Everything you need to know about Japanese knotweed

As soon as the temperatures rise, Japanese knotweed re-emerges. What starts as a few young shoots can quickly turn into a major infestation. Its rapid growth makes the plant very difficult to control. It is, after all, one of the most invasive weed species.  

How can you identify the plant?

Japanese knotweed is a plant that can grow rapidly, sometimes reaching heights of up to four metres. The stems are immediately striking: they are hollow, segmented and resemble bamboo, with a greenish colour and reddish patches. The leaves are large and have a distinctive heart shape. At the end of summer, usually in August and September, light, creamy-white clusters of flowers appear. In winter, the plant appears to have disappeared as the above-ground parts die back. However, it remains present underground. The rhizomes survive effortlessly and ensure that the plant sprouts again in spring.

What happens throughout the year?

Japanese knotweed has a distinct annual cycle. In spring, the young shoots shoot up rapidly, sometimes by several centimetres a day. In summer, the plant grows into dense, sturdy foliage with large leaves that easily crowd out other plants. By autumn, the plant withdraws all its energy back into the roots and tubers underground. Above ground, everything seems to have disappeared, but underground the plant simply carries on living. In winter, there is often nothing visible, but the roots remain ready to send up new shoots again the following spring. The problem seems to have gone, but it hasn’t...

Why is it so difficult to control?

The main problem lies underground. The roots can grow deep and far, and even a small piece left behind can grow into a new plant. Japanese knotweed also spreads easily via soil, garden tools or building materials. Simply pulling it up or mowing it down often doesn’t help: the plant usually just comes back. If you really want to see results, you need to check regularly, remove it carefully and make smart use of, for example, a specialised root barrier such as Dupont™ Plantex® Platinium.

When is the best time to control the plant?

Effective control primarily requires a consistent and long-term approach throughout the entire growing season. Covering with, for example, Platinium Plantex can be applied year-round and helps to structurally weaken the plant. By depriving it of light and growing space, development is inhibited and the root system gradually becomes exhausted.

Additional measures, such as regularly removing above-ground parts, can help to control growth. However, this method is labour-intensive and therefore costly, particularly when it needs to be maintained over a longer period.

An integrated approach, with an emphasis on continuous covering and maintaining this over several seasons, is essential to effectively control the plant.