Introduction

Reynoutria japonica, or its synonyms Fallopia japonica and Polygonum cuspidatum are commonly known as Japanese Knotweed. Sometimes it is called False Bamboo due to its superficial appearance to Bamboo though it is of a completely different and unrelated family. This was another one of my early discoveries of invasive species after Scotch Broom. On the Sunshine Coast of British Columbia where I grew up, False Bamboo infestations were particularly common along wet roadsides, riparian areas and wetlands. It is notoriously difficult to control Japanese Knotweed due to the vegetative reproduction that allows them to easily spread to nearby wild areas when left unchecked. I have seen firsthand the biodiversity loss that results when they invade a habitat. There are so many gorgeous native shrubs in North America, there is no need to risk planting this one in your yard. If you do, you may spend many years trying to get rid of it later.

Reynoutria japonica is on the IUCN’s list of the world’s most 100 invasive plants. There are two other plants in the same genus that look extremely similar, grow in the same habitats and ranges and can be equally invasive. For the most part, being able to distinguish the three species is not important since collectively all of the Reynoutria species in North America are invasive. That means that they should all be treated the same in terms of reporting infestations and implementing the control or removal of Japanese Knotweed species. This article discusses Reynoutria japonica but in the description section there are instructions on how to differentiate the species. There is one native species in the eastern part of North America that is sometimes misidentified. Please be sure that you have identified the plant as one of the Japanese Knotweed species before you remove it. It is also described below.

Description of False Bamboo Reynoutria japonica

Leaves & Stems of Reynoutria japonica

Reynoutria or Fallopia japonica is part of the Polygonaceae family in the Caryophyllales order of dicot angiosperms. It is a herbaceous perennial that dies back each fall and grows again every spring from its spreading rhizomes. Japanese Knotweed has a horizontal root system of rhizomes that can grow aggressively and spread 10 – 20 m away from the plant and up to 3 m deep. It then produces additional stems from its spreading rhizomes creating clonal colonies. Its primary mode of reproduction is this vegetative spreading of its rhizomes, making it a very aggressive and rapid invader.

Its purplish-green stems are slightly woody but hollow and jointed, similar to bamboo. It grows to 2 – 3 m tall each year, though it may be shorter with thinner and more solid stems in areas where it gets mowed back periodically. The rapidly growing stems can grow up to 8 cm per day or 1 m in 3 weeks. This allows it to rapidly produce large bamboo-like clumps of stems.

Reynoutria japonica has alternate leaves arranged in a characteristic zigzag pattern. The leaves are ovate to triangular in outline with an acute apex and a truncated base. Leaves possess a long petiole with a pulvinus, a swelling at the base of the joint where the petiole attaches to the stem. False Bamboo leaves are from 7 – 15 cm long and 5 -12 cm wide. They are red when young but unroll into a green leaf with reddish veins that fade with age. Its margins are entire and not wavy or toothed.

Flowers & Fruits of Reynoutria japonica

Japanese Knotweed flowers in July and August producing 6 – 15 cm long erect panicles. Each panicle consists of numerous small white-green flowers near the end of the stems and in the leaf axils. It is a dioecious plant meaning that it produces viable male and female flowers on separate plants and requires both sexes to cross-pollinate to produce viable seeds. Upon superficial examination, however, the flowers will look bisexual because the male and female flowers both have vestigial parts of the opposite sex. Vestigial parts are evolutionary remnants of the sex organs that still grow in the flower but are no longer functional. However, Reynoutria japonica spreads much more often by its aggressive rhizomes and relies far less on the sexual reproduction of viable seeds.

It produces fruits of winged seeds that are triangular, shiny, and very small, suitable for both wind and water dispersal.

Similar Species Frequently Confused With

There are multiple plants that are often mistaken for False Bamboo including docks, lilacs, dogwoods and others based on the appearance of the leaves. To make identification much more simple, if you are uncertain at all, wait for flowering. All those other species have different flowers and can easily be differentiated at that time.

