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Controlling Japanese Knotweed – Get Rid Of Japanese Knotweed

Controlling Japanese Knotweed – Get Rid Of Japanese Knotweed


By: Heather Rhoades

Though Japanese knotweed plant looks like bamboo (and is sometimes referred to as American bamboo, Japanese bamboo or Mexican bamboo), it is not a bamboo. But, while it may not be a true bamboo, it still acts like bamboo. It is also like bamboo in that control methods for Japanese knotweed are almost the same as for controlling bamboo. If Japanese knotwood has taken over a part of your yard, keep reading to learn more about how to kill Japanese knotweed.

Japanese Knotweed Identification

The Japanese knotweed plant (Fallopia japonica) tends to grow in clumps and can grow up to 13 feet (3.9 m.) tall in the right conditions, but is often smaller than this. The leaves are heart shaped and about the size of your hand, and have a red vein running down their center. Japanese knotweed stems are the easiest to identify, as they also give it its name. The stems are hollow and have “knots” or joints every few inches. Japanese knotweed flowers grow at the top of the plants, are cream colored and grow straight up. They are about 6-8 inches (15-20 cm.) tall.

Japanese knotweed plant grows best in damp areas, but will grow anywhere that their roots can find soil.

How to Get Rid of Japanese Knotweed

Japanese knotweed plant spreads by rhizomes under the ground. Because of this, killing Japanese knotweed is a slow process, and you must be diligent and persistent if you are to be successful.

The most common method for how to kill Japanese knotweed is using a non-selective herbicide. You will need to use it undiluted or at least at a high concentration on this weed. Remember that this is a tough plant and one application of herbicide will not kill Japanese knotweed, but will only weaken it. The idea is to spray it repeatedly until the plant uses up all of its energy reserves while trying to regrow repeatedly.

You can also try calling your local city hall or extension service. for advice Due to the highly invasive nature of this plant, some areas will provide free spraying of Japanese knotweed.

Another control method for Japanese knotweed is mowing. Chopping down the plants every few weeks will start to eat away at the plant’s energy reserves as well.

Another way to get rid of Japanese knotweed is to dig it out. You will want to dig out as much of the roots and rhizomes as possible. Japanese knotweed can and will regrow from any rhizomes left in the ground. No matter how well you dig up the roots, there is a good chance you will miss some of the rhizomes, so you will need to watch for it to start regrowing and dig it out again.

The most effect Japanese knotweed control is to combine methods. For example, mowing and then spraying weed killer will make your efforts at killing Japanese knotweed twice as effective.

Note: Chemical control should only be used as a last resort, as organic approaches are safer and much more environmentally friendly.

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Cornell Cooperative Extension

Japanese Knotweed flower and leaves


Vermont Invasives

Author: Andrea Shortsleeve, Private Lands Habitat Biologist , VT Fish and Wildlife Department

There’s a patch of Japanese knotweed growing on the edge of a town park near my house that I see every morning when I walk my dog. It hasn’t started sending up green shoots yet this spring, but there can’t be too many more days before it does. For the past few years, this patch has been there, steadily growing and spreading each season, slowly creeping more interior into the forest each summer.

Both Japanese and Giant knotweed (Fallopia japonica and sachalinensis), the two species found here in Vermont, are natives to East Asia. While these plants can grow and exploit a range of site conditions, they seem most comfortable along riverbanks and roadsides here in Vermont, causing severe damage to our ecosystems. As it grows, knotweed forms extensive, dense mats of roots and stems, eliminating native vegetation. The loss of native shrubs, trees, and flowers impacts the insect, bird, and mammal populations that depend on those plants for food and nectar. Additionally, as the insect populations declines, the fish, birds, and mammals that feed on those insects suffer as well. And as native plants are eliminated from along the riverbanks, their root systems are, too. The ground beneath the thick patches of knotweed leave bare soil, increasing the risk of soil erosion into the waterways. And finally, the impenetrable vegetative wall created along the riverbanks all but eliminates the ability of wildlife to travel along the river corridor, a space which many species rely upon to safely move across the landscape.

