Japanese Knotweed Identification and Treatment

What is Japanese Knotweed?

Japanese knotweed identification, first you need to identify the plant – so what is it? Japanese knotweed (Fallopia japonica) and its hybrids are a non-native, invasive species of plant. Since it was introduced into the UK as an ornamental garden plant in the mid-nineteenth century, it has spread across the UK, particularly along watercourses, transport routes and infested waste areas and of course private gardens.

Plants within their native range are usually controlled by a variety of natural pests and diseases. When plants such as this are introduced into new areas, that are free from these pests and diseases, they can become larger and more vigorous. They invade natural habitats and out-compete the native plants and animals that normally live there. Rivers, hedges, roadsides and railways form important corridors for native plants and animals to migrate, and large infestations of non-native weeds can block these routes for wildlife.

But how easy is Japanese knotweed identification?

Japanese knotweed is a rhizomatous (produces underground stems) perennial plant with distinctive branching, hollow, bamboo-like stems, covered in purple speckles, often reaching 2-3 m high. The leaves of the mature plant are up to 120 mm in length with a flattened base and pointed tip and are arranged on arching stems in a zig-zag pattern.

The plant flowers late in the season, August to October, with small creamy-white flowers. The rhizome system may extend to, and beyond, a depth of at least 2m and extend 7m laterally from a parent plant. During winter, the leaves die back to reveal orange/brown coloured woody stems which may stay erect for several years. Stem and leaf material decomposes slowly, leaving a deep layer of plant litter.

From March to April, the plant sends up new shoots, red/purple in colour with rolled back leaves. These shoots grow rapidly due to stored nutrients in the extensive rhizome system. Growth rates of up to 40 mm a day have been recorded. These features can help towards Japanese knotweed identification.

Winter

Japanese Knotweed does not enjoy the UK winter! After the first hard frost, the plant will loose its leaves and go into winter dormancy. The canes of Japanese Knotweed will continue to stand during winter but they will turn brown and become brittle.

Top Tip! – identifying Japanese knotweed is especially difficult in winter because of the lack of leaves and the canes looking ‘twig like’. You can also check the ground for fragments of old canes and also the crown of the plant for signs of growth areas!

Spring

Japanese Knotweed will start to grow as early as March in some places in the UK so it is important to be vigilant at all times of year! The growths will initially grow with triangular leaves and red stems. Sometimes they can look a little like asparagus!

Once the growths become more mature the leaves will ‘fill out’ and become less triangular and more ‘shield shape’ with a very flat base, smooth edges and a small tip. They will also lose their red colour and become a vibrant green, making Japanese knotweed identification much easier.

Early Summer

During summer, we have seen Japanese Knotweed grow upto 1 foot a week! The main canes of growth are primarily green with small red/purple specks on them. The stems that come off the main cane grow in a left/right zig/zag pattern. When identifying Japanese knotweed, remember the leaves are shield shape with a flat base, smooth edge and slight tip to the end of the leaf.

Late Summer

As the summer progresses and your Japanese Knotweed continues to grow taller and taller, it will get to a point where it stops growing in height and starts to flower in long creamy clusters. The flowering is designed for seed creation and dispersal but thankfully the seeds are not viable in the UK!

Japanese Knotweed is a fast growing, invasive plant and can cause problems in both Urban and Rural areas. The plant damages the urban environment such as gardens and road verges, by pushing up through tarmac and paving, out-competing other species in the area causing both aesthetic problems, such as ruining garden landscapes and also structural problems, such as when the plant grows into walls and cracks, pushing them apart. This is why Japanese knotweed identification is so important.

As well as rendering a property as ‘unmortgageable’ by the majority of the UK major lenders, the following are areas of residential property that can be adversely affected by the presence of Japanese Knotweed. Although these are the worst case scenarios it is worth considering when dealing with this invasive weed.

The plant can be very easily spread and in the UK the plant is mainly spread through rhizome fragments or cut stems. Greenhouse trials have shown that as little as 0.7 gram (finger nail) of rhizome material can produce a new plant within 10 days. Cut fresh stems have also been shown to produce shoots and roots from nodes when buried in soil or immersed in water. Once cut stem material has been allowed to dry out thoroughly and has reached the orange/brown ‘woody’ stage, there is no further regeneration. Rhizome material may take much longer to die and may remain dormant for long periods, possibly as long as 20 years.

The spread of Japanese knotweed is a serious threat to our countryside, and the native plants and animals that rely upon it. Tackling the plant should be taken very seriously with thorough planning and execution if we are to combat this invasive weed. If you are unsure whether you have been successful in identifying Japanese knotweed, get in touch with CYB Environmental.

A property is considered as being affected by Japanese Knotweed if it is found within the boundaries of a property or within 3 metres outside of the property boundary. To help understand the correct level of control, the Royal Institution of Chartered Surveyors (RICS) have created a framework, based on the management of Japanese Knotweed. This framework is the Japanese Knotweed Management Category assessment, categorised from A – D , with an approach on the management of Japanese Knotweed. The framework focuses on the damage that Japanese Knotweed may have caused, and the impact to loss of amenity space.

On-site burial and/or
encapsulation with membranes roots

Japanese Knotweed can be excavated and then buried on-site, but it must be covered with a specialist root barrier membrane to prevent any re-growth. A root barrier membrane can also be used to encapsulate Japanese Knotweed where space does not allow burial.