of light is the secret to successful control of perennial weeds, but this is, of course, easier said than done. In open areas, covering the ground with carpeting, newspapers or ground-cover cloth for a period of a year or more is often enough to kill persistent perennial weeds such as Japanese knotweed or Himalayan blackberry. The convention is to cover these materials with leaf or bark mulch to provide a "natural" look. Alternatively, if there is adequate sunshine, turf works well.

    Above-ground stems must first be removed before covering the ground with any material, and the best time to do this is when plants are in active growth. Pruning should happen before flowering or at least before fruit is formed, as seeds may compound eradication difficulties. Pruning while the plant is in active growth effectively weakens them (they have fewer stored food reserves).

    In shady areas where light exclusion techniques are more difficult to apply, repeated pruning may be the only non-chemical approach to control.
  3. Dear Mr. Justice,
    i am reading your site from the Limousin region of France. This is a rural area as you probably know. I have a small garden with around 8 sq.metres affected by knotweed. Two winters ago (2003- 2004)n I covered the entire area with a rubber backed carpet from November through May. I was here in February and March and there was no sign of growth. In fact there was also heavy snow. I went away for a month in April and upon my return in May the carpet had been pushed out of the way and teh knotweed was already at a height of around 2 metres! I treated the whole area first with RoundUp. After daily treatments for around three days some leaves turned brown. the smaller plants were uprooted and resprayed. As the rest gradually died I chopped them down and sprayed them. As they dried out I sprayed them with white spirit to then burn them. After the area was cleared I sprayed the ground with round up and then burnt all the remaining stubble.
    I then went away for four months and returned to find two metre high knotweed again! This time I followed the same procedure but with much higher concentrations of RoundUp. As the ground was gradually cleared I sprayed it with sodium chlorate as reccomended by an old French farmer.I burnt all the remaining stubble and before going away for the winter at the end of November I sprayed the area again with sodium chlorate at 1:5 for a total of 11 litres.
    I am now back and so far there is no sign of any above ground growth. I dug down to a depth of arund 30cm and found at least one healthy rootball so I know there will be many others. I have , once again, treated the area with sodium chlorate to be safe. I will shortly be going away again for the summer so I will see what happens.
    I realise that I have used alot of chemicals but once plant takes hold it becomes an obsession!
    keep up the good work
    Best Regards
    Jason Ward
  4. Ron B

    Ron B Paragon of Plants 10 Years

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    WA USA (Z8)
    If this material is what I think it is, and if you are planning to plant other plants there later you may first have to wait until all that sodium chlorate has sublimated. Check the label for precautions.
  5. Ralph Walton

    Ralph Walton Active Member 10 Years

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    Denman Island,BC
    Glyphosate (Roundup and others) works by absorbsion thru the growing leaves of a plant, followed by translocation thruout the plant where it prevents the synthesis of certain key proteins. The visible effects are (relative to some other herbicides) slow to appear, a week or more being fairly common. Green stems and new thin bark can allow some penetration, but growing leaves are the key entry point. Application to tilled earth will have virtually no effect. Once a treated plant is uprooted, no more chemical will be transmitted to the roots still in the ground.
    Your best approach with any unwanted plant is to spray with the label's recommended concentration, wait for the above-ground portion of the plant to brown off (re-spray if necessary), remove the dead material fairly close to the ground, wait for sprouting and spray again. Repeat this cycle every 4 to 6 weeks as long as you can in the growing season, and if necessary, hire someone to check up and possibly spray again during the "dormant" season if there are signs of re-growth.
    The keys are repetition, patience, and never give it a chance to build up strength.
    By the way, Sodium Chlorate is lsited as a soil sterilant, and will operate thru root absorbsion, so it will have some of the desired effect here. It does persist in the soil from 6 months to 5 years depending on soil type, temperature, moisture etc.. It will also move in the soil with the water movement, perhaps in an unwanted direction, so use it with caution.

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