Invasive species such as Japanese knotweed (Reynoutria japonica) are notoriously difficult to remove once established, so it’s important to identify them early and take action accordingly. The key to identifying knotweed is to carefully consider the plant’s appearance, particularly the shape of its leaves, stems and flowers. Using these visual cues, along with a few other key points of reference, you can be confident in your identification and take the right steps to eradicate knotweed from your land.
The defining characteristics of how to identify japanese knotweed are its thick, bamboo-like stems and branched sprays of white flowers. These clustered sprays of flowers grow from late summer into autumn and can be very dense. Despite having complementary male and female organs, the plants produce unisexual flowers which function as a seed dispersal mechanism. Infestations of Japanese knotweed can spread by seed (transmitted by wind, water, animals or people) and by shoots sprouting from its extensive system of rhizomes.
Unlike most weeds, the leaves of Japanese knotweed are bright green and feature a distinctive shield-shaped or heart-shaped pattern with pointed ends. They are also arranged in a zigzag fashion, unlike similar-looking plants which have their leaves positioned at staggered intervals alongside each other.
As they mature, Japanese knotweed stems become more and more rigid with pronounced dark purple speckles and clear nodes. This gives the plant a more bamboo-like appearance, and can result in them reaching up to 3 metres in height. Once mature, the plant produces a small bunch of creamy white flowers in late summer. This is another key identifier, as the flowers have a distinct fragrance and are a distinguishing mark against other plants that are in flower at the same time.
Once the flowers have died, the stems of Japanese knotweed will slowly degrade into hollow stems over winter and can even be seen growing in the middle of winter if exposed to the right conditions. It is not uncommon for knotweed to emerge from cracks in walls or brickwork, and it is important to be vigilant and check for the plant throughout the winter.
As you may have gathered from the information above, Japanese knotweed is extremely fast-growing and can easily overtake and overwhelm a garden, if not fully controlled. If you suspect that you have a Japanese knotweed infestation on your property, it is essential to contact a specialist and book in an identification survey. This will be much more effective than waiting until the plant reaches maturity and begins to flower in late summer, as this will significantly delay treatment and cause the problem to escalate. To help with the identification process, you can use an incredible smartphone app called PictureThis which is a free-to-use plant encyclopaedia and identifier that can recognise thousands of different plants and their characteristics in seconds! To get started, simply photograph your suspected plant and then upload the image into the app. You can learn more about this incredible app here.