Below we will answer the question; ‘How does Japanese knotweed spread?’ While also covering the plant’s characteristics and the legal responsibilities we face should we find Japanese knotweed on our land. We will also focus on some other important sub-questions including ‘How fast does knotweed grow?’ and ‘How far does knotweed spread?’ This information is essential in helping us understand, effectively treat, remove and remove the invasive weed.
How Does Japanese Knotweed Spread? A Complete Guide
Japanese knotweed is an invasive plant that can quickly take over gardens and landscapes if left unchecked. Understanding how this aggressive weed spreads is key to controlling and eradicating it. In this complete guide we’ll look at the origins of Japanese knotweed how it reproduces, and methods to stop its proliferation.
What is Japanese Knotweed?
Native to Japan, Taiwan and northern China, Japanese knotweed (Fallopia japonica) was introduced to Europe and North America in the 19th century as an ornamental plant. With its dense thickets and attractive heart-shaped leaves, it was seen as a decorative addition to gardens. However, Japanese knotweed grows and spreads very rapidly, crowding out native plants. It’s now considered an invasive weed in many areas across the UK, Europe and North America.
How Does Japanese Knotweed Spread?
Unlike many weeds that spread by seeds, Japanese knotweed spreads vegetatively. Its main means of reproduction and expansion are via its underground rhizome root system and the dispersal of plant fragments.
Underground Rhizome Networks
The primary method Japanese knotweed uses to spread is through the expansion of its underground rhizomes. These modified stems grow horizontally outward from the parent plant, up to 3 meters deep and 7 meters sideways. Even small fragments broken off from the rhizomes can regenerate into new plants. Pieces as tiny as 2mm have the ability to form new plants if dispersed to hospitable ground.
Digging out or otherwise disturbing the root system, such as during construction work, spreads knotweed by relocating fragments of the rhizomes. Screening soil carefully from these sites is essential to avoid spreading rhizome sections to uninfested areas.
Plant Fragments
Stems, crowns and canes from Japanese knotweed plants can also take root and generate new growth if dispersed to suitable ground. Cutting or striking the hollow, bamboo-like stems of knotweed in an effort to control it can unintentionally spread viable plant fragments. Properly disposing of any cut pieces rather than leaving them onsite is critical.
Fragments can also stick to tools, footwear, tires and other surfaces and be transported to initiate new infestations. Meticulous decontamination practices are key when working in or near Japanese knotweed.
Intentional Planting
While less common today, some continue to intentionally plant Japanese knotweed for aesthetics or erosion control, not recognizing its weedy invasive tendencies. It’s imperative to educate landscapers and homeowners to avoid buying or spreading this troublesome plant. Proper identification of knotweed is the first step.
Soil Movement
A common way knotweed infestations start is by relocating soil containing rhizome fragments. Dumping contaminated soil from an infested site can seed rapid Japanese knotweed growth if it includes any sections of root stock. Likewise, using material from a site with knotweed as fill or for landscaping purposes can spread the infestation. Careful screening of soil is essential.
Waterway Transport
The rhizomes of Japanese knotweed can break free and be transported downstream to colonize new areas along river banks, drainage systems and other riparian corridors. This has been a major factor in knotweed’s extensive distribution.
Vehicles, Footwear and Tools
Knotweed rhizome fragments in infested soil can readily stick to footwear, vehicle tires, tracks and tools. If these are subsequently moved offsite, the nearly invisible fragments knock loose and start new infestations wherever they land. During any soil disturbance, rigorous footwear and tool decontamination protocols are key to preventing spread.
How Quickly Can Japanese Knotweed Spread?
The underground nature of much of Japanese knotweed’s growth makes measuring its precise rate of spread difficult. But visible above-ground growth and proliferation can be extremely rapid:
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Emerges early spring and quickly achieves growth rates up to 10 cm per day once established.
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Can form dense, impenetrable thickets up to 4 meters tall during peak summer growth.
