Japanese Knotweed (Reynoutria japonica) is a perennial plant that can grow up to 10 feet tall and 15 feet wide. It has invasive roots, green leaves with five lobes, and white flowers that hang in clusters.
Many people consider this plant to be one of the most problematic plants in North America because it crowds out native species and grows quickly, often causing damage to buildings and roads during heavy rains.
The history of Japanese knotweed dates back hundreds of years when it was first introduced into Europe from Asia as an ornamental landscape plant for its attractive foliage.
However, it wasn’t long before people realized that the plant was difficult to control and it began popping up everywhere. Eventually, Japanese knotweed spread throughout much of North America, the United States and other countries, such as the United Kingdom.
It is now considered one of the most problematic invasive species in existence.
Why is Japanese knotweed so problematic?
So how did Japanese knotweed become such a widespread horticultural landscape plant?
First of all, this plant thrives in a variety of habitats, including disturbed areas like roadsides, ditches, stream banks, and open fields. This makes it an easy plant to spread if you’re not careful.
On top of that, knotweed can reproduce by both rhizomes (underground stems) and seeds. It is thought that the flowers on the plants act a little like a fly-wheel, carrying the seeds to new areas and depositing them into new habitats.
Oh, and did we mention that it is resistant to herbicides? Yup! It can grow back from an underground stem after being sprayed with chemicals.
If you look closely at the flower clusters on knotweed, you might notice tiny pieces of stem-like material attached to the flowers. These pieces are actually small rhizomes that detach from the parent plant. When they hit the ground, they develop into new knotweed plants with relative ease!
Where is Japanese knotweed originally from?
Japanese knotweed is native to Japan, but it is also found in China and Korea. The plant is one of a number of species that have been discovered growing on volcanoes’ sides. Japanese knotweed may reproduce spontaneously in its native region and benefit from a large underground rhizome system.
Knotweed is kept in control in its natural environment by natural predators such as fungi and insects, as well as other plants and an environment that can be far more hostile than the United Kingdom.
Who discovered Japanese knotweed?
The plant was found in the 18th century by Dutch scientist Maarten Houttuyn, who named it Reynoutria japonica, which it still bears today. Unfortunately, the records of this original finding were lost for a long time, so the plant was found and given a new name by a Bavarian botanist, Phillip von Siebold, when additional European botanists began studying Japan 150 years later.
The plant was given the name Polygonum cuspidatum by Siebold and his collaborator Zuccarini. It wasn’t until the start of the twentieth century that Makino, a Japanese botanist, determined that Houttuyn and Siebold’s plants were identical.
Who brought Japanese knotweed to the UK?
In 1850, Philip von Siebold brought Japanese knotweed to the United Kingdom, unaware of the environmental consequences. Botany and plant gardening were popular pastimes among the aristocratic classes at the time.
Botanists like Siebold could pay for their research excursions by returning their discoveries to nurseries on the European continent, where they would be nurtured and sold to commercial nurseries and botanical gardens all over the world.
Why was Japanese knotweed imported to the UK?
Japanese knotweed was brought to the United Kingdom for commercial purposes as well as botanical cultivation. Siebold collected and catalogued approximately 1,000 different plants during his time in Japan, often planting them in the garden behind his research facility on Dejima, an artificial island near Nagasaki.
Siebold would send a number of samples back to his home in the Netherlands on a regular basis. These plants would be carefully grown and then packaged for botanical gardens in Belgium and the United Kingdom from their new home in Leiden, with an unlabelled box containing Japanese knotweed arriving at London’s Kew Gardens in 1850.
These boxes of new plants were frequently used by Kew Gardens to maintain their own collection diverse and exciting for visitors. Horticulturists in Britain recognised Japanese knotweed as a plant that could be easily farmed and sold to aspiring gardeners after finding how quickly it could grow there.
Japanese Knotweed arrived in the UK thanks to German botanist and explorer Phillipp Franz Von Siebold, who brought the plant over to cultivate and sell as an adornment to the rich classes, as Japanese Knotweed’s bloom and bizarrely shaped leaves were the rage.
The plant was quickly supplied to garden nurseries across the country, and delighted gardeners shared cuttings. The plant began to expand on its own, occupying river banks and finding new paths to the rest of the country along roads and newly built railway lines, in addition to being purposefully disseminated by people.
How did Japanese knotweed spread throughout the UK?
Japanese knotweed spreads incredibly easily. It rarely produces viable seeds (those produced are generally not true Japanese knotweed seeds but hybrids which seldom survive) but is spread by its rhizome fragments (roots) and cut stems.
This mainly occurs through fly-tipping of contaminated garden waste containing its cut stem or rhizomes or through transporting soil from infested sites during construction work. It can grow in virtually any habitat and is very difficult to control once it is established.
Similarly, in river catchments, fragments of rhizomes or cut stems are often washed into watercourses which can then form new plants downstream.
The Japanese knotweed’s robust, almost woody stalks grew up to 20cm each day, at an unheard-of rate. This hardworking machinery proved to be particularly valuable during a period of strong industrialisation and development. Hundreds of these plants soon lined the gardens, bolstering banks along potential railway lines, and being joyously shared among gardeners.
Despite the worrisome rate of growth of this new discovery, Victorians thought they had it under control. The fact that only one gender of the plant was brought gave them confidence. Gardeners and scientists congratulated themselves on their resourcefulness in obtaining control of such a useful creature, but they hadn’t considered the potential that this alien plant may reproduce itself in another way.
Known as the ‘largest female in Britain’ by some horticulturists, every single Japanese knotweed plant is a clone of the original female plant that was introduced to Britain over a century ago.
On a cellular level, the plant’s outstanding dispersion across the country can be attributed to its architecture. Rhizomes are fragments of the plant’s roots that can travel far from their source and grow into completely new plants on their own.
The history of Japanese Knotweed is long and complicated. Whilst we now know a lot more regarding this invasive weed, it is clear to see that it will take decades to control its spread, let alone try and eliminate it permanently from most areas of the UK.
Understandably, Japanese knotweed will continue to thrive in the UK with its warm climate for many years to come. All we can hope and work towards is finding a more effective way to remove it.
Research continues at an increased rate due to how severe it is in damaging properties and its growing resistance to herbicides.
Japanese knotweed and its history will continue to be written about for a long time to come.
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