Japanese Knotweed: Tamar NRM Takes Action
Japanese knotweed may look like an ordinary garden plant, but it can have a significant impact if left untreated. Tamar NRM will soon be undertaking control work to tackle localised infestations around the Tamar, preventing further spread. Read on to find out more:
Japanese knotweed (Fallopia japonica) is native to Japan, Korea, Taiwan and eastern China, though has become a serious weed in parts of Europe. It is widespread across the UK, where it was introduced as an ornamental in the early 19th century.
Under legislation in England, Wales and Scotland, it is an offence to cause Japanese knotweed to grow in the wild. A man in England was fined £200,000 in damages for failing to declare it was present on his property when he sold it! Why? Because Japanese Knotweed is remarkably difficult to kill, recolonising and stabilising the newly bare earth exposed following volcanic activity. It is perfectly capable of staying dormant underground for 20 years, using its rhizome like a battery before sending shoots through cracks in concrete, bricks or asphalt.
The plant is extremely difficult to control. Large infestations often need to be fully excavated, then sealed in a membrane and wrapped in copper foil before being reburied, effectively imprisoned and unable to re-shoot. Some estimates suggest knotweed infested properties in the UK have lost a collective £20bn from their value, not to mention the high costs of ongoing control. As this weed is considered emerging in Tasmania, the situation is far from being that dire, though it may represent a vision of the future if this plant is left unmanaged.
Let’s go through a few key identifying features:
Spade/shield-shaped leaves: It is important to note the flat base where the stem is joined to the leaf. The leaves also grow alternately along the stem. The leaves are 10-15cm long and a lush green colour through summer.
Purple speckles on the main stem: self-explanatory, be on the lookout for bamboo-like nodes as well.
Zigzag lateral stems: Japanese knotweed has a main stem not dissimilar to a bamboo cane, reaching up to 3 metres in height. The lateral stems that shoot off from the main stem have a slight zigzag pattern, although it is important to note this may not be as apparent in smaller, less mature plants.
Creamy-white flowers: Japanese knotweed flowers are tiny and emerge in late summer on a flower spike generally 10cm long.
Japanese knotweed will then die back in winter, leaving dead canes. During spring, shoots will emerge that are somewhat asparagus-like in appearance. As they grow, they will often have a reddish-purple colour with rolled back leaves. Summer is a time of dense growth, often gaining metres in height seemingly overnight. Flowers will then form during late summer to early autumn. Somewhat mercifully, Japanese knotweed in Australia is a clone of the UK variety and does not produce seed. This means that the predominant method of spread is related to human activity.
In the Tamar, Japanese knotweed is currently restricted to a small number of private properties in Invermay, Launceston, West Launceston, Lilydale and Beauty Point. In collaboration with Biosecurity Tasmania, Tamar NRM will be visiting these properties in late November to begin the first round of control. Any Japanese knotweed will be cut at the base, with herbicide applied directly to the stem, allowing it to draw down into the rhizome. The sites will be revisited in March of 2027 for monitoring and a second round of control work. Following this, Tamar NRM will be empowering the property owners with Japanese knotweed infestations to undertake their own ongoing control. As we’ve mentioned, this plant often has an extensive underground rhizome which allows it to weather surface disturbance. Due to this, ongoing control and community collaboration will be key to mitigating the spread of Japanese Knotweed across the Tamar.
As a community member, there are a few ways you can help! Keep an eye out for this plant, especially if you are within the hotspots noted. If you find something that resembles Japanese knotweed, please contact us at admin@tamarnrm.com.au, or report the plant to Biosecurity Tasmania at invasivespecies@nre.tas.gov.au for identification (sending through good quality images of the leaf, stem and overall growth habit will help a lot!). In the meantime, do not disturb, cut or move any part of the plant or soil until further advice has been given.
Invasive plants have a serious impact on Tasmania’s environments, agriculture and economy. Many of them now persist in densities impossible to control with our current collective resources. Japanese knotweed is still at a manageable scale in the Tamar, giving us an opportunity for early intervention. Through control, monitoring and empowering the community, Tamar NRM and Biosecurity Tasmania are working to prevent this emerging weed from becoming a much larger problem.
Alongside Japanese Knotweed, we run a variety of other weed management programs guided by Tamar NRM's Tamar Valley Weed Management Strategic Plan. We welcome you to contact us at admin@tamarnrm.com.au for weed related enquiries or guidance on control.
Sources:
GB Non-Native Species Secretariat (n.d.) Japanese Knotweed (Fallopia Japonica): identification. Available at: https://www.nonnativespecies.org/assets/Uploads/ID_Fallopia_japonica_Japanese_knotweed.pdf (Accessed: 9 November 2026).
Japanese Knotweed Agency (n.d.) Japanese knotweed. Available at: https://japaneseknotweedagency.co.uk/japanese-knotweed/ (accessed 10 November 2026).
PBA Solutions (n.d.) ‘Five easy ways to identify Japanese Knotweed’, PBA Solutions. Available at: https://www.pba-solutions.com/japanese-knotweed-news/five-easy-ways-to-identify-knotweed/ (Accessed 9 November 2026).
The Guardian (2023) ‘The war on Japanese knotweed’, the Guardian. 16 May. Available at: https://www.theguardian.com/environment/2023/may/16/the-war-on-japanese-knotweed (Accessed: 9 November 2026).
