Question 1

What is Japanese knotweed?

Accepted Answer

Japanese knotweed is a contentious plant that can depreciate the value of a property and cause conflict between neighbours. The invasive plant is known for spreading quickly and for monopolising gardens. Knotweed grows in large patches that can quickly push out any other competing plant life. Thankfully, Japanese knotweed is easy to identify, so homeowners can waste no time in getting a treatment plan in place to get rid of the plant.

Question 2

Where is Japanese knotweed originally from?

Accepted Answer

Japanese knotweed is originally from Japan and is also native to China and Korea. The plant is one of many species of plants that have been discovered to be growing on the side of volcanoes. In its native land, Japanese knotweed can reproduce naturally and also benefits from a prodigious underground system of rhizomes. In its native environment, knotweed is kept in check by natural predators in the form of fungi and insects, not to mention other plants and an environment that can be much more hostile than the United Kingdom.

Question 3

Who discovered Japanese knotweed?

Accepted Answer

The plant was originally discovered by Dutch naturalist Maarten Houttuyn in the 18th-Century, he named it Reynoutria japonica, a name which it still goes by today. Unfortunately, the records of this original discovery were lost for some time, so when more European botanists started exploring Japan some 150 years later, the plant was rediscovered and given a new name by a Bavarian botanist, Phillip von Siebold. Siebold and his partner Zuccarini, named the plant Polygonum cuspidatum. It wasn’t until the turn of the 20th century that a Japanese botanist by the name of Makino, discovered that Houttuyn and Siebold’s plant were one and the same.

Question 4

Who brought Japanese knotweed to the UK?

Accepted Answer

Philip von Siebold brought Japanese knotweed to the UK in 1850, unaware of the impact that it would go on to have on the environment. At the time, botany and the cultivation of plants was a popular interest of the upper classes. Botanist like Siebold would be able to fund their research trips by sending their discoveries back to nurseries on the European continent, where they would then be cultivated and sold onto commercial nurseries or botanical gardens around the world.

Question 5

Why was Japanese knotweed imported to the UK?

Accepted Answer

Japanese knotweed was imported to the UK for commercial sale and botanical cultivation. Over the time that he spent in Japan, Siebold collected and recorded over 1,000 different plants, often choosing to plant them in the garden behind his research base on Dejima, an artificial island near Nagasaki. Periodically, Siebold would box up a number of samples and send them back to his home in the Netherland. From their new home in Leiden, these plants would be carefully grown and the packaged up for botanical gardens in Belgium and Britain, with an unmarked box containing Japanese knotweed arriving at London’s Kew Gardens in 1850.

Kew Gardens often relied on these boxes of new plants to keep their own collections diverse and interesting for visitors. After discovering how quickly the plant could grow in Britain, the horticulturists there recognised Japanese knotweed as a plant that could be easily cultivated and sold on to budding gardeners. Soon the plant was being distributed to garden nurseries around the country and then shared in cuttings by enthusiastic gardeners. In addition to being intentionally spread by people, the plant also began to spread by its own means, populating river banks and finding new routes to the rest of the country along roads and newly built railway lines.

Question 6

How did Japanese knotweed spread throughout the UK?

Accepted Answer

The strong, almost woody stems of the Japanese knotweed grew up to 20cm a day: a rate previously unheard of. At a time of intense industrialisation and construction, this hard-working plant proved to be extremely useful. Soon scores of these plants were lining the gardens, shoring up banks along prospective railway lines and being enthusiastically shared between gardeners.

Despite the alarming rate at which this new discovery grew the Victorians believed that they had it under control. Their confidence was derived from the fact that only one gender of the plant was imported. Gardeners and scientists applauded themselves with their ingenuity in gaining control over such a useful organism, but they hadn’t considered the possibility that this alien plant might have another way of reproducing itself.

Known in some horticulturist circles as ‘the largest female in Britain’; to this day, every single Japanese knotweed plant exists as a clone of the original female plant that was brought to Britain well over 100 years ago. The plant’s impressive dispersion across the country can be contributed to its construction on a cellular level. Fragments of the plants’ roots, known as rhizomes, are all capable of travelling far from their source and growing into entirely new plants by themselves.

Question 7

Is there Japanese knotweed in my area?

Accepted Answer

Japanese knotweed has since been designated as an invasive plant by the UK government making, improper disposal or transporting of it a criminal offence. That hasn’t stopped it spreading further across the UK, with only settlements in the Northern most region of the countries remaining unaffected.

It’s difficult to pin down how far the plant has spread, as knotweed often remains dormant in root systems beneath the ground during colder months. An infestation discovered in the lead up to the London 2012 Olympics led to the government shelling out an estimated £70 million, whilst Swansea has been besieged for decades with the local council struggling against an ever-expanding 250 acres of knotweed.

Question 8

Where does Japanese knotweed grow in UK?

Accepted Answer

Japanese knotweed now grows in almost every area of the UK. The plant was given free rein to spread throughout the country for over a hundred years before being recognised as invasive by the government. By the time Japanese knotweed was identified as an invasive threat in 1981, it had already been spread by gardeners, dug up and transported by urban development projects and even used to stabilise soil on the sides of railway lines.

The plant is frequently found growing alongside public watercourses such as canals, as well as near railway lines, motorways and public footpaths. Japanese knotweed thrives in most conditions in the UK and is often found growing on land that has been abandoned or left unattended. For this reason, large infestations can often be discovered in overgrown back gardens, or on abandoned industrial sites.

Question 9

How common is Japanese knotweed in the UK?

Accepted Answer

Japanese knotweed is a very common sight in the UK. Since the government has made the spread of Japanese knotweed a more pressing concern, efforts have been made to track where it has been growing throughout the country. Using these statistics, it’s been possible to recognise the areas of the country that have been most affected by Japanese knotweed. Bolton, Bristol, Conwy, Rotherham and Nottingham are areas in the UK that have been affected the most, however exact numbers are difficult to assess

Question 10

What does it cost to remove Japanese knotweed?

Accepted Answer

Costs for Japanese knotweed removal vary depending on the area that has been affected and the form of treatment that you opt for. In order to manually remove the rhizomes from beneath 4 square yards of the ground you would have to dig up 20 square yards. Take into account the removal of any paving or pipes and the costs could be up to £1,000 per visible square yard.

Another option is to attack the plant with an industrial-strength weedkiller. Although this option is undeniably cheaper it’s a long-term solution—the costs of around £2,000 to £5,000 are spread out over a number of years. These may seem like big investments but it’s better than simply ignoring your problem. Left unchallenged Japanese knotweed can spread like a virus across land and into buildings, leeching anywhere from 5-20% off the value of your home.

Question 11

What do I do if I find Japanese knotweed?

Accepted Answer

If you find Japanese knotweed it’s important to ascertain where the plant has come from and to what extent the plant affects your property. You may want to consider contacting a Japanese knotweed specialist to make a positive identification before you move forward with talking to any neighbours who might also have the plant on their land. Ignoring the infestation is not advised, as the plant can quickly multiply, leading to a larger problem to deal with and potentially more costs, if you find that it’s not possible to claim compensation for the cost of treatment.

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Japanese Knotweed UK Map: Is there Japanese knotweed in my area?