Japanese knotweed is an aggressive semi-woody perennial plant that is native to eastern Asia. In the 1800’s it was introduced to North America as an ornamental species and also planted for erosion control. It has since spread throughout the United States and Canada. Japanese knotweed is often mistaken for bamboo; however it is easily distinguished by its broad leaves and its ability to survive Ontario winters. Japanese knotweed is especially persistent due to its vigorous root system, which can spread nearly 10 metres from the parent stem and grow through concrete and asphalt. This invader is very persistent and once it becomes established, is incredibly difficult to control.
In Canada, Japanese knotweed is established from Ontario to Newfoundland and is also found in British Columbia. In Ontario, it is mostly established in southern and central areas of the province where it mostly grows in gardens, along roadsides and near old buildings or former building sites.
Impacts of Japanese Knotweed
- Spreads quickly, creating dense thickets that degrade wildlife habitats.
- Reduces plant biodiversity by competing with other native vegetation. Thick layers of decomposing stems and leaves on the ground make it difficult for native plant species to establish.
- Aggressive plant with a strong root system that has been known to break through asphalt and concrete.
- Plant populations are extremely persistent. Plants are able to survive severe floods and recolonize areas.
- It can establish along riverbanks, where pieces of roots can break off and float downstream to start new populations.
How to Identify Japanese Knotweed
- Semi-woody perennial plant capable of reaching 1-3 metres in height.
- Stems are round, reddish-purple, smooth and have a bamboo-like appearance.
- Leaves are ovate with a flat base, reaching 3-6 inches long and 2-5 inches wide with pointed tips.
- Flowers are greenish-white.
- Fruit is small and white with wings that help to disperse seeds to new sites.
- Seeds are brown and shiny.
What You Can Do
- Learn how to identify Japanese knotweed and how to avoid accidentally spreading this invasive plant through its root fragments and seeds. This is especially important if you are planning to do work in an area which contains Japanese knotweed.
- Learn how to effectively manage Japanese knotweed on your property. The guide to Best Management Practices for Japanese knotweed describes the most effective and environmentally safe control practices for this species.
- Never buy or plant Japanese knotweed. It is against the law to buy, sell, trade, propagate or purposely grow Japanese knotweed.
- Stay on designated trails and keep pets on a leash. Leaving trails or entering areas containing Japanese knotweed can encourage the spread of this plant.
- When leaving an area containing Japanese knotweed, inspect, clean and remove mud, seeds and plant parts from clothing, pets (and horses), vehicles (including bicycles and ATVs), and equipment such as mowers and tools.
- Do not compost Japanese knotweed in your backyard composter. Both seeds and rhizomes (horizontal plant stems growing underground) could survive and grow in compost, unless high enough temperatures are reached to kill the reproducing structures. Contact your local municipality to determine if plant material can be brought to their composting facility. Ontario composting facilities monitor the compost process and meet provincially regulated temperature requirements.
- If you have any information about the illegal importing, distribution or sale of Japanese knotweed, report it immediately to the NDMNRF TIPS line at 1-877-TIPS-MNR (847-7667) toll-free anytime. You can also call Crime Stoppers anonymously at 1-800-222-TIPS (8477).
- If you’ve seen Japanese Knotweed or other invasive species in the wild, please contact the Invading Species Hotline at 1-800-563-7711, or visit EDDMapS Ontario to report a sighting.
OFAH/ONDMNRF Invading Species Awareness Program. (2021). Japanese Knotweed. Retrieved from: www.invadingspecies.com.
This factsheet may be reproduced for non-commercial purposes.
Header photo by Francine Macdonald, NDMNRF
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