Japanese Knotweed a brief history looks at how an ornamental plant prized by collectors has become an invasive weed and a major UK problem.
What is Japanese Knotweed?
Reynoutria japonica, Fallopia japonica, Polygonum cuspidatum, Asian knotweed are all names by which Japanese Knotweed is known.
Fallopia japonica is a rhizomatous perennial plant. This means that it has an underground root system, which, in the case of knotweed in its native habitat, allowed it to survive on the limited nutrients available in the inhospitable environs of a volcano. No wonder it has found the rich soils of European gardens such a feast!
In appearance, the young green and pink stems in spring look slightly rhubarb-like. (Another of its common names is donkey rhubarb). Like rhubarb stalks, these young knotweed stems are edible when they are pliable. It is regularly eaten in Japan and Vietnam and these young stems are a good source of vitamins and minerals.
Note to those in the UK who would like to try this delicacy –
it is against the law to remove knotweed from its site of origin without taking it to a registered landfill site. And, health-wise, there is a strong likelihood that herbicide will have been used to treat the stand of knotweed.
Knotweed grows quickly and where there are no natural predators forms a dense leafy stand, or clump, which shades the ground below. The leaves when they fall prevent competition from other plants by exuding a hormone. Other plants use this method too, known as allelopathy, for example walnut trees and sunflowers.
As well as being edible in spring, the flowers in late summer and autumn are full of pollen. Bees loves it. Odd to think of public enemy number one as being a plant for pollinators! Luckily for us, there are only female plants in Europe and the USA (so far). All the plants in Britain are from a single female clone. Research suggests that this is the same clone present in Europe and the USA. On one level, that makes them all one plant. I’ll leave you with that imagery…
Japanese Knotweed A Brief History – In the Beginning was Seibold
The story is that Japanese knotweed was introduced into Europe by Siebold, who had seen it when in Japan in the 1820s. He saw the possibilities of various plants he collected as exotic additions for rich European gardeners. But he wasn’t led purely by commercial reasons. Seibold was a doctor as well as a botanist, which is why he was in Japan with the East India Company. He studied Japanese plants in their native habitat at a time when few Westerners were allowed to do so. His private collection later formed the basis of more than one public collection, so important were the specimens. In fact, you may have a “seiboldii” in your garden as, among others, he brought in Acer, Hosta, Clematis and Magnolia.
Or was the beginning Seibold? He had knotweed plants at his nursery in Holland in the 1840s and was selling them in Europe before any documented sales in Britain. Seibold named his plant Polygonum cuspidatum; the Royal Botanic Gardens Kew received a parcel of this in 1850. The Royal Botanic Gardens Edinburgh received plants named Polygonum seiboldii in 1854. From this point, commercial sales of knotweed took place across Britain. However, this plant was originally described by Houttuyn in 1777 as Reynoutria japonica.
Two points we should note. Firstly, that the confusion over botanical names and lost names for knotweed wasn’t resolved until 1901 by the Japanese botanist Makino, who realised that the plants were the same. Secondly, a plant named Houttuynia had been introduced and grown in the Horticultural Society’s garden in London since 1825. It was growing in an artificial swamp as it was thought to be the Chinese plant that likes damp conditions. Indeed identification of Japanese knotweed can still be problematical as there similar looking plants, of which Houttuynia cordata is one.
Japanese Knotweed A Brief History of an Exotic Ornamental
In 1847 the Society of Agriculture & Horticulture at Utrecht awarded a gold medal for the most interesting new ornamental plant of the year to Japanese knotweed.
By the latter half of the 19th century, cattle farmers were growing knotweed to feed to their livestock. Those young shoots were highly nutritious in spring before the meadow grass was ready for grazing. The ornamental value of colourful spring stems and autumn flowers made knotweed popular with gardening gurus.
William Robinson author of “The Wild Garden” (1870) recommended it for inclusion as one of those plants having
“large or graceful foliage suitable for naturalisation”Mrs C. W. Earle, another famous Victorian gardener, in 1897 called it
“the handsomest, easiest grown, hardiest, most useful plant for London gardens”John Wood in his 1884 book “Hardy Perennials and Old Fashioned Flowers”, referred to knotweed under the synonym Polygonum cuspidatum:
“If the roots are allowed to run in their own way for two or three years they form a charming thicket, which must prove pleasant for any large garden.”
