UNITED KINGDOM — Homeowners in the U.K. are warned not to mistake autumn dieback for eradication as the invasive plant begins to become less visible
- Japanese knotweed begins to yellow and die back above ground during autumn
- Its underground rhizome system can remain alive even when visible growth disappears
- Homeowners could mistakenly assume an infestation has resolved itself as gardens change with the season
- Experts advise anyone who suspects knotweed is present not to cut, dig up or disturb the plant
Homeowners are being warned not to assume Japanese knotweed has disappeared simply because the invasive plant begins to yellow and die back as autumn approaches.
As the UK moves through September and temperatures begin to finally fall, Japanese knotweed will gradually become less prominent above ground, with its leaves yellowing and its stems eventually dying back.
Although the visible parts of the plant die back during autumn and winter, its underground rhizome system can remain alive and capable of producing new growth when conditions improve.
This means a patch of Japanese knotweed that appears to have vanished from a garden, boundary or overgrown area may still be present beneath the surface.
Following periods of extreme hot and dry weather over the summer, some homeowners may already have noticed stressed or weakened-looking vegetation. Experts say the arrival of autumn could make Japanese knotweed even easier to overlook, particularly for people who assume dying foliage means the problem has resolved itself.
Anyone who suspects Japanese knotweed is growing on or near their property is advised to avoid disturbing the area and seek professional identification before attempting any treatment or removal.
Why does Japanese knotweed appear to ‘disappear’ in autumn?
Japanese knotweed produces vigorous above-ground growth during spring and summer.
At the height of the growing season, established plants can develop tall, hollow stems, broad green leaves and dense foliage, while clusters of small white or cream flowers can also appear from late summer.
As autumn progresses, however, that growth begins to change.
Leaves can turn yellow before falling away, while the stems gradually die back and become brown and brittle through winter.
This seasonal dieback can make an established infestation much less obvious than it was just weeks earlier.
Crucially, however, the disappearance of visible growth does not mean the plant has been eradicated.
The rhizome system beneath the ground can remain alive throughout the dormant period and produce new shoots again the following growing season.
The original JKWS guidance already highlights this distinction: visible stems die back during autumn and winter while the underground system remains alive.
The signs homeowners may notice this autumn
Homeowners who have seen suspicious growth during the summer may notice several changes as the season turns:
Yellowing leaves
The broad green leaves associated with Japanese knotweed can begin to yellow as the plant enters its seasonal dieback.
Thinning foliage
Dense patches that were highly visible during summer may start to lose leaves and appear much less substantial.
Dying stems
The plant’s hollow, bamboo-like stems gradually die back and can become brown and brittle as autumn progresses.
Homeowners who recognise these changes in an area where Japanese knotweed may have been growing should not assume the plant has died permanently.
What should homeowners not do?
Suspected Japanese knotweed should not be treated in the same way as ordinary garden vegetation.
Homeowners are advised not to:
- Cut, mow or strim suspected knotweed
- Dig up the plant without an appropriate treatment or removal plan
- Move soil that may contain pieces of rhizome
- Place suspected stems, roots or contaminated soil in household or garden waste
- Assume that the disappearance of visible growth means an established infestation has gone
- Ignore suspected knotweed simply because it becomes harder to see during autumn or winter
These precautions carry directly through from Japanese Knotweed Specialist’s existing summer guidance, which warns that fragments can spread and small sections of rhizome can regenerate.
When handled professionally, Japanese knotweed can be controlled or removed using an appropriate treatment plan. The correct approach will depend on the location, extent of the growth and any plans for the affected property or surrounding land.
Vicki Marchbanks from Japanese Knotweed Specialists said:
"Autumn can be misleading for homeowners because Japanese knotweed naturally starts to die back above ground. If you have been watching a suspicious patch during the summer, seeing the leaves yellow and the stems begin to disappear can make it look as though the problem is resolving itself.
"Unfortunately, what you can see above ground is only part of the plant. The underground rhizome system can remain alive even after the visible growth has died away, which means the knotweed may return when the growing season begins again.
"If you think you may have Japanese knotweed, the safest approach is to take clear photographs, avoid cutting or disturbing the area and seek professional identification. The fact that a plant becomes less visible during autumn does not necessarily mean it is no longer there."
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