Young Japanese knotweed (Fallopia japonica) looks pretty, with bamboo-like stems and little white flowers. But don’t be fooled – this is an aggressive invasive plant that’s difficult to eradicate once it gets established. Its rhizomes are long and knobbly, and it’s able to mould itself to the surface it grows through, creating thickets that outgrow surrounding plants. It can even break through concrete and tarmac, spreading its roots into the new ground.
It’s important to recognise the growth pattern of this weed, which is very distinctive and easy to spot. Early spring is when it first appears, growing from the ground in a very fast-growing way. It often grows a couple of centimetres each day in warm weather, and can quickly outgrow adjacent plants. It usually begins life as asparagus-like shoots that have a reddish colour and can grow to be the length of your forearm.
As the season progresses, the shoots become a darker shade and develop purple speckles. They can grow to be 2-3 metres tall. The stems have a node-based structure, similar to bamboo, and they can be snapped easily in the hand.
The leaves also have a distinct shape, with a shield-like appearance and a lime green colour. They also feature purple speckles, and a distinct zigzag growth pattern as shown in the picture below.
In late summer, the weed produces small bunches of flowers which are often described as looking like tiny foxgloves. The flowers are followed by seed pods that burst open to release a black-brown ‘frill’ which is often described as being very similar to bamboo seeds.
This weed usually stays dormant over winter, but it’s still very much alive beneath the soil and can re-grow once it’s warm again. If you have a large infestation you may notice that it has a dead appearance over winter, with the canes becoming dry and brown.
Once the warmer weather comes, the plants will begin to re-grow, and by mid-spring they will be at their full height of between 1.9-3 metres tall. The leaves become less green and more rigid, with a dark muddier shade and deeper purple speckles. The stems have clear nodes and can be snapped easily in the hand. It is also possible to see a distinct silvery grey/blue colour to the canes if the plant has been exposed to light. During this time, it is also possible to see the rhizome system that makes the plant so invasive, with long lengths of clumping, dark brown, knobbly material sprouting from the ground. This is one of the key indicators that you are dealing with a mature young japanese knotweed plant. This is an invasive species that can cause serious damage to native flora and fauna, and should be removed as soon as it’s spotted. Please see our article on the eradication of Japanese knotweed for more advice and information.