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The creeping rhizome network extends outward up to 7 meters horizontally and down 3 meters deep from each parent plant.
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Tiny rhizome fragments just 2mm in size can generate new infestations when dispersed.
Once introduced to an area, Japanese knotweed will quickly take over any unmanaged open ground. Its aggressive accelerated growth allows it to overwhelm competing vegetation in just a few seasons.
How Far Can Japanese Knotweed Spread?
Left uncontrolled, Japanese knotweed can spread indefinitely given adequate space and growth conditions. Unlike plants that spread by seed, knotweed has no innate limits or controls on the distance its running rhizome roots can extend. In the UK it has colonized large swaths of the countryside, assisted by soil movement and transport along waterways.
The size of individual knotweed stands is restrained only by proximate man-made boundaries like buildings, roads and competing plant life. But rhizome fragments can easily circumvent these barriers when spread by human activity or water. This staccato, leapfrog expansion allows Japanese knotweed to invade areas far from the original infestation.
Does Japanese Knotweed Spread by Seed in the UK?
In Japan, Japanese knotweed relies on seeds for much of its reproduction. But in the UK and most of Europe, it does not effectively spread by seed. Japanese knotweed has both male and female variants, and needs both present to pollinate and produce viable seed. But the UK only received the female form originally. With no male Japanese knotweed plants present in Europe to fertilize the females, seed propagation is extremely rare.
Stopping the underground rhizome growth is the key to controlling knotweed, since seeds do not play a meaningful role. However, small isolated instances of spread by seed have been reported, so vigilance is still warranted.
Can Animals or Birds Spread Japanese Knotweed?
Animals and birds can potentially spread Japanese knotweed. Grazing creatures like cattle, sheep, goats and horses may ingest young shoots and canes that could later regenerate if deposited in manure undigested. These animals could also unintentionally transport rhizome fragments caught in their fur, hooves or claws to new areas.
Rodents like rats, mice and voles that burrow and dig in contaminated ground may also unearth and move pieces of rhizome to spread knotweed. While not a primary vector, animal spread is possible, especially where infestations are disturbed.
Can Japanese Knotweed Spread on Shoes or Vehicles?
As outlined previously, shoes, boots, tools, tires and other surfaces or objects can easily pick up microscopic knotweed fragments when moving through contaminated soil. These miniscule pieces can then be transported far from the original site before falling off and potential initiating fresh growth.
During any earthworks, construction, gardening or other soil disturbance at sites with Japanese knotweed, rigorous cleaning protocols on people, tools and vehicles are essential to avoid spread. Dedicated washing stations and decontamination areas should be implemented to eliminate any fragments clinging to anything exiting the site.
Stopping the Spread of Japanese Knotweed
Preventing Japanese knotweed from advancing takes an integrated approach:
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Learn to identify knotweed. Be vigilant for any new growth.
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Follow all guidelines for safe disposal of knotweed and contaminated soil. Avoid moving soil offsite.
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Use screens, barriers and membranes to isolate and contain infestations.
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Implement professional chemical treatment plans using approved herbicides. Persistence is key.
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Excavate very small infestations if feasible, but properly dispose of all removed plant parts and soil.
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Monitor sites closely for several years post-treatment and quickly re-treat any regrowth.
With vigilance and commitment to best practices, the spread of invasive Japanese knotweed can be stopped and eliminated from infested sites. But preventing its advance requires dedication and persistence. Don’t let aggressive knotweed take over your property!
Can Japanese knotweed just appear?
Japanese knotweed doesn’t appear from thin air. Like any other plant, its origins should always be able to be traced back to an original place. Discovering the source of a Japanese knotweed infestation is almost as important as making the initial positive Japanese Knotweed identification. In order to determine where the plant has come from and when it first entered your land, you may need to consider whether the plant can be found anywhere in your local vicinity (in a neighbour’s garden or on adjacent publicly owned land). If there’s no sign of any knotweed near your land, then it’s possible that the plant may have originated from a batch of contaminated soil that was dumped on the land, or accidentally transferred from footwear or a vehicle.
