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Japanese Knotweed (Fallopia Japonica)
What is it, where did it come from?
Fallopia Japonica, or Japanese Knotweed as it is more commonly known is listed on the world conservation union’s one hundred worst invasive species list. Japanese Knotweed is a large, herbaceous perennial plant that is native to Eastern Asia, in Japan, China & Korea. It was first recorded in Great Britain in 1825, where it was brought over as an ornamental plant and became prized for its grand size and sprays of white creamy flowers. By 1886 it was established in the wild and now it has become widely established in the British Isles.
The main spread of these plants especially Japanese Knotweed through the UK has been through the watercourses, transport routes and infested waste areas. It is an invasive non-native plant and in the UK it has none of its natural pests, such as insects and fungi that feed off the plant to keep it at bay. This as well as its invasive and highly competitive nature has facilitated its spread through the UK
Japanese Knotweed spreads by means of its stems and rhizomes, new shoots can arise from tiny fragments, which is one of the reasons it can spread so quickly and is often so difficult to eradicate. The rate of spread is dependent on the density and composition of the soil but rhizome growth can extend 7m laterally and 2 to 3m in depth, although in reality rarely gets to these distances. Lengths of shoot can also reproduce new plants. The plant itself can reach a height of 3m. It has shield-shaped leaves which are flat at the base and are carried of zigzag stems. The stems are bamboo like sturdy, purple spotted and hollow with regular spaced nodes.
The plant flowers late in the season and they are creamy white coloured and are formed in drooping clusters up to 12cm in length. In winter, the leaves die back to leave orange/brown coloured woody stems which can stay erect for many years. In spring new emerging stems are green to red/purple with rolled leaves that unfurl as the shoot extends. The plant grows quickly through nutrients stored in the rhizomes of up to 40mm per day. The underground rhizomes are thick and woody with a knotty appearance and when broken reveal a bright orange coloured center.
FAQ's
There is a lot of information about Japanese Knotweed on the web, unfortunately its not all factual. Please look at our FAQ's page to learn the facts NOT fiction!
Identifying Japanese Knotweed
Spring
Spring
Spring
In Spring, little red buds emerge from the crown, once the weather warms up from here asparagus type shoots will grow at a vigorous rate, often with red and purple speckles on the green stems, with leaves still unfurled.
Summer
Spring
Spring
Most recognisable in the summer months Japanese Knotweed stems that have been untreated can reach 2-3m tall. The stem nodes grow in a zig-zag growth pattern with vibrant green spade shaped leaves. In late Summer around August time the plant flowers with creamy white cluster flowers.
Autumn
Autumn
Autumn
In Autumn it will start to naturally die back being a perennial plant, the leaves will wilt and turn brown eventually dropping off leaving the winter canes behind.
Winter
Autumn
Autumn
Just the winter canes will remain during the colder months. Often the zig-zag pattern is clearly visible. Although the canes are dormant, the rhizome below ground is still present and may continue to be viable through this time.
Treatment Methods
Herbicide
Herbicide treatment is applying herbicide to Japanese Knotweed in situ. It is the most cost effective means of treating Japanese Knotweed and can quickly reduce the capacity of the plant to spread. However it can takes years to achieve acceptable results as management plans must include at least 2 years of monitoring after all evidence of regrowth has stopped.
Stem Injection
The stem injection spray method ensures that only the plant targeted is treated, so that other more desired foliage can be kept and given the chance to thrive once again. The stem injection delivers a calibrated dose of herbicide directly into the bottom node of the stem, ensuring the herbicide is delivered to the crown and root system (rhizome) with no native species being affected. It can be particularly effective if the Japanese Knotweed is near a water course or if it is growing adjacent to specimen plants that you don't want to be affected by spraying a systemic trans-locating herbicide.
Membrane Root Barrier Systems
Vertical root barriers are used to minimise the risk of an infestation from an adjoining property spreading across the boundary. Other uses of root barriers - are to protect buildings or structures from being damaged ‐ particularly to line foundations if knotweed is in the vicinity. However although vertical barriers comply with Environment Agency recommendations, this method would only be advisable used in conjunction with an appropriate herbicide programme.
Excavation and Bunding
More suited towards new build schemes and construction projects, Japanese Knotweed excavation and removal solution removes the constraints of the typical length of time a knotweed herbicide treatment may take by using plant machinery to physically excavate the infected soil due for development. A temporary or permanent bund can be created on-site if space permits where the infected soil can be treated with a herbicide programme. Or the soil can be removed completely by transporting the Japanese Knotweed to a licensed landfill site. Due to this soil be categorised as controlled waste (Environmental Protection Act 1990) the costs associated with this method can be very expensive .
Stem injection treatment for Japanese Knotweed is very effective and only targets the plant treated.