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Japanese knotweed spread
Japanese knotweed is a prolific and widespread plant, having conquered every 10 square km patch of the UK since its introduction into the country in the 1840s. But how has it done it? The answer may surprise you!
How does Japanese knotweed grow?
Japanese knotweed is a perennial plant that grows up from the ground each year, dying right back in winter. Beneath the soil is where the real power house of the plant is located. Japanese knotweed has an extensive rhizome system that is the key to its reproductive success.
The plant’s growth starts with its rhizomes, which spread horizontally underground. These rhizomes can extend metres away from the parent plant, sending up new shoots as they grow. The rhizomes store nutrients, allowing the plant to survive in a wide range of conditions.
In early spring, the rhizomes produce fast-growing shoots that break through the soil. These shoots resemble reddish-purple bamboo-like stems, quickly growing up to 2-3 metres tall.
What is Japanese knotweed rhizome?
Rhizomes are specialised underground stems that grow horizontally beneath the soil. Unlike typical roots, rhizomes serve not only to anchor the plant but also to store nutrients and help it spread. They have nodes from which new shoots and roots can emerge, allowing the plant to propagate and expand its range.
Rhizomes differ from regular roots in that they can produce new plants. If a section of a rhizome is broken off, it can grow into a separate plant. This makes rhizomes an efficient means of reproduction, especially for invasive species like Japanese knotweed. They allow plants to survive harsh conditions, such as frost or drought, by storing food and water, and enable quick regeneration when conditions become favourable. Examples of other plants with rhizomes include ginger, bamboo, and ferns.
Why does Japanese knotweed spread so much?
The rhizome system of Japanese knotweed is particularly successful at helping to spread the plant for s3 key reasons.
- Extensive Spread – The rhizome system can spread horizontally for several meters. This wide-reaching system enables the plant to colonise large areas quickly. Even if the visible part of the plant is removed, the rhizomes remain underground and can sprout new shoots.
- Resilience and Regeneration – The rhizome system is highly resilient and capable of regeneration from tiny fragments. As little as 0.7 grams of rhizome can produce a new plant. This is why controlling Japanese Knotweed is so challenging: even small pieces left behind during excavation, construction, or disturbance can lead to new infestations.
- Dormancy and Survival – Rhizomes can remain dormant in the soil for extended periods, surviving extreme conditions such as frost, drought, or poor soil quality. Once conditions improve, they can spring back to life, regenerating the above-ground shoots even after long periods of inactivity.
How does Japanese knotweed spread in the UK?
Since its introduction in the 1840s, Japanese knotweed has become a notorious invasive plant that’s a major concern across the UK. Its relentless spread can cause significant damage to buildings, infrastructure, and the natural environment. Understanding the ways in which this resilient plant spreads is crucial to controlling its proliferation and protecting the landscapes of Britain.
There are 3 main pathways for knotweed to spread in the UK.
Natural Spread
Waterways
Rivers, streams, and other bodies of water can transport rhizome fragments downstream, where they can take root along riverbanks and floodplains. This mode of dispersal is particularly concerning because it allows the plant to colonise large areas relatively quickly, often in locations that are difficult to access and treat.
Human Activity
Human activities are a significant factor in the spread of Japanese knotweed. Gardeners who unknowingly move contaminated soil or plant material can cause new infestations. Fly-tipping, the illegal dumping of waste, is another major contributor. It can also be inadvertently spread during infrastructure projects, such as road building or railway maintenance, where soil is disturbed and relocated.
FAQ
Yes, Japanese Knotweed can potentially be spread on shoes. If small fragments of the plant’s rhizomes or stems get stuck in mud or soil on your shoes, they can be transported to new locations. Since even tiny pieces of rhizome, as little as 0.7 grams, can grow into a new plant, walking through an area with knotweed and then tracking soil elsewhere can cause the plant to establish in a new spot. To prevent this, it’s important to thoroughly clean shoes after being in areas known to have Japanese knotweed, particularly if you’ve been in disturbed soil.
There is little evidence that mowing knotweed will cause knotweed to spread from stems, but it’s not impossible for fragments, especially rhizome pieces, to regenerate into new plants if they land in favourable conditions. Mowing can therefore unintentionally cause the plant to spread to new areas, both within the same location and through the movement of contaminated equipment.
Mowing more commonly causes knotweed to spread by encouraging the growth of new rhizomes over a wider area as the plant looks to survive by extending out beneath the ground to search for more favourable growing conditions.
Yes, Japanese knotweed can spread on tools. If gardening, construction, or landscaping tools come into contact with the plant, particularly its rhizomes or stem fragments, those small pieces can cling to the tools. If the contaminated tools are then used in a different area, the plant material can be introduced to a new location, potentially leading to new growth.
To prevent this, it’s essential to clean tools thoroughly after working in areas with Japanese knotweed. This helps minimise the risk of unintentionally spreading the plant to new sites.
No, Japanese knotweed does not typically spread by seed in the UK. While the plant does produce seeds, they are generally not viable outside its native range in East Asia as all the plants in the UK are female. Japanese Knotweed spreads almost exclusively through its vegetative means, primarily via its rhizomes (underground root system) and stem fragments. These pieces can regenerate into new plants, making the spread of the plant largely reliant on the movement of contaminated soil, plant material, or fragments.
No, Japanese Knotweed does not spread by spores. Unlike fungi or ferns, which reproduce via spores, Japanese knotweed spreads through its rhizomes (underground root system) and stem fragments. In the UK, it doesn’t reproduce through viable seeds, so its primary method of spreading is vegetative.
No, Japanese knotweed does not spread by spores or seed, so it cannot blow into your garden on the air. It can however sneak over your boundary underground, as the creeping rhizomes of knotweed do not respect property boundaries. Find out more about Japanese knotweed encroachment here.
It is impossible for birds or insects to spread Japanese knotweed in the UK. Burrowing animals like badgers and rabbits do however move soil around in large quantities which can cause knotweed to spread. Their disturbance of the soil, and fragmentation of the rhizome system can also break the dormancy of herbicide – causing plants to regrow.
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