Japanese Knotweed – Fallopia japonica

Synonyms: Reynoutria japonica, Polygonum cuspidatum 

What is it?

Also known as Mexican bamboo, Fleeceflower, Japanese Polygonum or Huzhang, and many other names, the Japanese Knotweed is a shrub-like semi-woody perennial plant native to East Asia known for its resilience, and attractive bamboo-like stems with large broad leaves and clusters of flowers. 

Photo of Japanese Knotweed by W.carter, CC0, via Wikimedia Commons

First introduced to North America in the 1800s, with the first recorded sighting in 1901 in Niagara Falls, Ontario, the Japanese Knotweed was initially planted as an ornamental species and to control erosion. It has since spread throughout the eastern seaboard, particularly in Ontario where it is most established in the southern and central areas, and as far as Newfoundland, especially in open woods, floodplains, and forest edges. Some specimens were found in British Columbia, though limited records made tracking its spread difficult. Likewise, little public attention has been paid to this alien plant except in the last two decades. 

As an ornamental plant, grown for its aesthetic qualities, the Japanese Knotweed mostly grows in gardens, and is a less prominent invasive species than most others. Outside of your backyard, and your grandmother’s garden, the Japanese Knotweed is mostly found in two types of environments: along stream banks, where it grows best in the wetland conditions, and in highly disturbed and derelict sites, such as along roadsides, railroads, ditches, and near old or former building sites, especially in abandoned material like soil, and aggregates etc.

Once established, Japanese Knotweed is known for being incredibly persistent and difficult to remove, with at least one dense cluster in Toronto surviving multiple repeated attempts at pruning, digging, and herbicide uses over years. This is achieved by their aggressive, highly dense and vigorous root systems. Notably, unlike many other invasive species, the Japanese Knotweed has difficulty making inroads in Undisturbed forested areas. Additionally, the Japanese Knotweed is vulnerable to persistent freezing conditions, and our harsh Canadian winters have ensured that the Japanese Knotweed is not as common as you might think. Nevertheless, the warming climate may soon change this. 

Why you should care

Like the Norway maple, the Japanese Knotweed has only recently been recognized as an invasive species. Due to its aggressive and lively root system, which can grow as far as 10m in every direction from the parent system, it can grow through concrete and asphalt, damaging urban infrastructure, building foundations, and increasing maintenance costs. This root system is so aggressive and persistent that it can survive even severe floods, and disasters! 

Japanese Knotweed
Japanese Knotweed growing through concrete. Photo by Kerry Wixted via Flickr.

In fact, one way Japanese Knotweed can spread without human intervention is during floods; pieces of the root system can break off in a flood, and start a new population where it lands. Ironically, despite being planted to prevent erosion by many settlers, the plant is not as good as many native plants at preventing erosion, and typically makes stream banks where it is common less stable, and more vulnerable to erosion and flooding in heavy storms. Near such waterways, thickets of Japanese Knotweed can also impede access, preventing recreational activities like canoeing, boating, angling, and swimming on affected riverbanks and shorelines.   

Japanese Knotweed along a stream bank. Original photo found here.

Outside of the gardens it is mostly found in, the Japanese Knotweed can very quickly and aggressively grow into dense thickets preventing 90% of sunlight from reaching the forest floor, enabling it to outcompete and suppress the growth of native plants. The Japanese Knotweed can grow as quickly as 8 cm per day, or 1m in three weeks. There is a reason why it is often compared to bamboo. Wildlife, including insects, rarely feed on the plant, so its rapid growth can quickly reduce native biodiversity and food supplies for local amphibian, reptile, bird, and mammal populations, effectively degrading the entire ecosystem. Additionally, the Japanese Knotweed is suspected of releasing chemical compounds that can alter soil chemistry and/ or inhibit the growth of other plants. Worst of all, even when they (eventually) die, its thick layers of decomposing plant matter can continue to suppress native plant growth for years.    

What can we do?

Given its reputation for near immortality, do not despair dear readers, there is much we can and should do about Japanese knotweed! Although it is almost indestructible, with at least one case of volunteers still battling an infestation as of 2017, and beginning in 2010, this monstrous hydra of plant CAN and will be brought to heel through perseverance and hard work. 

Or you know, just don’t plant it in the first place. 

Unlike a lot of other invasive plants, there is still room for us to prevent widespread colonization by Japanese knotweed, and it is always easier to remove newer plants than established thickets. If you live by a waterway, and you’re in the market for an erosion resisting plant, try native plants that are way better than Japanese knotweed like these:

  • Field pussytoes
  • Silverweed
  • Virginia waterleaf
  • Barren strawberry
  • And many more!

Otherwise, if you can, volunteer with efforts to help remove it where it can be found because they’ll need all the help they can get. When doing so, or going on a walk through locations known to be infested by this immortal alien hydra, stay on designated trails, avoid walking through thickets of Japanese knotweed, and always check your clothing and equipment for fragments or seeds that might have broken off can be carried away and spread. Never buy or plant Japanese knotweed, and never compost it as even the smallest root fragments and seeds can grow into new thickets. Instead, bring them to your local composting facility which can monitor the process while meeting provincial regulated temperature requirements.   

However, in order to help out, you’ll first need to know how to identify Japanese knotweed! 

 How to identify Japanese Knotweed

  • Height
    • Can grow 1 – 5 m in height
  • Stem
    • Dense clusters of tall straight bamboo-stems that may bend over under its own weight when extremely tall
    • Each stem can be between 2.5 – 3 cm thick 
    • Stems are hollow-jointed with reddish brown solid nodes with a papery sheath
    • Younger stems are green with purple splotches, resembling asparagus
    • Older stems can be reddish brown 
    • In winter, they appear as bare, grey, or straw colored hollow stalks  
    • New stems can emerge in late March to early April from an overwintering root system that can grow as far as 10 m away underground from the surface plant. 
  • Leaves
    • Oval heart to triangular shaped with a pointed tip
    • 8 – 10 cm wide and 10 – 17 cm in length 
    • Grows in alternating patterns from purple-red stems branching off of the main stem 
    • Green, but may have purple blotches on the edges and veins 
  • Flowers
    • Blossoms in the spring and up to July or August 
    • Found only at the end of the stems resembling wheat stalks
    • Attractive clusters of small white or green flowers growing along the tip of the stems in plumes 
  • Fruit
    • Produces fleshless winged oval shaped seeds that point into the stem 
    • Shiny beige or off white papery sheath covers the darker seed 

Can you remove Japanese Knotweed? 

But of course! Some would even go so far as to say there is a kill on sight order on these. However, be prepared dear reader to be in it for the long haul, perhaps years or even decades at worst, should you attempt to tackle these hydras of plants, for they have a reputation for immortality for a reason! As always, the best protection is prevention and avoid planting it in your backyard. 

If you do happen to find these bamboo like menaces in your garden, some of the best control methods include:

  • cutting off the flower heads to prevent seed production 
  • cutting off stems and applying herbicides to the roots and cut stems – best done in summer and fall 
  • digging up the entire root system 
  • smothering – best done in spring

However, using one of these methods is not usually helpful; the more combinations and techniques you use to fight this hydra of a plant, the more successful you might be. Most importantly, never give up, no matter how long it takes. Other methods and techniques can be found here:

If you see any plants that match the description above, please contact the Invading Species Hotline at 1-800-563-7711 or the Natural Resources Information Support Centre at 1-800-667-1940 or 1-800-387-7011 to submit your observations! Also consider visiting the EDDMapS Ontario which has partnered with both organizations above to report sightings!

Interested in learning more? See also:

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