A 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.

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.

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).

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 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. The scheme is currently reaching the end of a four-year period of research and trials, and could see the imminent release of biological control agents (such as a sap-sucking Aphalara psyllid or a Mycosphaerella leafspot fungus) that will feed specifically on Japanese knotweed plants. If government consent is granted, these control agents could be released by early 2009.

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.