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Old 22nd Apr 2012, 16:13
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G-CPTN
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Join Date: Aug 2005
Location: N54 58 34 W02 01 21
Age: 75
Posts: 1,859
As a fully-trained (and certificated) knotweed operative, the answer is high-concentration Glyphosate (Roundup) injected into the stem when they are large enough to accept a needle. You could buy (or beg) a syringe from a pharmacist, however we use custom-made equipment based on that used for injecting sheep:-
Japanese Knotweed Removal by Glyphosate Herbicide Injection

The mixture that we use (neat - not diluted) is commercial grade strength not normally available to the public - though you might be able to get some from a farmer or agricultural chemical supplier.

Full strength glyphosate for total weed control - Weed Killers - Total Weed Killers - Roundup Pro Biactive 360 5LT - Progreen Weed Control Solutions, suppliers of agricultural, horticultural and amenity chemicals

We don't use sprays as the stands are alongside a river which is a main water-course and the Environment Agency won't permit spraying as there is risk of run-off and it is deemed harmful to fish.

You probably know that you cannot move any pieces off-site - not in any form, and certainly not in compost - they can be burned, but don't allow anything to 'float away' as this will risk it spreading.

Take care that you don't carry pieces away on your boots or on vehicle tyres.

It's better to leave the plants intact.

Spraying (as Rads is doing) would be the best solution for small plants that cannot be injected. Do it repeatedly until you win.

What you have to realize is that Knotweed propagates from underground rhizomes, so the above-ground vegetation is simply the 'flower',

You need to kill the rhizomes by weakening them season after season (don't even consider digging them up unless you can dig deep enough to be absolutely certain that you are way way beneath them - any broken bits will regenerate and grow). Treating the above-ground vegetation with Glyphosate at a time when growth is receding will draw the chemical back down into the rhizome and weaken it.

We get 85-90% success (so 10-15% regrowth the following season).

Keep at it!

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Last edited by G-CPTN; 22nd Apr 2012 at 16:45.
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