These detrimental impacts of this invasive plant are well-known, and even so, I’ve worked with landowners who are reluctant to treat knotweed on their property. Some landowners like the way it grows because it provides a vegetative privacy fence between their houses and a road or a neighboring property. Some people believe knotweed flowers make exceptional honey, and some espouse how delicious the fresh shoots are to eat. Frankly, I feel that looking at knotweed in these ways can trivialize an incredibly damaging invasive plant and are often short-sighted. There is a variety of native and non-invasive plants available to provide those same benefits without creating ecological damage. However, I do recognize that hyping these aspects of knotweed is a way for landowners find a silver lining in having an infestation on their properties, an often-overwhelming situation.

Knotweed is notoriously difficult to control and often seems indifferent to many treatment methods. VTInvasives.org is a great resource to look at different methods to treat knotweed, but each suggested method offers varying levels of success calling for a variety of resources and time.

This year, I’ve decided to do something about the little patch of knotweed on my morning walk, and I’m planning to try a new method to treat it. The method has been offered commercially for a few years in Great Britain, a country where Japanese knotweed has significant economic and environmental impacts. Knotweed in Britain routinely cracks the pavement in roads, causes damage to retaining walls and building foundations, and lowers property values. Mortgage companies have begun to refuse to lend on properties that are affected by Japanese knotweed due to the high threat of damage to residential homes the country has seen. Luckily, we are not in as grave of a situation here across the pond, but my point is that if Great Britain has been seeing success with a treatment method, it’s probably worth looking at.

Full disclosure: I’ve never actually tried this method, nor do I personally know anyone who has, either. However, it strikes me as a low-cost, low-energy, low-risk method to try and tackle an invasive that has brought many weed warriors to their knees. So, what’s the harm? This method, called MeshTech, was developed by Dr. Eric Connelly and Japanese Knotweed Control Ltd (This is not a promotion, but I want to give credit when it’s due). A simple Google search reveals much more in-depth information about this technique and its development than I can provide here, but I’ll run through a short explanation.

This new (new-to-me) method involves selecting a patch of knotweed and clearing out the surface growth from previous growing seasons. The cleared-out section is then covered with a wire fence mesh with ½ inch x ½ inch openings. The fencing material will have to be fastened down in some method so it will stay put throughout the growing season. And that’s all you do. Simple.

The theory is that the knotweed stems will emerge from buds on the underground rhizome in early spring and will grow through the steel mesh. As the stems continue to grow, they will expand in diameter. At a certain point, the stems will push against the steel mesh and girdle themselves. The surface growth (stems and leaves) of the plant wilts and will eventually die, but the rhizome will continue to push new stems up through the ground (and wire mesh). This will continually kill the stems and will lead to the depletion of rhizome carbohydrate stores, which are required by the stems for growth. This cycle will repeat until the end of the growing season, when the plant enters a period of dormancy. Normally, the knotweed will have a plentiful store of carbohydrates stored in its rhizome to begin growing again in the spring. However, after a season of trying to outsmart the steel mesh on top of it, the knotweed will not have any energy stored up to continue growing in the next season.

The company Japanese Knotweed Control, Ltd indicates that there will be a noticeable difference in the vigor and growth of the plant after the first year of trying the MeshTech method, but total control of the plant will still take over 5 years. Other control methods have similar timelines to see semi-permanent results, but this method could have potential to make a difference in knotweed infestations where other methods such as chemical or mechanical treatments are not appropriate. I know that I’ll be able to handle this method while I juggle all of life’s other responsibilities, and I encourage readers to try this out and let me know how it works for them. It seems to be a low-cost and low-effort method to obtain promising results on troublesome knotweed patches. And that sounds, as the British would say, brilliant.

Photo Credits: Japanese Knotweed Solutions Ltd


Why is a new method needed?

‘All sorts of things have been tried, but complete pest control is extremely difficult and very expensive. We will have to combine various methods to get the Asian knotweed under control. We know from the Japanese knotweed psyllid that it can kill young shoots and slow down or even stop the growth of the plant by sucking up sap – nutrition – from the plant.’