In flowering, there are four species that Reynoutria japonica is often confused with. Two are from the same genus, look very similar, grow in the same habitats, are equally as invasive and are often called by the same common name. For the purpose of dealing with them as an invasive species all Reynoutria in North America is invasive and can be treated accordingly. The other two species are related being in the same family, but one is native and one is not. They can all be differentiated as follows:

• Reynoutria × bohemica is a hybrid species formed from a cross between Reynoutria sachalinensis and Reynoutria japonica. It is found in much the same range in North America. They look very similar with the hybrid having features that are intermediate between its two parent species. The hybrid is often misidentified as Reynoutria japonica but can be distinguished by the short pubescence on the veins on the abaxial (under) surface of young leaves in the spring. R. x bohemica hairs are unicellular, acute and less than 0.1 mm long. Hairs of R. sachalinensis are multicellular, twisted, acute to acuminate, and 0.2 – 0.6 mm long. Hairs on R. japonica are unicellular, blunt, and barely raised, giving the veins a scabrous appearance as opposed to hairy or pubescent.
• Reynoutria sachalinensis sometimes called Giant Japanese Knotweed is also found in much the same range throughout North America and also looks very similar. They can be differentiated by its very large cordate (heart-shaped) leaves that grow 15 – 40 cm long and 10 – 28 cm wide with undulate (wavy creased) margins. It also grows to 4 m tall as opposed to 2 – 3 m tall.
• Fallopia baldschuanica the Russian Vine is introduced in North America and so far has limited but scattered distribution here. It appears superficially similar but it is a climbing vine that does not possess the erect stems of Reynoutria spp. Its leaves are typically much more narrowly ovate than the broad ovate of Reynoutria japonica. Also, its flowers that also appear in panicle-like inflorescences are usually pink or pinkish instead of greenish-white.
• Fallopia cilinodis is native to eastern North America. It is superficially similar but can be easily differentiated by its arrow-shaped sagittate leaves with a deep cordate base and reddish veins. It also has a scandent and sprawling habit as opposed to the erect shrub of the Reynoutria genus.

Native Distribution of Reynoutria japonica

Reynoutria japonica, Polygonatum cuspidatum or Fallopia japonica is originally native to the eastern Asian countries of Japan, Korea, China and Taiwan.

Habitat Types Where Japanese Knotweed is Found

Japanese Knotweed in its native environment prefers rich, moist soil types in wetland and riparian areas. It also grows in moist fields, meadows, hillsides, and forest edges.

It prefers full sun and while it can grow in shade, its growth and reproduction is reduced there. Due to its reduced growth in shade, it is never able to dominate in forest habitats. It has a wide tolerance to soil type, pH, and salinity. The roots can survive in temperatures down to -35 C so it is capable of invading warm and cool temperate zones alike.

Human Uses of Japanese Knotweed or False Bamboo

Japanese Knotweed used to be widely used as an easy-to-grow ornamental plant and still sometimes is used this way. It can be used as a hedge plant due to its rapid growth.

The young stems are eaten in spring and taste similar to rhubarb. To eat them peel the fibrous skin off and soak them in water for half a day before cooking them to make them less bitter.

Beekeepers use the plants for their abundant nectar to make honey.

Medicinal Uses of Reynoutria japonica

Japanese Knotweed is used in traditional Chinese and Japanese medicine to treat various disorders through the actions of resveratrol, a secondary metabolite produced by plants in response to disease or injury. The levels of resveratrol are highest in the roots of Reynoutria japonica and this is usually extracted at the end of the growing season. If you want to harvest root for medicine then harvest it in the fall and remember that when dealing with aggressive invasive plants like this one the rules of ethical wildcrafting do not apply. Harvest to your heart’s content.

