Plant of the Week – 26th Sept. 2022 – Japanese Knotweed (Reynoutria japonica Houtt.)

This alien plant belongs to the family of rhubarb and docks (the Polygonaceae). Its history is well known (Bailey and Conolly 2000, Drazan et al. 2021) – the plant was collected in Japan by Philipp Franz von Siebold, first taken to Holland (Leiden) and then introduced to the Royal Botanic Garden Kew in 1850. It escaped, and rapidly colonised Britain and Ireland. It has also colonised much of Europe (Alberternst and Böhmer 2011). Now it is widely considered to be one of the most troublesome of all invasive plant species. It’s one of those plants that people like to hate!

R. japonica growing against a wall at Alexandria,  West Dunbartonshire, Scotland. This individual has necrotic spots on its leaves (fungal leaf-spot?) but thrives nevertheless. Photo: John Grace.

The peculiar feature of the British and European plants is that all are female. No pollen is produced and so the species cannot form viable seeds as it does in its native Japan (where male plants are present). However, it spreads rapidly by means of fragments of rhizome, which it does when soil is moved from one place to another. Even more peculiar is the fact that genetic analysis shows that all ‘our’ plants are identical to each other, i.e. we have a single clone derived from the original introduction. Moreover, plants in North America seem to be of the same clone as the British ones.

R. japonica, flowering spike. These flowers are functionally female (they have an ovary with three short stigmata, but the stamens are not properly developed and they produce no pollen, see Beerling 1994). The flowers produce nectar and are visited mostly by flies and honey bees. Photo: John Grace.

It is definitely ‘invasive’. The Wildlife and Countryside Act of 1981 recognised the need to control invasive plants. The first to be outlawed were Giant Hogweed and Japanese Knotweed. Now there are many more. The penalties for growing these species are severe (£5,000 fine or two years in prison).

R. japonica at Dalry Cemetery, Edinburgh, thriving in light shade. Young plants were a few metres away; presumably they sprouted from rhizomes of this ‘mother’ plant. Without control, the plant will spread by rhizome-sprouting, and hinder access to gravestones. Photo: John Grace.

Victorian gardeners liked to grow Reynoutria japonica because it is highly decorative. It forms a large bush, 2-3 metres tall, with creamy white spikes of tiny flowers and it scrambles against rough walls and fences, ‘softening’ hard edges. It is however, hard to get rid of. If uncontrolled, its roots may undermine walls, pavements, roads and flood defences. When growing on residential land it may affect the saleability of property; in the UK several specialist companies have sprung up, promising to remove it. In Japan however it is not much trouble. Perhaps that is because it has many natural enemies there, which are absent in Europe. Much European research has been done regarding control using herbicides and/or cutting. Its growth can be checked using glyphosate or repeated mowing but it always comes back. I have read that its control costs the UK economy around 166 million pounds each year. Recent attempts at control include the release of a Japanese sap-sucking insect (a psyllid) called Aphalara itadori. This insect is one of Japan’s Reynoutria specialists. However, it is not yet clear whether it can thrive under British conditions; the climate and native predators may not suit it. Moreover, the introduction of any alien sap-sucker is not without risk to native species and crops.

R. japonica, showing arrangement of leaves on the stem and their characteristic shape. Not heart-shaped (like the related R. sachalinensis) but more ‘spade shaped’ (fairly flat at the leaf base not lobed like a heart). Leaves are about 12 x 10 cm. Photo: John Grace.

Grime at al. (1988) record the habitats as soil heaps, river banks, road verges, railway banks, wasteland, coal-mine spoil and cinder tips. In Japan it is an early colonist on volcanic soils, larval flows and river gravel. It can tolerate high concentrations of heavy-metal pollutants in the soil, and some authors even claim it grows best on polluted soils.  I often see it growing near the sea, just above the high tide mark but subject to salt spray, demonstrating some tolerance to salt. I know it too from a woodland site in Edinburgh – so it has some tolerance of shade.

R. japonica, growing in woodland (Dreghorn Link, Edinburgh). It is somewhat less floriferous than open-grown specimens but still healthy and vigorous. Photo: John Grace.

The rhizomes typically extend laterally to four metres from the main stems and the shoots can sprout through asphalt. It dies back in the winter and regrows in the spring from these extensive below-ground rhizomes. The growing spikes can achieve rates of 4 cm per day, and are said to be edible and good.

