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.
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...
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.
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.
This is what I know of as hogweed in the UK: Hogweed (Wikipedia).
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.
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.
Disposal of knotweed is regulated in the UK. The best approach is to see if your neighbour will split the cost of control or removal by a professional contractor. If it is outside a 7m radius of your house you can employ a pesticide-licensed gardener with experience of control - he will have access to glyphosate in strengths not available to lay people, should know relevant disposal regulations (make sure he does and is prepared to guarantee their application) and will have access to tools such as stem guns.
If it is within a 7m radius you will have to contract a specialist knotweed firm, of which there are a number.
In all cases treatment will run over at least 3 seasons.
There are advantages to employing professionals:
There is no law I know of which says you cannot also treat it yourself, and this will be cheaper, but there are laws you can break easily through lack of knowledge if you go down that route. Japanese Knotweed plant material and contaminated topsoil is classed as a controlled waste, and stems, leaves, rhizomes and topsoil can only be disposed of at licensed landfill sites or burned in the contaminated area. It is advisable to read the latest environment agency advice and make sure you understand it before DIYing.
If you DIY give two treatments each season over several seasons in May and August. If the Knotweed is not near plants you wish to live then you can spray it from a knapsack with commercially available glyphosate at a concentration designed to kill tree stumps. Many of these will have specific instructions for use on Knotweed, so ask your garden centre for advice, but both Roundup and Bayer make brands which can be used on Knotweed. Try to get a good dose on the underside of the leaves, where uptake is better. Combine this with basal stem injection of glyphosate, near the crown, at the recommended manufacturer concentration for Knotweed. A stem gun costs about £130.
If the knotweed is near plants you do not wish to kill, then you will have to combine stem injection with leaf wiping, which is much more laborious. Use a sponge such as you would use to clean dishes to wipe Glyphosate onto the upside and underside of the leaves then stem inject near the crown.
Wear protective clothing recommended by the herbicide manufacturer when you do this and choose a day when no rain is forecast because Glyphosate takes up to 6 hours to penetrate the leaf membrane.
You can cut back the dead growth following treatment but it must be contained, dried and burned on site or sent to licensed landfill. Read the latest envinroment agency advice on this before you start.
Whatever approach you choose, you must accept that you are in for at least 3 seasons of fun and games before you get it under control.
Also, try not to get too stressed about it. I've dealt with it domestically and succesfully mixed DIY with professional help to control it - the problem is sometimes hyped and overstated. It can affect a house valuation, but that's not an immediate problem unless you're selling. If not you have time to get rid of or control it, and you will as long as you do the right thing.
Most people won't end up with it growing through their floorboards - For sure it can do this if left, but don't forget it was an ornamental garden plant long before it was an invasive weed! The most immediate problem is that it crowds anything else in the garden so if you leave it you won't even have a lawn after a while.
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.