Extracts are used to treat a number of ailments including as a diuretic, as a laxative, to treat flus, acute hepatitis and appendicitis and gynecological problems like menstrual irregularities. They are also used to treat injuries, infections, and poisonous snakebites. There is research being done into its possible anti-tumor activity.

Leaves can also be crushed and applied externally as a poultice to treat abscesses, cuts, burns and boils.

Distribution of Reynoutria japonica in North America

The species was first brought to North America in the late 1800s as an ornamental plant for gardens.

In Canada, Reynoutria japonica has been recorded in British Columbia, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland Island (but not Labrador).

In the USA, Japanese Knotweed is found in most of the continental USA as well as Alaska. The states where it has so far been reported include Washington, Oregon, California, Montana, Idaho, Utah, Colorado, South Dakota, Nebraska, Kansas, Oklahoma, Minnesota, Iowa, Missouri, Arkansas, Louisiana, Michigan, Illinois, Kentucky, Tennessee, Mississippi, Georgia, South Carolina, North Carolina, Virginia, Washington DC, Maryland, Delaware, West Virginia, Indiana, Ohio, Pennsylvania, New Jersey, New York, Connecticut, Rhode Island, Massachusetts, Vermont, New Hampshire and Maine.

In Mexico Reynoutria japonica so far has only been confirmed in Oaxaca. It is possibly also present in Chihuahua, Nuevo Leon, Jalisco and Guanajuato.

Japanese Knotweed has been introduced to every continent except Antarctica.

How Japanese Knotweed Spreads

Japanese Knotweed is primarily spread through long distances by deliberate human introductions as garden ornamentals. It is still sold in garden stores and online despite widespread knowledge of its invasiveness.

Short distance dispersal occurs primarily through vegetative reproduction of its rapidly growing and spreading rhizomes. Short distance dispersal is also common in carelessly discarded “yard waste” piles which frequently are seen to sprout False Bamboo from both stem and rhizome fragments that were not properly solarized before disposal. See Physical Removal of Japanese Knotweed below on how to properly dispose of stems and rhizomes.

Transportation of seeds by wind, water, pets, livestock, humans etc can also be a source of spreading both long and short distances. However, by far most spread is from the aggressive vegetative reproduction of Reynoutria japonica.

Habitats at Risk of Invasion in North America

False Bamboo or Japanese Knotweed will easily invade any riparian areas, wetlands, roadsides, ditches and wet or moist forest edges it gains a foothold in. It is common in disturbed sites, empty lots and abandoned gardens. It is very common in waste areas, especially near yard waste piles.

Due to its preference for sun, it is not able to grow well in dense forests, though forest edges are at risk. It also seldom grows in arid and semi-arid areas unless in a riparian habitat.

Impacts of Invasion by False Bamboo

Japanese Knotweed forms dense thickets of bamboo-like vegetation that easily and aggressively out-competes native vegetation. This reduces biodiversity, wildlife values and causes a multitude of other negative impacts in wetland and riparian areas. The water carrying capacity of wetlands and riparian areas is often reduced in areas of False Bamboo infestation. The dense thickets reduce sunlight around them by 90% or more, shading out native plants and out-competing them for sunlight. Its thick mats of decaying vegetation it produces every fall when its abundant herbaceous stems and leaves die back build a thick mulch layer that also prevents other plants from growing with it.

Reduced plant and invertebrate densities are significant in established populations of Japanese Knotweed. Furthermore, native amphibian, bird, reptile and mammal populations are also much reduced in the dense stands of False Bamboo. It is also suspected that the aggressive roots of Japanese Knotweed are allelopathic meaning that they produce compounds that may alter soil chemistry and further negatively impact the growth of other nearby plant species.

Japanese Knotweed is capable of growing through cracks in cement, damaging sidewalks, driveways, roads, and house foundations. It can puncture its way through cement and asphalt 8 cm thick.