How Does Japanese Knotweed Spread?
Japanese Knotweed spreads via dispersal of its rhizome fragments, stems and crown. UK Japanese Knotweed is typically spread when soil or water infested with knotweed fragments is relocated to a new area that was previously uninhabited by knotweed. This can happen accidentally or intentionally.
Japanese Knotweed can produce seeds, but it does not spread through seed dispersion because it is extremely rare for these seeds to germinate. Despite not being able to propagate via its natural method, Japanese knotweed has been able to spread throughout most of the UK via its stems, roots and crown since its introduction in 1840.
Introduced to the country via horticulturists in the late 19th-century from Japan [1], the plant has no controlling organisms to keep it in check. Whilst the climate in the UK is different from that of its native land, Japanese knotweed has been able to spread unchecked at the expense of native species, often commandeering large swathes of land.
Each Japanese knotweed plant is supported by an underground system comprised of stems, crowns and rhizomes which spread underground and periodically send shoots to the surface in order to fuel further growth. An entire plant can grow from a fragment of rhizome fragments as small as 10mm, therefore, if any soil beneath a Japanese knotweed patch is moved or dumped, a new crop can be expected to grow on the new site.
We can answer your Japanese knotweed questions!
How does Japanese knotweed spread?
FAQ
How easily does Japanese knotweed spread?
The rate of growth is very rapid. Consequently, Japanese knotweed can quickly colonise large areas of land if not controlled. Japanese knotweed spreads by disturbance and spreading of the root system that stimulates re-growth. It is often quoted that a fragment weighing just 0.7g can produce a new plant.
What kills Japanese knotweed permanently?
Glyphosate is the herbicide of choice for controlling knotweed. It is effective, has no soil activity, it is readily available, and somewhat inexpensive.
How do you prevent Japanese knotweed from spreading?
Spray with chemicals
Spraying or injecting the stems with chemicals can be an effective treatment to stop knotweeds spreading. You must only use approved herbicides. You’ll have to respray. It usually takes at least 3 years to treat Japanese knotweed.
How did Japanese knotweed get in my yard?
… spreads primarily by seed (transported by wind, water, animals, humans, or as a soil contaminant), stem fragments, and by shoots sprouting from its system …
How does Japanese knotweed spread in the UK?
In the UK, Japanese knotweed is most commonly spread via the dispersal of rhizome fragments. How does Japanese knotweed spread? It can happen in several different ways.
How does Japanese knotweed grow?
Japanese knotweed exhibits vigorous growth through the soil from a fibrous rhizome (creeping root system, much like underground stems). Soil can be contaminated with reproductive knotweed material up to a depth of 3m and a radius of over 7m (or wider in certain instances) from the main clump of stems. The rate of growth is very rapid.
Does Japanese knotweed spread through seed dispersion?
Japanese Knotweed can produce seeds, but it does not spread through seed dispersion because it is extremely rare for these seeds to germinate. Despite not being able to propagate via its natural method, Japanese knotweed has been able to spread throughout most of the UK via its stems, roots and crown since its introduction in 1840.
How do you spread Japanese knotweed?
You cannot spread Japanese knotweed simply by touching it. In order to spread it you would need to break off a piece of rhizome/root and place it in the ground somewhere. Fragments of stem can also cause new growth, but this is less likely than from rhizome fragments.
Does knotweed spread by seed?
In the UK, knotweed almost never spreads by seed. This is because only female plants were originally introduced in the 1800s, so there is no knotweed pollen. However, some plants can produce hybrid seed from closely related Russian vine pollen. Thankfully, this does not result in viable seedlings. Can I spread Japanese knotweed by touching it?
What causes Japanese knotweed?
Therefore the main culprit in the spread of Japanese knotweed is the inadvertent transportation of soils containing rhizome fragments. It is therefore vital that if you suspect you have Japanese knotweed you should get specialist advice as soon as possible.