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I have Japanese knotweed growing into my garden from an adjoining neighbour's garden.

Does anyone please have any advice of how to get rid of this awful plant?

I'm in the UK.

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5 Answers

Japanese knotweed (Polygonum cuspidatum) and hogweed (Heracleum spp.) are two completely different things. Knotweed kind of looks like bamboo, it has a hollow stem and takes over like crazy. Hogweed is an Apiaceae and has flowers that look a bit like Queen Anne's Lace.

If you've got knotweed, it's awful stuff to get rid of. This fact sheet from the NH DOT describes some of the practices they're supposed to be using to avoid spreading it around. (Too bad they still mow it and spread it around...)

This page describes a technique using tarps to smother it. If you try this, let us know how it works -- I live in fear of the japanese knotweed around us invading our yard...

Edit:

Since you can't smother it, you can try one of two other approaches. See this page by the Alien Plant Working Group of the U.S. National Park Service:

Grubbing is effective for small initial populations or environmentally sensitive areas where herbicides cannot be used. Using a pulaski or similar digging tool, remove the entire plant including all roots and runners. Juvenile plants can be hand pulled depending on soil conditions and root development. Any portions of the root system not removed will potentially resprout. All plant parts (including mature fruit) should be bagged and disposed of in a trash dumpster to prevent reestablishment.

Do not let any part of the plant get away from you! Bag it and toss it in the dumpster.

I generally try to avoid resorting to chemical warfare. For Japanese knotweed I might make an exception. If you want to go this route, make sure you read the label and follow directions:

Cut the stem about 2 inches above ground level. Immediately apply a 25% solution of glyphosate (e.g., Roundup®, or use Rodeo® if applying in or near wetland areas) or triclopyr (e.g., Garlon) and water to the cross-section of the stem. A subsequent foliar application of glyphosate may be require to control new seedlings and resprouts.

You should talk to your neighbor to see if you can work together to get rid of it. If you don't, it will probably migrate back into your yard later anyway.

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It has the hollow stalks, so must be knotweed. It is very quick to grow back, and it just looks awful. As it isn't in my garden, I'm not going to be able to use the smothering technique unfortunately. –  Paul Jun 12 '11 at 9:54
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It will keep coming in as long as your neighbor has it.

Patience and persistence will win, or at least get you to a standoff in the bad-neighbor case. Roundup and digging work. For full-grown plants, the root bole can be quite hard to get out - use a D-handle gardening fork, or even a pickax.

If you don't cut the plant first, Roundup often just stunts the plant, making it produce a lot of small foliage. Dig it out then.

Even if you don't have the wherewithal to dig it all out, just keeping it cut down will slow it down a lot - don't just let it go because you don't have time to get rid of all of it.

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This is what I know of as hogweed in the UK: http://en.wikipedia.org/wiki/Hogweed

When people complained of knotweed I assumed they meant something else (but I could be wrong).

Some hogweeds are native, but the Giant one was imported as an ornamental and escaped. From what I remember, it has shallow roots and pulls out fairly easy? I do remember my primary school back in the 80s getting paranoid about it and telling us not to touch it! in reality the problem is only the sap. Some people are allergic to the sap, and it can also result in skin burns/reaction in the sun. So if this is the stuff, be sure to use gloves.

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Clearly the OP is referring to knotweed. I was pretty certain they were different but wasn't sure. I'll keep the above answer in place, in-case it is useful for other people in the future. –  winwaed Jun 12 '11 at 13:22
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Some of these answers sound a lot like home remedies. Knotweed should NEVER be cut before being treated with Roundup or any glyphosate-based product as these products are SYSTEMIC which means they use the plant's own leaf system to kill it. It cannot be smothered - it can grow through concrete and tarmac. It should NEVER be dumped - this only spreads it as it will grow again from the tiniest living fragment. It should be cut an appropriate time after spraying (see pack instructions), left to dry then burned - all on the contaminated land. After burning, the contaminated land will have to be dug over to weaken all surviving and uncollected parts of the plant. This procedure will have to be repeated for at least three seasons. If it's growing next door, have a word with your neighbour before starting.

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Applying glyphosate to fresh-cut stalks is pretty well-known to be effective (see the link in @bstpierre's answer to the National Park Service recommendations), is much less likely to cause collateral damage to surrounding plants, and, at least in my experience, is a lot more effective than leaf-spraying - particularly when considering the much smaller amount that must be applied. The fleshy, moist, hollow stalks take up glyphosate quite well because of their unusual structure. –  Ed Staub Jun 15 '13 at 17:47
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If, as I think you said, you're in the UK, talk to your local Council environmental department about it first - they may not deal with it because it's not on public land, but they may be able to give you some advice about what you should do. Second, if you own your house and you have a mortgage with buildings insurance, inform the insurance company and the mortgage company that it is coming from beneath your neighbour's house. Unfortunately, this means the insurance company will take proceedings against next door, but this problem must be dealt with before it starts appearing inside your own living room, which it will, if left untreated. If your house is owned by a housing association or Council from whom you rent, inform them as well and request they take action. Trying to deal with it yourself when its so close to a house is impossible - roots go down 9 feet and, as someone else says, any fragment will grow again. The issue of Japanese knotweed is now so serious that some mortgage companies will not give a mortgage on any property where this plant is within 30 metres.

If you're renting privately, and you're able to, move.

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