Japanese Knotweed

Japanese Knotweed (Reynoutria japonica), formerly Fallopia Japonica, is one of the most invasive weeds in the world. It produces extensive and powerful networks of rhizomes below the soil surface, so powerful that they can damage foundations and building structures, not to mention choke out other vegetation. The rhizomes burrow down up to 3 m (10ft.) in depth and span between 2 to 7 m (7-23 ft.) horizontally with a thickness of up to 20cm (9”). There are nodes all along the rhizomes from which new plants can spring up. This gives it the ability to form very dense colonies of 3 m (10′) tall plants. Unless every fragment of rhizome is removed new plants can form. In Ontario, as in many regions of the world, it is “illegal to buy, sell, trade, propagate or purposely grow Japanese knotweed” (Ontario’s Invading Species (n.d.)). Additionally it is a criminal offence in some regions, such as the U.K., if you do not control Japanese knotweed on your property and it spreads to a neighbouring property (Gordon Brown Law, (n.d.)). Following is some information and photos to help you identify Japanese knotweed and some recommendations for managing it.

How to Identify Japanese Knotweed

Overall Growth Habit: plants form dense colonies of tall hedge-like clumps.

Japanese knotweed in early May. Photo by Kimberley Pacholko
Japanese knotweed in mid-June in Whitby, Ontario. Photo by Kimberley Pacholko
Japanese knotweed in mid-July. Plants can grow 3 to 3 1/2 m tall. Most of this patch of knotweed has been cut back but they missed a few behind the fence. Photo by Kimberley Pacholko
Japanese knotweed in mid-August.

Stems: are hollow, 2.5-5 cm (1-2″) thick and up to 3-3.5 m (10-11 1/2′ ft.) tall, with red speckles or streaks and distinctive leaf nodes (a red band running across the stem where the leaf joins and a whitish ocrea just above that). These distinctive leaf nodes give the stems a bamboo-like appearance.

Japanese knotweed in early May. Stems have red speckles and distinctive leaf nodes with a red band running across the stem where the leaf joins and a whitish ocrea just above that. Photo by Kimberley Pacholko.
Japanese knotweed in early May. Notice the hollow stem from being cut back last year. Photo by Kimberley Pacholko
Stems of Japanese knotweed at maturity (mid-July). Photo by Kimberley Pacholko

Leaves are heart or spade shaped, with smooth edges and measure to 12cm to 20cm long. New leaves are red, then turn green with red veins, then finally fully green at maturity with red petioles. The leaves alternate up the stem in a zig zag pattern. Lower leaves typically begin dying off as plants grow taller.

Japanese knotweed leaves in early May. Leaves start off red then begin turning green with red veins. Photo by Kimberley Pacholko
Japanese knotweed leaves are heart or spade shaped, fully green at maturity with red petioles and entire margins. They can measure about 12cm- 20cm in length. Photo by Kimberley Pacholko

Roots are underground rhizomes that can become quite woody and reach depths of up to 3 m (10’) with a 20cm (9”) thickness and spread between 2 to 7 m horizontally. These rhizomes can become so densely packed that they form what are called ‘crowns’. These crowns are like tree trunks and usually require heavy equipment to remove. Japanese knotweed rhizomes are woody and brown on the outside and orange on the inside. New plants arise from nodes along the rhizomes causing dense colonies. The underground network of rhizomes can become so extensive, and powerful that they can damage foundations and building structures.

Flowers are white, fragrant, finger-like projections (clustered panicles) that are about 10 cm (4”) long in late summer.

Japanese knotweed in bloom at the beginning of September.
Japanese knotweed in bloom at the beginning of September.

Reproduction and spread: new plants form almost exclusively by rhizomes and fragments of stem, or crowns. They rarely reproduce from the heart shaped seed produced by the plant.

Management and Control of Japanese Knotweed

This is one of the most difficult weeds to eradicate. Often professionals need to be called in for removal. There are however several things homeowners can do to help manage this invasive weed, especially if the population is small and the plants are young.

Digging: this only works for very young plants with minimal root structure and very small populations. Every fragment of rhizome must be removed to prevent regrowth. After removal monitor the area regularly for regrowth. DO NOT compost or brown bag these removed plants, as composters do not get hot enough to kill them. Black bag them instead and put them in the sun to dry out and dispose of them according to your local laws.

Grazing: is not a very effective means of control but the weed is edible for livestock and it can help suppress the weed early in the season.

Mowing/Cutting: helps to weaken the rhizomes by depleting them of carbohydrates. Each time they are cut back the rhizomes have to expend more energy and food reserves to send up new growth. This can be a slow process requiring several years. The younger the plants and the smaller the population the more effective this method is. A word of caution here though; the cut or mowed fragments must all be collected and black bagged and disposed of in accordance with local laws.

Tarping (solarizing): is used to cook the rhizomes to death. The area must be in full sun. Heavy light blocking tarps, (like the poly tarps or heavy landscape cloth) are spread loosely over the patch of cut down Japanese knotweed and secured in place. The tarps must extend well beyond the patch. The area must be monitored regularly for tears and sprouting at edges. Again it may require several years.

