Japanese Knotweed

Written by Henry Lau

http://apps.rhs.org.uk/Advice/ACEImages//WSYD0014562_767775.jpgOnce considered an ornamental plant, the Japanese knotweed is now more known as a ‘terrorist among weeds’ in gardeners’ circles, and can be found throughout Renfrew Ravine. The Japanese knotweed, or Fallopia japonica, originates from the sides of Japan’s volcanoes and was first brought to Britain in the 1800s, because of its pleasant look and ease of growth. Since then, it has been brought to several parts of the world, including 6 provinces in Canada. Although innocent-looking, this invasive species definitely has earned its moniker and poses a great threat towards the biodiversity of Renfrew Ravine.

The Japanese knotweed has several distinct characteristics which can be used to help identify it. Quick to note, this knotweed has a similar appearance to bamboo, because of its bamboo-like stem that can extend up to 10 ft., but actually is a part of the same family as buckwheat, hence the membrane-like coat along the stalk. The stems are reddish-brown, hollow, and highly fibrous. The leathery leaves are heart-shaped and can grow up to 6 inches in length. There are several different knotweeds in BC, but the Japanese variety will have a unique zigzag placement of the leaves. In the late summer and early autumn, knotweeds have clusters of white flowers that can also grow up to 6 inches in length. Beneath the ground, the Japanese knotweed has an extensive rhizome network that can spread up to 20 m. in length and 3 m. deep.

The versatile nature of Japanese knotweed allows for it to thrive in various locations, but it does prefer well-lit and moist areas, such as roadsides and wetlands. Of note, the knotweed thrives especially well in riparian settings, not unlike the Renfrew Ravine. In the winter, the plant dies down, but maintains its presence underground, and by early summer, stems can already be up to 3 m. in height. The rhizomes are also able to propagate into new plants quite a ways away from the original plant, which helps to give it its invasive nature.http://warehouse1.indicia.org.uk/upload/Knotweed,%20Japanese%20(Fallopia%20japonica)%20Stanton%20Road%20Sapcote%20SP%204887%209374%20(taken%206.9.2006).JPG

In Japan, this variety of knotweed has a natural predator which maintains the plant population; however, that insect is specific to Japan. Currently in Renfrew Ravine, the knotweed can be found in various locations, of note being along the corridor from 29th avenue to 22nd avenue. The knotweed poses several problems, because of its extremely proliferative growth and spread. As many people who have tried to remove the plant have discovered, it only takes a few millimetres of the rhizome to propagate into a whole new plant. Often, fragments are transferred during construction work or when fragments are broken off and sent downstream. Its rapid growth allows it to outcompete for resources with native plants and even other invasives in the ravine, thus hampering the restoration effort of the community extending back to the 1990s. By limiting the biodiversity of the ravine, the Japanese knotweed effectively harms the whole ecosystem by replacing plants that provide homes for the local wildlife, as well eliminating all the other benefits native plants bring.

In the coming years, the problems of the knotweed could very well extend beyond the riparian area of the ravine, as very little stops this plant. Several reports from Britain have placed blame on the knotweed for lowering property values, and elsewhere, the knotweed is being held responsible for road and drainage damage; this is because the knotweed can very easily penetrate up to 4 inches of concrete. In addition, the tall growth of the knotweed blocks visibility and access to areas where it grows. Because of these issues, the Japanese knotweed poses not only ecological problems, but also significant economic repercussions.

Currently, the only proven way of eliminating Japanese knotweed is the application of glyphosate, more commonly known as Roundup, on the leaves or injected into the stem. The glyphosate gets taking up by the plant and prevents the rhizome from absorbing nutrients. However, even this process takes several seasons to exterminate the plant. This is the route many invasive species specialists go with, and if done properly, will not affect the local flora. It is not recommended to dig out the plant, as this might cause the plant to spread, but it is a possibility. Removing the plant generally takes a few years of consistent management to ensure the knotweed does not re-establish and eventually starves itself out. Any removed materials needs to either be burned or sent to a special disposal site, since it may propagate if sent to a regular landfill.

Currently in the UK, trials are also being done with the controlled release of psyllids, similar to aphids, in various sites. These insects have had positive results in quelling the spread of Japanese knotweed and are highly specific to the plants, so pose little risk to other native plants. However, the introduction of a non-native species should always be carefully analyzed, hence the strenuous trial conditions. With this biological approach, ecologists recognize that the knotweed will most likely not be eradicated, but this intervention will slow down the voracity of the knotweed and bring it towards equilibrium with the local ecosystems. Another approach has been to actually eat the knotweed. From mid-April to early May, before the plant gets too fibrous, the young shoots and unfurled leaves are edible and taste like rhubarb with a hint of lemon. Several people have come up with recipes to include knotweed in soups, jams, and pies. Interestingly enough, the Japanese knotweed has a relatively good nutrient profile, being a good source for vitamin A and C, as well as potassium, zinc, and phosphorus.


At Renfrew Ravine, a prudent approach is being taken to ensure the plant does not spread any further, while also researching the best way to deal with it. As with many challenges, prevention is the best approach to dealing with the Japanese knotweed problem. This can be done by noting any knotweeds and contacting knowledgeable sources in terms of removing it, not purchasing or growing it, preventing spread of its plant materials, and maintaining healthy native plant communities. By making the conditions conducive for native plants to thrive in the ravine, the knotweed won’t go away, but the spreading of it will at least be slowed down.


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