Japanese Knotweed roots are aggressive and strong but they are not as dense as those of native plants and they do not hold onto the soil nearly as well. As a result in riparian areas where Japanese Knotweed invades it can make the stream banks unstable and more vulnerable to erosion and flooding. This causes a loss of soil and space for vegetation, making the area further susceptible to future flooding and erosion.

Due to its impact on riparian areas False Bamboo also threatens salmon streams and stream reclamation projects.

Potential Benefits of Invasion

Wild birds are known to eat the seeds and bees use the flowers for their abundant nectar.

Methods to Remove Japanese Knotweed

As always prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in most local garden stores. Do not buy or transport any Japanese Knotweed. Do not plant it in your yard. Plant native species instead that will require little to no maintenance once established, provide wildlife and biodiversity values and likely will not need to be ‘controlled’.

If you see them being sold online or in your local garden stores please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Once Japanese Knotweed is established it is one of the most notoriously difficult plants to eradicate. There are no methods known today that will remove Japanese Knotweed in the short term. Long term removal of Japanese Knotweed is labor-intensive no matter what the method used. And long term eradication is seldom successful due to the aggressive nature of Japanese Knotweed. Diligence and hard work, integrated management and rigorous ongoing monitoring is the only possible ways to remove Japanese Knotweed.

Physical Control of Japanese Knotweed

Once already established physical control is notoriously difficult. While physical control is labor-intensive and time-consuming it still usually causes the least amount of environmental damage. Physical control is particularly difficult with False Bamboo due to the way it spreads vegetatively through its rapidly spreading rhizomes that grow up to 20 m long and 3 m deep. This makes removal particularly challenging and many people still turn to chemical methods. All methods however require multiple treatments over multiple years so physical removal of Japanese Knotweed is the least environmentally damaging option.

Physical methods to remove Japanese Knotweed generally involve the repeated cutting of the above-ground stems throughout the growing season for multiple years in a row. Cut it first in the spring after it reaches 0.5 – 1 m tall and keep going back to cut it again each time it reaches this height. It is a rapid-growing plant that can grow 1 m tall in three weeks if the conditions are right so in spring and early summer this will require multiple visits. As the soil dries and it gets hotter later in the summer it will not grow as rapidly and cutting frequency can be reduced. This will eventually weaken the aggressive root system as the plant is unable to conduct photosynthesis to store energy in its roots for subsequent years’ growth. This is very time-consuming but it will eventually kill it after several years of treatment.

Another method some people have tried with smaller patches is the cut all the above-ground growth and then solarize the entire patch with a heavy black tarp. This works for small infestations where the tarps can extend far beyond the stems. This is because the rhizomes can grow up to 25 m so the plants will tend to regrow just outside the edges of the tarp. This can still be done, however, as you can simply keep returning to the spot to cut any growth attempting to start outside the tarped area, as described above for stem removal.

Where land is to be cleared the best method may be to dig up the entire patch to remove all the rhizomes. Use caution however as any piece of rhizome left can grow and start an entirely new infestation. Proper disposal is also an issue in this case as there will be large volumes of soil needing to be properly disposed of so that it does not create a new infestation. Bringing it to a garbage dump and informing them of its aggressive invasive nature is one option. Phone ahead to make sure they have the means to bury it immediately, and deep enough (more than 5 m) to ensure that it does not grow again.

Disposal of the Shrubs Once Removed

Stems and rhizomes should never simply be disposed of as they will rapidly grow and invade any site they are delivered to. They can be burned on the spot if there are open fires allowed in your area. Burning is usually a good option in spring before fire bans have begun.

In the summer, however, solarization may be the only option. Place stems and rhizomes either under a heavy thick black tarp or into thick black garbage bags and leave them in the full sun for a good 10 weeks to be sure that all roots, stems and seeds are no longer viable. Many people recommend shorter solarization periods but due to the differential heats achieved in garbage bags and under tarps I recommend solarizing for as long as possible. Even after solarization you should bring them to the garbage dump and inform them that it is an invasive species so that they can be disposed of appropriately.

