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Invasive Plants

By Lisa Scott

This is one of the worst

Despite its exotic visual appeal, Japanese knotweed hasmade the top 100 list of the world’smost unpopular alien plant invaders, and is found in many parts of B.C.

nvasive knotweeds are one of the most impressive of our province’s top 13 ‘unwanted’ horticulture plants. Although the primary knotweed species found in British Columbia is Japanese knotweed (Fallopia japonica), Bohemian knotweed (Fallopia x bohemica), Giant knotweed (Fallopia sachalenensis), and Himalayan knotweed (Polygonum polystachyum) are also present, and all four species are considered invasive. Since all four species are similar in appearance, biology, impacts, distribution and methods of control, they will be discussed under the general title of ‘knotweeds’.


Invasive knotweeds are native to eastern Asia in Japan, China and Korea, and have spread at a rapid rate because of their lush ornamental appeal to gardeners, persistent root system and ability to adapt to a wide range of habitats. There is no doubt that invasive knotweeds have an exotic appeal. The vivid green foliage and hollow stems with

distinct raised nodes give knotweed the appearance of bamboo, which it is commonly marketed as, although it is not closely related. But don’t be deceived; knotweeds are feisty, forceful and resilient to most control treatments. Invasive knotweeds are herbaceous perennials that prefer moist habitats and freshly disturbed soils, but are capable of thriving in gravel and pavement landscapes and in sun or shade. The hollow stem is covered in a reddish- brown speckled papery sheath and will grow one to five metres in height. They will reach their full height by the end of June.

Knotweeds have small white-greenish or light pinkish flowers in showy plume-like branched clusters. Japanese knotweed leaves are heart or triangle-shaped and zigzag along the arching stems whereas Himalayan knotweed has long-tapered leaves.

Japanese knotweed has received the greatest attention around the world and is listed amongst the “World's 100 Worst Invasive Alien Species” by the World Conservation Union. It can be found in 39 U.S. states and in six Canadian provinces. In the U.S. it is listed as an invasive weed in Ohio, Vermont, Virginia, New York, Alaska, Pennsylvania, Oregon and Washington State. This species


is also common in Europe. In the United Kingdom it was made illegal to spread Japanese knotweed by the Wildlife and Countryside Act of 1981.

Why has Japanese knotweed earned such a notorious status? Here are some facts that are mind-boggling. First off, it is capable of growing as much as 10 centimetres per day. It is a highly invasive plant and is capable of exposing weaknesses in buildings, foundations, concrete and tarmac. Japanese knotweed has the capability to regenerate from rhizome as small as 0.4 g. Therefore, it poses a massive risk of spreading via groundwork and disturbance.

All knotweeds are very efficient at spreading in part because they use their root system and can propagate vegetatively. A broken piece of a rhizome, as small as 2 cm, can form a new plant colony with shoots that sprout all along its rhizomes.

Think of how many new shoots a plant can have if its roots can grow three metres vertically and up to 20 metres horizontally. Interestingly, even though in their native range they reproduce by normal bisexual flowers, they can also have plants that are solely female and that do not propagate by seed. Seed dispersal is minimal as there are more female than male plants and each species of knotweed is predominantly one sex.

Japanese knotweed here in B.C. and all over Britain is an example of all the plant parts being female. So essentially a thicket of Japanese knotweed is one single female!

In B.C., knotweeds are currently found in the southwest British Columbia FRUIT GROWER • Summer 2011 19

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