Root barriers: block and prevent the weed from spreading into neighbouring properties.

Excavation: may be required for large populations and mature plants with extensive root systems. The soil will have to be excavated to a 2 m depth and the soil disposed of according to local laws. Monitor the area regularly for any new growth.

Chemicals: such glyphosate that work systemically are helpful in controlling this weed. Several applications may need to be applied. For foliar spraying the Japanese knotweed is first cutdown then allowed to grow back for 8 weeks then sprayed. Professional removal companies may instead do stem injections of the glyphosate. There are also other chemicals available, check with your local municipality to see what is permitted in your area.

Different Types of Knotweed

In this article I am focusing on Japanese Knotweed (Reynoutria japonica), formerly Fallopia Japonica. However there are other types of Knotweed.

  • Giant Knotweed (Reynoutria sachalinensis, formally Fallopia sachalinensis) grows up to 4- 5 m tall and has much larger leaves that have long wavy hairs down the center vein of the underside of the leaf.
  • Himalayan Knotweed (Persicaria Wallichii) grows about 2 m tall. The leaves are narrow, about half as wide as they are long, rather than being heart or spade shaped. Flower clusters have a pinkish hue.
  • Lesser Knotweed (Persicaria campanulata): grows to a similar height but has smaller flower clusters that are pink rather than white and leaves are long thin ovate.
  • Dwarf knotweed (Fallopia japonica var. compacta) formerly Polygonum pictum, Polygonum compactum and Polygonum reynoutria. It grows to about 60 cm tall (5ft.). The leaves are darker green and have a more leathery appearance, wavy-like margins and reddish veins. Flowers start off white or pale pink and age to rose or red. Foliage turns red in the fall rather than yellow.
  • Bohemian knotweed (Fallopia × bohemica) is a hybrid cross between Japanese knotweed and Giant knotweed. The plant grows 2-4 m tall and leaves are larger then Japanese knotweed and smaller than Giant knotweed. Leaves are also thicker and rougher than than Giant knotweed and less so than Japanese knotweed. On the underside of the leaves center vein are short hairs that are wide at the base and taper upwards, like a triangle.

Japanese Knotweed Look-a-likes

Japanese knotweed is sometimes confused with bamboo due to both having thick sturdy, hollow stems with distinctive leaf nodes (joints) and in the case of running bamboo, both can spread wildly. The most distinct difference is the leaves. Bamboo leaves are lanceolate (long and narrow, tapering to a point at each end). Another difference is that Japanese knotweed stems will snap when bent but bamboo is difficult to snap.

Another plant that is commonly misidentified as Japanese knotweed is Pokeweed. Below is a summary table of the differences between the two.

Summary

Japanese knotweed is a tall, dense and fast growing perennial weed that aggressively spreads vegetatively by rhizomes, stem or crown fragments or by nodes sent up by the root system. Eradicating this weed involves the removal of the entire root system or killing it off, in order to prevent regrowth. It is illegal to intentionally grow this plant or cause it to spread.

Report sightings to your local invasive species hotlines. To do this take some photos of the weed and record the location. In Ontario call 1-800-563-7711, or go on-line to EDDMapS Ontario or http://www.invadingspecies.com to report the sighting.

References:

Anderson, Hayley. 2012. (Ontario invasive Plant Council). Invasive Japanese Knotweed (Fallopia japonica (Houtt.) Best Management
Practices in Ontario. Ontario Invasive Plant Council, Peterborough, ON. Printed April 2013. https://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/06/OIPC_BMP_JapaneseKnotweed.pdf.

Gordon Brown Law, (n.d.). IS JAPANESE KNOTWEED ILLEGAL?. https://www.gordonbrownlaw.co.uk/blog/is-japanese-knotweed-illegal/

Japanese Knotweed Specialists, (2021). JAPANESE KNOTWEED ROOTS: WHAT’S THE BIG DEAL?. https://www.japaneseknotweedspecialists.com/news/japanese-knotweed-roots

Martini P. (2019 updated: May 2022). Japanese Knotweed: How Does It Spread? Japanese Knotweed News | Knotweed Help. https://www.knotweedhelp.com/japanese-knotweed-guide/how-does-knotweed-spread/

Northwest Michigan Invasive Species Network, (n.d.). Look-alikes:  Invasive Knotweeds and Native Pokeweed. https://www.habitatmatters.org/knotweeds-v-pokeweed.html

Ontario’s Invading Species, (2021.). Japanese Knotweed Fallopia japonica. http://www.invadingspecies.com/invaders/plants/japanese-knotweed/

The Knotweed Killers, (2021). Bohemian knotweed Invasive Species Information. https://www.japaneseknotweedkillers.com/fallopia-x-bohemica

Wurzbacher S., Gover A., Jackson D.R., and Templeton S. (updated: Feb. 24, 2020). Japanese Knotweed. Penn State Extension. https://extension.psu.edu/japanese-knotweed

Photo credits: all photos taken by the author.

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