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Giant Knotweed vs Japanese Knotweed
Feb 02, 2021

What is giant knotweed?

Giant knotweed is the largest of three closely related invasive knotweeds that are found in this area and are listed as noxious weeds. They all share habitat and can occasionally be found growing together. In North America, these imported knotweeds are not held in check by natural enemies and is capable of thriving and spreading in a wide range of conditions, especially riverbanks, roadsides and other moist, disturbed areas. Containment and control of all the invasive knotweeds is highly challenging but very important in order to protect uninfested areas from the damage caused by this group of plants.

japanese knotweed

What is japanese knotweed?

Japanese knotweed (Polygonum cuspidatum Sieb. & Zucc.), a member of the buckwheat family, was introduced into the U.S. from Eastern Asia (Japan, China, Korea) as an ornamental on estates in the late-1800s. It has also been used as an erosion control plant. By the mid-1890s, it was reported near Philadelphia, PA, Schenectady, NY, and in New Jersey. Although once sold through seed and plant catalogs, by the late-1930s knotweed was already being viewed as a problematic pest. The plant, which can grow from three to 15 feet tall, has bamboo-like stems and is sometimes called Japanese bamboo. As with many invasive plants, knotweed thrives in disturbed areas and once established can spread rapidly, creating monoculture stands that threaten native plant communities. Japanese knotweed can tolerate deep shade, high temperatures, high soil salinity and drought. It is commonly found along streams and rivers, in low-lying areas, disturbed areas such as rights-of-way, and around old home and farmsteads.


Is giant knotweed the same as japanese knotweed?

Giant knotweed and Japanese knotweed both arrived in Europe and the UK as exotic Asiatic plants to be used in fashionable gardens. The two plants share similarities but have distinct differences.It should be understood that when Japanese knotweed was brought to Europe by a noted Dutch horticulturist in the 1800s, only one female plants was imported.


This means that the rapid reproduction of Japanese knotweed in Europe is the result of the plant’s ability to adopt to different partners. Giant knotweed is one of the preferred reproductive partners for Japanese knotweed.


Giant Knotweed

The Latin name form Giant knotweed is Fallopia sachalinensis. The species is indigenous to the island of Sakahlin, an island north of Japan. Like Japanese knotweed, it was brought to Europe to provide a tempting option to high end gardening enthusiasts.


Giant knotweed shares some physical similarities with its cousin, Japanese knotweed. It can grow to heights of 4 meters and has leaves that range from 20 to 40 centimetres.


Like Japanese knotweed, Giant knotweed also has flowering leaves that are greener and more rounded than the leaves of its cousin. The leaves of Giant knotweed plants are rounded at the base and often have scattered hairs on the underside.

Giant Knotweed

Giant knotweed has reddish shoots that first emerge in the early spring. The shoots grow rapidly and can achieve heights of 4 meters quite quickly. At the same time, the roots of Giant knotweed extend deep into the ground, about 2 meters vertically and range horizontally up to 15 or 20 meters. The plant is a voracious eater. Food is stored in the stout rhizomes so that as the shoots are hindered by certain natural elements like frost, the food reserves in the rhizomes are unaffected.


This allows the plant to survive UK winters and come back quickly and wildly.


The rhizomes develop pink coloured modules in early spring. From these modules, shoots begin to sprout as early as April. However, Giant knotweed stems from previous years may still be present and can generate new shoots for up to another 12 months.


The more established the Giant knotweed, the woodier the stocks. These seem to increase in mass as the plants ages. They continue to generate horizontal spreading and creeping rhizomes that can spread under the surface of the ground.


The Spread of Giant Knotweed

Giant knotweed only reproduces for vegetative purposes. Studies of the plant indicate that the species can produce viable seeds but the more conventional way for the plant to spread is by encroaching on established clumps or by the spread of small pieces of the plants. Once Giant knotweed invades an area, in even the most minute manner, it is stubborn and will take root.


If pieces of the plant float downstream it will often grow where it takes settles. Like Japanese knotweed, Giant knotweed will take over an area forcing native plants to decline and eventually fail.


Also, like Japanese knotweed, the handling of Giant knotweed is a stressful and careful undertaking. Only licensed landfills can accept the plant for waste. There is currently no easy way to dispose of either species.


Gardeners that attempt to remove the plants are in for a considerable ongoing battle, especially during the growing season. The tenacious character of the two plants is undeniable as they have disrupted public water and sewer lines and caused foundations to be compromised.


Detection and Elimination

In the UK, the best way to handle Giant knotweed and Japanese knotweed is to contact a licensed professional as soon as the plants are identified. If you think the plants have arrived at your property, in your water lines or are beginning to surface in your foundations, do not hesitate to contact a professional. The quicker you act, the better your chance of effective eradication.

giant knotweed vs japanese knotweed

How much does it cost to get rid of japanese knotweed?

What other potential costs of Japanese knotweed could there be to property owners and landowners? Japanese knotweed is able to ‘spread like wildfire’, so it’s the responsibility of every individual to contain it within their property once its growth has been discovered. It’s not against the law to have Japanese knotweed on your property, but inaction that allows it to spread to any surrounding land is an offence that could incur fines of up to £20,000 if you are a business. 


The sheer scale of the Japanese knotweed problem in the UK has pushed the government to intervene and help to control its growth and spread. Currently, there is legislation in place to control the weed, with the government working with various organisations specialising in its removal. The aggressive and invasive nature of Japanese knotweed has led to the rapid decline of property value for many homeowners. Surveyors are well aware of the costly damage that the weed can cause, so it is not uncommon for mortgages and planning permissions to be refused on this basis. 


Aside from the costs of removing Japanese knotweed itself, it is clear there are additional ‘costs’ to property owners, and these will only increase the longer the infestation is left unchecked. By acting now will not only avoid additional damage, but also the costs of any potential legal action. 

For more information about Giant Knotweed or Japanese Knotweed, get in touch with us today. Contact us at email: [email protected]


References:https://www.kingcounty.gov/services/environment/animals-and-plants/noxious-weeds/weed-identification/invasive-knotweeds/giant-knotweed.aspx

https://tpknotweed.com/guidance/giant-knotweed-vs-japanese-knotweed/

http://nyis.info/invasive_species/japanese-knotweed/

https://www.jksl.com/how-much-does-it-cost-to-treat-japanese-knotweed

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