What is Japanese Knotweed

What is Japanese Knotweed?

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.

Japanese knotweed was first discovered by Swedish naturalist Carl Peter Thunberg in the late 1700’s.

Distribution throughout Europe 1820-1850

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 Japanese 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 Japanese 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 Societyto 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).

Japanese 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" Japanese 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. Aphalaraitadori (as psyllid) has been released in controlled areas to naturally manage knotweed. However, at scientists have said, five years after the release of the bugs, that 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.

How does Japanese knotweed grow?

New Japanese knotweed growthIn the spring, new growth emerges as rapidly growing soft red/purple shoots reminiscent of asparagus spears. The stems are hollow and bamboo-like and can grow as much as 2 cm per day. As the shoots grow into canes the leaves unfurl and the plant turns green, with stem diameters of up to 40 mm (1.6 inches). The petioles, or leaf stems, are distinctive, with leaf growth on alternate sides producing a zigzag pattern in the stem.

The extensive rhizome system allows new shoots to be produced from any part of the rooting system in the spring, with a 1m² stand of knotweed capable of producing as many as 238 new shoots. This new growth forms dense thickets (which are known as ‘stands’) with new shoots remaining interlinked back to the parent plant. This means the shoots have access to the stored energy of the parent plant, allowing the new growth to possess sufficient energy to break through hard paved material. Established stands of knotweed with a large reserve of stored energy contained within the rhizome system can be vigorous enough to penetrate hard surfacing such as bitumen macadam and concrete. Rhizomes have a dark brown ‘knotty’ appearance and are bright orange inside. Snapping a fresh rhizome can be quite reminiscent in both sound and colour of snapping a carrot. The actual roots produced by the rhizome are fine, white and threadlike.

Japanese kntoweed elongated clusters of creamy-white flowersElongated clusters of creamy-white flowers appear towards the end of August and results in the production of seeds, which are currently non-viable in the UK. During late autumn/winter senescence occurs following pollination, when all of the moisture and nutrients are pulled down by the plant from the top growth into the crowns and rhizomes. Consequently, the canes die off and turn brittle. The crown, rhizome and root below ground remain dormant but alive throughout the winter. Rhizomes have been recorded surviving in frozen soil for a number of months. The crowns are hardier still and can survive composting and drying out. The stems die quickly after the first frost, leaving an unsightly mass of dead vegetation. The orange to brown coloured, woody, stems persist erect throughout the winter and the slow decomposition of leaf litter forms a deep organic layer preventing native seeds from germinating.

Find out more on our Japanese Knotweed Identification page.

How does Japanese knotweed reproduce?

Japanese knotweed reproduction is primarily by vegetative regeneration of rhizomes and fresh stemsReproduction is primarily by vegetative regeneration of rhizomes and fresh stems. In some cases the rhizome system could extend from a parent plant up to 7 metres (23 feet) laterally and up to 2-3 metres (6-10 feet) deep, although it is more common to find much smaller distances – unless particularly soft soil conditions or high bedrock encourages greater spread. Very small fragments of rhizome (as little as 0.7 gram – about the size of a small fingernail) can give rise to new plants. Fresh stems produce shoots and roots when buried in a soil medium or floated in water. The plant thrives where there is an available water supply for it to tap into and will be at its most prolific along open stream courses. Stem fragments in water may produce viable plants within 6 days. Japanese knotweeds’ ideal environment is in open damp positions with full sun, but it can grow with no discernible signs of stress in a variety of soil types, no matter how poor, with pH values ranging from 4.5 to 7.4. Common locations to find knotweed are railway lines, riverbanks, roads, footpaths, graveyards, disturbed/derelict sites or anywhere it has been dumped, dropped or deposited.

It thrives on disturbance and has been spread by both natural means and by human activity. In riparian areas, high water flows disperse fragments of the plant downstream where new colonies form. In the past, fly-tipping and transportation of soil containing rhizome fragments have been a major cause of spread, particularly in the urban environment. As landfill charges continue to increase, it is a sad thing to acknowledge that fly-tipping of knotweed is likely to continue.

Dealing with Japanese knotweed

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


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