What is Japanese Knotweed

What is Japanese Knotweed?

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Japanese knotweed is an invasive and resilient weed. Its roots and rhizomes can grow to a depth of 2m. Even after herbicide treatment has “eradicated” the aerial and surface growth, the deep underground rhizomes can remain in a viable state and may do so for up to twenty years. It can re-emerge and re-grow on its own accord at any time and especially if the contaminated ground is disturbed.

If knotweed is left to grow untreated for a number of years it has the potential to cause damage to drains, paving, paths, driveways and poorly constructed boundary walls. For this reason it should not be ignored.

History of knotweed

Japanese knotweed was first discovered by Swedish naturalist Carl Peter Thunberg in the late 1700’s. It was rediscovered between 1823-1829 by Philipp Franz Balthasar von Siebold, a Bavarian doctor and naturalist working for the Dutch army. From a nursery he set up in Leiden in Holland, von Siebold distributed the plant throughout Europe from around 1848. Taking advantage of its success following its award of the Gold Medal from the Society of Agriculture and Horticulture at Utrecht in 1847, von Siebold aggressively marketed the plant. Using its vigorous growth habit and its penchant to form dense screens as major selling points, knotweed was sold as an ornamental fodder plant with the ability to stabilise sand dunes. Sales were widespread.

How did knotweed get to the UK?

In 1850, the Leiden nursery despatched an unsolicited parcel of plants, including Japanese knotweed, to the Royal Botanical Gardens at Kew. In 1854 a knotweed specimen arrived at the Royal Botanic Gardens in Edinburgh. This is the same year that a nursery in Kingston became the earliest recorded nursery to offer knotweed for sale in Britain. Thereafter it was sold and distributed by a large number of commercial nurseries and amateur enthusiasts and was reported as becoming naturalised in the UK by 1886.

Dangers of knotweed identified late 19th century

By the end of the Nineteenth Century and the beginning of the Twentieth, the dangers of the plant were becoming apparent. An account of the flora of Alexandra Park in Oldham, published in 1887, observed how knotweed kept appearing “unexpectedly in nearly every piece of cultivated ground.” Gertrude Jekyll, previously an ardent admirer of Dwarf Japanese knotweed (Fallopia japonica var. compacta), admitted in 1899 that knotweed should be planted with caution and that her beloved Dwarf knotweed must be reduced. In 1905, the Royal Horticultural Society urged readers of their Journal of the Royal Horticultural Society to not plant knotweed unless it was "most carefully kept in check".

By this time it was too late to reverse the impact of knotweed and the plants’ current status is increasing and invasive. Despite the warnings it was not until as late as 1981 that the British government saw fit to produce legislation that specifically controlled its sale and spread (The Wildlife and Countryside Act).

Knotweed a "menace" outside its natural environment

It has only become a worldwide menace since leaving its natural environment in Asia. In its native habitat it is kept in check by natural means – in Japan at least 30 species of insect and 6 species of fungus feed on the plant. Outside of its natural habitat, these species do not exist and, with no natural predators, the plant is thriving.

Trials to control knotweed growth

Trials have been conducted in the UK to enable the safe release of some of knotweed’s natural predators from Japan into Britain – without causing a knock-on effect on any other species. Aphalara itadori (a psyllid) has been released in controlled areas to naturally manage knotweed. However, as scientists have said five years after the release of the bugs, more time is needed to assess whether the project has been successful.

Once released, they will take several years to have any real effect (typically 5-10 years). It is important to note that these agents will ensure control only rather then eradication and that they will only feed on Japanese knotweed and not any of its hybrids or related species.

Dealing with knotweed

Below is a video published by the Property Care Association (PCA). Find out more about the PCA here.

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