‘If the psyllid can establish, reproduce and spread, and do the damage we see in the breeding trials, it can hopefully inhibit the growth and spread of Asian knotweed. Then you have a very cheap and environmentally-friendly solution with many years of effect that you can combine with the more expensive methods.’


Invasive of the Week: Japanese Knotweed

Japanese knotweed, Fallopia japonica (also named Polygonum cuspidatum) and nicknamed Japanese bamboo and fleeceflower, is listed as a prohibited species in Michigan. Native to Japan and used in traditional medicine, it has contemporary herbal use as it contains the antioxidant trans-resveratrol: similar to the compound found in red grapes, but more readily absorbed and utilized. Introduced as an ornamental plant, Japanese knotweed is devastatingly invasive, because it is allelopathic and spreads rapidly, aggressively forcing out other species and creating decimated monocultures.

The Global Invasive Species Database lists Japanese knotweed on its “100 of the World’s Worst Invasive Alien Species” list. In the United Kingdom, sellers have to disclose the presence of Japanese knotweed on deeds. Banks require a written management plan if it is present on or nearby property–and will not underwrite a mortgage without one. Researchers determined that the UK infestation derived from one single plant.

Japanese knotweed is a perennial herbaceous shrub which can grow from 3-10 feet tall (often taller than other woody shrubs–helpful to ID it) though its mottled, upright stems die back, hollow stalks remain in winter, looking like bamboo (see image 2 below). Leaves are simple, alternate, oval, wide, up to 6” long and 5” wide, with a flat base and pointed tip. Creamy white, numerous flower spikes cluster on stalks ends in August-September in Michigan fruits are three-winged with dark glossy seeds (see image 4 below). Its inherent danger as an invasive species derives from the extensive root system of globulous rhizomes which store up to two-thirds of a mature plant’s mass. Rhizomes can grow to 3 inches in diameter (!), reach seven feet deep in some soils, and extend up to 70 feet. Older infestations have massive root systems.

Found in wetlands, riverbanks, and uplands, knotweed prefers sun but can grow in shade and a range of soil conditions. It thrives in disturbed natural areas or sites such as roadsides (see image 3 below) or construction areas. Powerful roots can infiltrate asphalt and cracks in concrete, and lead to erosion along stream banks.

Reproducing vegetatively by rhizomes, even the smallest of cut root and stem fragments can resprout with a vengeance–including those up to a meter deep. Along riverbanks, small root pieces spread rampantly through water and after rainy-season floods. In Michigan, roadside mowing equipment has inadvertently spread cut parts and spurred growth, and has also spread unintentionally through movement of contaminated construction site fill. It can develop fertile hybrids with similar giant knotweed (Polygonum sachalininese) and some of these hybrid US populations produce copious quantities of viable seeds.

Control methods: Work focuses on a combination of mechanical and chemical control paired with vigilance and follow up for 5-10 years: like the undead, rhizomes not completely eradicated can grow shoots up to three years later! Several strategies include applying herbicide to foliage early in the season with repeat efforts later after cutting it back, and herbicide injections–time and labor intensive. It’s imperative to take into consideration the nearby plants and water features when planning control and determining appropriate targeted herbicide use. Excavating the plants’ intertwined, elaborate root system is important–and incredibly difficult. All cut or dug parts must be removed and disposed of in the trash. (Some sites even say not to leave small pulled plants near soil.) The knotweed aphid, Aphalara itadori, may be useful as a biological control, though research in the UK showed mixed results when anthocorids (minute pirate bugs or flower bugs) consumed the aphid eggs.


Japanese Knotweed Plant: Control Methods For Japanese Knotweed - garden

This technical article provides additional guidance on Japanese knotweed.

Japanese knotweed (Fallopia japonica) was first introduced to Britain by the Victorians as an ornamental plant. The 2006 code of practice produced by the Environment Agency on managing Japanese knotweed was amended in 2013. Japanese knotweed is considered to be one of the most invasive plants in Britain.

