What are the characteristics of knotweed?

As one of the most invasive plants on the planet, Japanese knotweed can cause major damage to buildings and infrastructure as well as lowering property values. This is why it’s important to recognise this plant as early as possible and take the right action to eradicate it. The best way to identify knotweed is to look at the distinctive appearance of its leaves, stems and flowers. However, it can be difficult to see the tell-tale signs of this plant as it can change its appearance depending on the season.

As the weather warms up in spring Japanese identify japanese knotweed knotweed shoots begin to emerge from its underground rhizome network. The shoots have a bamboo-like appearance and can grow up to 2-3 metres tall. It also has a distinctive spade or shovel-shaped leaf that can be up to 14cm (5.5in) long and is shaped in a zigzag pattern. The leaf is rolled back at the base and a deep green in colour with purple or red speckles.

From early to mid-summer, the plant goes through a period of exponential growth. The plant can form dense clumps of foliage and at this time, it can be easy to spot due to its height. In late summer and autumn, the plant produces distinctive creamy-white flowers that are arranged in conspicuous clusters. Once the flowers have faded, the plant will begin to shed seed cases. By the end of the autumn, the leaves will turn yellow and then brown, before becoming brittle and turning to a straw colour in winter.

Unlike many other weeds, Japanese knotweed spreads mainly by its seeds which can be carried by water, animals, people or as soil contamination. It can also spread through its branched sprays of tiny, winged fruits or by stem fragments. This plant is able to survive and thrive in a range of conditions, including poor soil, shade, extreme temperatures and low light levels. It grows in a wide variety of sites including riverbanks, canal banks and railway embankments, and around old homes and farmsteads.

Knotweed can grow in dense stands and can cause serious problems for surrounding vegetation, infrastructure and properties. It is highly invasive and can spread rapidly, taking over large areas of land. It can also smother and degrade native vegetation, block drainage channels and disrupt wildlife habitats.

The invasive nature of Japanese knotweed means that it can be very hard to eradicate. This is because it is a rhizomatous plant which means that the root system will remain in the ground underground even after the shoots have died back in the autumn. The only way to fully remove a stand of Japanese knotweed is to dig out all of the rhizomes, which is impractical and costly.

The invasive plant management team at Acadia National Park actively manages a number of stands of Japanese knotweed, which is primarily achieved through the yearly cutting and re-establishment control. The park’s invasive plant team also use herbicide treatments to prevent any new re-growth. The team have a hesitancy to fully dig up the root systems as this can damage adjacent native plants, increase soil disturbance and is often not effective in preventing regrowth.