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A brief guide to the troublesome characteristics of Japanese knotweed

Prolific, resilient and fecund. These qualities give Japanese knotweed its notorious reputation as one of the most destructive species of non-native flora in the UK. This article outlines the ecological qualities that make Japanese knotweed so problematic in the UK.

First; prolific growth

Japanese knotweed is a strong competitor with prolific growth-rates; this damages local biodiversity through outcompeting native plants. Knotweed also behaves like a ruderal species, meaning it is often the first and only plant to colonise bare ground or newly disturbed land. This is unusual as most ruderal species make a lot of seeds; knotweeds produce seeds, however most don't grow into new plants. Instead, knotweed mainly spreads vegetatively, by parts of the plant breaking off and regenerating into new plants.

An ability to colonise bare ground rapidly and block-out other plants results in large semi-rural plots and urban spaces, such as railway sidings and riverbanks, effectively becoming bio-uniform ‘knotweed forests’.

Second, resilience

Knotweed’s resilience is due to a huge rhizome system - a below-ground horizontal stem that is highly resistant to environmental pressures, such as low winter temperatures.

The rhizome makes killing knotweed by surface application of herbicides very difficult and serves as a parent store of energy that new shoots are able to access resulting in vigorous potentially destructive growth.

Access to the energy reserve in the rhizome is how knotweed is able to grow through tarmac and gaps and cracks in brick and concrete. Therefore, Japanese knotweed growing on densely built up urban land, and development sites in particular, can result in costly disruption if left unmanaged.

Third; fecundity

Third, knotweed is exceptionally fecund in an unconventional way. New plants can potentially grow in four ways;

  • From rhizome: even fragments weighing > 1 gram, can develop into a new plant. Breaking up the rhizome also stimulates red bud production, each of which in turn, grows into a new stem.
  • From crown: this can survive drying or composting, the crown also produces new canes when in contact with soil or water.
  • From stem and leaves: new plants can grow from nodes of green stem (and occasionally leaf) pieces in contact with soil or water  - these are  often spread by traditional mechanical weed control tools such as strimmers or flails.
  • From seed: although it is unlikely these will germinate in the wild - all Japanese knotweed plants in the UK are female clones descended from a single specimen - the Sieboldii plant. 

Conclusion

An unusual degree of natural fecundity, resilience and prolific growth rates, combined with the myriad of human and natural vectors of dispersal, explains why knotweed has spread so rapidly across the UK.

The potential to generate major disruption, downstream costs and loss of value to property has elevated the technical challenge of controlling Japanese knotweed into an ethical and increasingly legal question.
 

At this stage of invasion, Japanese knotweed management at the national level is less about eradication and more about control. We aim to steer clients through these potential problems and orient them around a model of control that is long-lasting; the only one worth using.
 

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