Chemical Control of Japanese Knotweed

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used species riches significantly increases.

Furthermore, there are no chemical control methods that effectively target only Japanese Knotweed. And due to the aggressive nature and deep root system of Japanese Knotweed, multiple applications during the year and in subsequent years are always needed.

However, if it is a very large patch of Reynoutria japonica and physical removal is not a viable option sometimes chemical control of Japanese Knotweed may be necessary. If it is near a riparian area or wetland, however, chemical application cannot be used due to the proximity to water. In those cases, physical removal is the only viable option.

Chemicals registered for use vary from area to area so if you are considering this method please research what is legal to use in your area and take into consideration proximity to water.

Biological Control of Japanese Knotweed False Bamboo

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. This is because the control agent would need to effectively destroy over 99% of the plants in order to be an effective control method. Results this high are rarely seen in the field. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective. Following is a list of biological control methods that have been used to help control Japanese Knotweed.

• Aphalara itadori the Japanese knotweed psyllid insect feeds only on Japanese knotweed. It has been tested in the United Kingdom with some success.
• Aphalara itadori leaf fleas that suck up the sap out of the plant have been recently released in Amsterdam. Results are not yet available.
• Mycosphaerella leaf spot fungus is currently being researched as a mycoherbicide due to its effect on plants in Japan. Research, however, is still preliminary and no results are available for its use outside of Japan.
• Goats are a great method to control Japanese Knotweed as they will feed on all the above-ground growth. If you have a small patch that you want to get rid of simply build a goat containment area around it and let the goats live there for several years. Eventually, the plants will cease to grow as the goats continually graze on all the above-ground growth. Some people recommend the use of pigs following the goats as the pigs will root out and eat the rhizomes. However, considering that the rhizomes can grow to 3 m below ground it is unlikely that the pigs will be able to deal with all the roots. Again, leaving them there for several years to keep removing any above-ground growth that appears can be a successful method.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form this involves physical removal methods, possibly biological control methods, replanting and ongoing monitoring. Integrated management recognizes the fact that when dealing with invasive species one method alone is almost never successful, especially in the long term.

Replanting With Native Species is Crucial

After removing any native species from an area replanting with native species is important to prevent the area from becoming reinfested with that or another invasive species. This is somewhat complicated in the case of Japanese Knotweed removal. This is due to the aggressive nature of the deep and extensive rhizomes which are its primary method of reproduction and spread. If the area was heavily infested and replanted immediately it will seldom be successful as the new growth of Japanese Knotweed from its extensive rhizomes will quickly take over. Replanting of native species should instead be planned for the following year(s) once the patch truly seems under control.

Ongoing Monitoring is Essential

In all cases of invasive Japanese Knotweed removal, ongoing monitoring is absolutely essential. More so for Japanese Knotweed removal than almost any other invasive species. Japanese Knotweed is one of the most aggressive invasive species known and eradication is notoriously challenging.

Yearly monitoring is not adequate due to the rapid and aggressive growth. Instead, monitoring should be done every 3 – 4 weeks during the growing season which would then include physical removal of any sprouts or seedlings found. Then this would need to be repeated yearly for at least 4 – 7 years, depending on how many individuals grow up each year. Yearly monitoring programs should be put in place after that for a few more years just to ensure that nothing has returned.

References and Resources

CABI on Fallopia japonica https://www.cabi.org/isc/datasheet/23875

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

eflora.org Flora of North America http://www.efloras.org/browse.aspx?flora_id=1

iNaturalist Plant Search https://www.inaturalist.org/home

Invasive Species Center on Japanese Knotweed https://www.invasivespeciescentre.ca/invasive-species/meet-the-species/invasive-plants/japanese-knotweed/

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia on Reynoutria japonica https://en.wikipedia.org/wiki/Reynoutria_japonica

Willis, Lyrae (2021).  Plant Families of North America. Not yet published.

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