Even Gertrude Jekyll (one of my gardening heroes) included it in many of her planting lists. But both Jekyll and Robinson became more circumspect, once they appreciated its domineering habits. It’s difficult to be precise as to exactly when Knotweed escaped into the wild when a plant reproduces from a small piece of rhizome or stem. Although it reduced house prices in Cornwall as early as 1930, Japanese knotweed was still available and mentioned as a suitable garden plant as late as the 1960s.
Japanese Knotweed A Brief History of the Human Response
The spread of Japanese knotweed has been swift and, so far, unstoppable. It got a mention at the 1992 Earth Summit and is considered one of the most invasive plant species worldwide.
In 2018, whilst there is some variation across the UK, including updates and amendments, the primary legislation used is the Countryside and Wildlife Act 1981. This states that it is classified as an invasive species and as such it is an offence to: –
- plant it in the wild
- cause it to grow in the wild
- allow it to spread to neighbouring properties
- and, although this could be open to different interpretation, encourage it to grow, as in cultivate it on your own land
However, there is currently no legal requirement to report the presence of, treat or remove knotweed which is growing in your garden.
The off-site disposal of knotweed comes under the Environmental Protection Act 1990. It is classed as controlled waste and ideally should be disposed of by a licensed waste carrier to a licensed facility. The situation is further complicated by the use of pesticide treatment before disposal.
Japanese Knotweed A Brief History – the End is Nigh?
The massive network of underground rhizomes which enable knotweed to survive and spread is one problem. The hybridisation with related species brings another.
There may be hope in the ongoing battle. Ground breaking research led by Leicester University over the last 40 years has given us some methods of restricting the spread of knotweed if not yet eradicating it. there are three main methods of control, as with any weed. These are: –
- cultural / mechanical
- chemical
- biological
There’s not the room here to go into detail, but biological looks like being the main contender for control moving forwards. In its native habitat there are some 186 bugs and about 40 fungi that are natural predators or otherwise feed off knotweed.
The key to successful biological control is to find an insect of fungus which will predate only on knotweed. 2010 -14 saw the safe testing of the psyllid (tiny sap sucking insect) Aphalara itadori which is a native predator. The trials went well, with the psyllids surviving over winter. It will take some time for the population to both establish and grow, but the release of the psyllid to other sites has been approved. These and other biological controls are likely to be our best resource. Not least because they do not damage the environment and native species as chemicals can.
Do You have Japanese Knotweed?
If you would like professional, independent advice on whether you have invasive weeds in your garden, do please get in contact. We do not treat Knotweed but we can recognise it, and give an opinion on how you could progress with reducing the problem. Plews expertise is in recognising the particular needs of you and your garden and seeing the whole picture.
Remember that firms which claim to ‘totally eradicate’ Japanese Knotweed from your garden may be playing around with the truth. The firm you use to treat Japanese Knotweed must operate within the legal requirements for the treatment and disposal of this invasive non-native.(see some external links below)
On a lighter note, if you don’t have Reynoutria japonica, Plews can also give you advice and ideas on how to de-weed and de-pest your garden in an easy, environmental way giving yourself more time just to sit and enjoy the sunshine.
And for further gardening advice and inspiration, check out Plews Potting Shed blogs, including the selection below and our monthly Tipsheet – You could come and find us on Instagram Pinterest and Facebook too.
Related Gardening Blogs from the award winning Plews Potting Shed
Uses for Weeds – Liquid Fertiliser
Edible Weeds
Nettles – Weed or Herb?
What makes a Garden Consultant different to a Garden Designer?
Humphry Repton – the First Landscape Gardener
External links on Japanese Knotweed
Whilst we do not recommend any particular firm, you should find these links will give you further information.
The UK Government website gives the up to date legal situation on preventing the spread of knotweed, treatment and disposal
For FAQs on the psyllid, have a look at the Japanese Knotweed Alliance
Many of the reputable firms dealing with knotweed control will be members of the INNSA