Since the publication of the superseded 2006 code, various soil screening and sieving methods have become popular methods for rhizome removal. Where conditions are appropriate for these methods, screening can provide an effective means of rhizome removal. Screened soil must still be regarded as potentially containing viable knotweed rhizome and must not be reused off-site or sold for re-use. If soil is taken off-site, it should be disposed of at an approved landfill as Japanese knotweed is classed as a controlled waste.

For warranty purposes, there is no cover provided under the New Homes Policy as Japanese knotweed is not a notifiable contaminant.

Managing Japanese knotweed is the responsibility of the owner/occupier of a site. The owner/occupier of the site has a legal obligation not to allow it to spread. Japanese knotweed spreads easily via rhizomes (root systems) and cut stems or crowns. It has a vigorous growth and is difficult to eradicate.

Japanese knotweed commonly grows up to 3m tall at a rate of 75mm per day. Its rhizomes can extend up to 3m deep and 7m out from the parent plant. Japanese knotweed thrives on disturbance.

There are various control methods available from companies specialising in Japanese knotweed management on development sites and some offer insurance-backed guarantees for its eradication. Therefore, careful consideration should be given to products and methods that claim to quickly eradicate the plant and the guidance for treatment of knotweed recommended in the Environment Agency’s Code of Practice should be followed.

The Code of Practice describes both chemical and non-chemical methods of control along with methods of disposal both on site and to licenced tips. Japanese knotweed can stay dormant for up to twenty years.

If left untreated or is incorrectly treated then the potential for regrowth can be considered to be extremely high. Japanese knotweed can grow through and cause damage to paved and tarmac surfaces and also cause damage to drainage. It is not known for growing through concrete, however it can grow through cracks and gaps in and around concrete finishes/floors both internally and externally of a property. In addition, Japanese knotweed has also been found to grow within external wall cavities and within sub-floor voids.]

Summary

If Japanese knotweed is found on the site, the developer should ensure:

  • The area is cordoned off where the knotweed is situated in order to prevent machinery/foot traffic from accidentally spreading the material across the site
  • The builder/developer seeks specialist guidance, referring the builder/developer to the Environment Agency’s code of practice for dealing with Japanese knotweed on development sites
  • That a copy of the Japanese Knotweed Management Plan should be obtained and followed
  • If herbicide treatment is to be carried out it is essential the contractor is a competent and qualified person and must have appropriate National Proficiency Test Council certification
  • That a copy of any insurance-backed treatment offered is obtained

Note: The 2006 Code of Practice produced by the Environment Agency on managing Japanese knotweed had been amended in 2013 but has now been withdrawn. Developers should follow the guidance on Gov.uk website.

Reference

Every care was taken to ensure information in this article was correct at the time of writing (March 2021). Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant building regulations or applicable technical standards. For the most up to date LABC Warranty technical guidance please refer to your risk management surveyor and the latest version of the LABC Warranty Technical Manual.


Additional Questions about How to Kill Japanese Knotweed

Still unsure about what you’re dealing with or why you should even bother handling this weed at all? These final tips and answers will give you some insight into why Knotweed control is so important.

What is Japanese Knotweed, and why does it need to go?

Japanese Knotweed is known by many names, including Polygonum cuspidatum, Fallopia japonica, and Reynoutria japonica. This plant is a fast-growing weed with the potential to spread as much as four inches every day. This invasive weed proliferates alongside riverbanks and is particularly harmful to native species.

It can even grow in sidewalks and will take advantage of the slightest cracks in concrete walls. This overgrowth is one of the many reasons why the plant needs to be eliminated from lawns as soon as the weed is visible. Lawsuits have even been fought over this plant, surrounding its upkeep and the damage it causes to neighboring properties.

Many places even have restrictions on how to interact, remove, and dispose of Knotweed due to concerns about it spreading. If you are unsure which way to tackle Knotweed removal, consult a professional first.

Now you have several different strategies to choose from when it comes to implementing the perfect Japanese Knotweed removal regimen. Many of these options work throughout the year in warmer regions and go exceptionally well together, too.

Choose the best one based on your circumstances and level of infestation. And, always follow instructions to prevent future outbreaks.

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