There is a related species, the Giant Knotweed Reynoutria sachalinensis which comes from Japan and Russia. It was also introduced to Britain, but much later than R. japonica (in the 1980s). Its appearance and general biology are similar, except that its leaves are much larger and not shaped like a spade. It can grow to 4 metres tall. It can produce pollen and it does form hybrids with F. japonica. The hybrid form was first described in the Czech Republic but is also recorded in Britain and called Bohemian Knotweed Reynoutria × bohemica. For a long time the hybrid was not recognised in Europe, and it has spread even faster than R. japonica. For those interested in the genetics (and polyploidy) within the genus, the fascinating story is told in Stace and Crawley’s book Alien Plants.

Reynoutria sachalinensis at Culzean in Ayrshire. It has elongated leaves much larger than R. japonica (up to 38 x 28 cm), grows to 3 metres tall, and flowers on this specimen have stamens (although female plants are said to be more common). Photo: John Grace.

Reynoutria japonica has 88 chromosomes and all Reynoutria species have chromosome numbers that are multiples of 11. The inference is that R. japonica is an octoploid. There is also a dwarf form ‘compacta’ which can be cultivated as an ornamental (see below). It has only 44 chromosomes. This one is considered as merely a varietal form of Reynoutria japonica but some authorities say it is a different species.

R. japonica var. compacta. It has only 44 chromosomes, smaller thicker somewhat-folded leaves with undulating leaf margins, and its flowers have stamens. It’s only up to 1.5 metres tall. We found it beside the A77 near Prestwick in Ayrshire, where it forms a broad band several hundred metres long. The image on the left shows it beginning to encroach onto the road. Photos: John Grace.

I do see the point of legislating against invasive aliens. But there is a something like ‘racism’ at play in people’s reactions: they exaggerate the differences between invasive aliens and natives that are sometimes just as invasive. Take for example the native ivy Hedera helix. It dominates habitats just like Reynoutria japonica: it scrambles up walls even more efficiently, its roots can break walls and it can smother ground flora. No-one seems to object.

A thorough analysis of the impact of Reynoutria on plants, animals and fungi was conducted by Lavoie (2017). He found 44 articles in peer-reviewed journals. There were ‘winners’ and ‘losers’. The losers were soil bacteria, most arthropods and gastropods, some frogs and birds. The winners were most fungi, detritivorous arthropods, aquatic shredders, a few birds. He concludes “Although there is some evidence that knotweed invasions have negative effects on the environment, the research to date remains modest and a more extensive effort is needed to better define the environmental impacts of these plant invaders”.

Can Reynoutria japonica be ‘tamed’ and brought into service? I can think of five possibilities. (i) The species produces nectar and is visited by bees. Japanese knotweed honey is well-known and much-liked in Japan. The dark red honey sells well in parts of the USA. (ii) The species has been used in herbal preparations: its roots can be made into tincture which sells at £16 per 100 ml. at Napiers of Edinburgh. (iii) Biomass yields of as much as 30 tons dry mass per hectare are claimed – that’s over twice as much as the fastest growing tree species. The biomass could be made into briquettes and used in wood-burning power stations; in Britain it could replace some of the 1.4 millions tonnes of North American wood pellets that are imported and fed to the Drax power station (Yorkshire) each year. (iv) Its roots take up toxic metals from soil, and so it could be used for decontamination of soils (v) Over the next decade we expect new polymers will be developed to replace plastics. The obvious feedstock for the production of biodegradable packaging is lignocellulose – and it could come from sustainable cropping of Japanese Knotweed.

Global distribution of Reynoutria japonica according to GBIF.

Regular readers of this blog will know that plant names can change because of research discoveries, misunderstandings, and (in the early days) a lack of knowledge of what other botanists had done. This species was first described by Maartin Houttuyn (1720–1798) from a plant brought back from Japan. He called it Reynoutria japonica but his work was not widely known. In 1846 German botanists Philipp von Siebold and Joseph Gerhard Zuccarinimore called it Polygonum cuspidatum. However, some thought it should be placed in the genus Fallopia (named after the Italian botanist Gabriele Falloppio). In the first edition (1991) of Stace’s New Flora of the British Isles its name is Fallopia japonica but in the latest (fourth) edition (2019) it reverts to its original Reynoutria japonica. Confused? Me too. Name changes are frustrating for botanical recorders and members of the public with an interest in plant life.

One last thought (and possibly a dangerous one). What would happen if the male of the species were to be introduced into Britain to enable pollination and the production of viable seed? Perhaps our female clone just happens to be especially vigorous whilst seed-derived offspring are less of a pest.


Alberternst B & Böhmer HJ (2011) NOBANIS – Invasive Alien Species Fact Sheet – Fallopia japonica. – From:Online Database of the European Network on Invasive Alien Species – NOBANIS

Bailey JP and Conolly AP (2000) Prize-winners to pariahs – A history of Japanese Knotweed s.l. (Polygonaceae) in the British Isles. Watsonia 23, 93-110.

Beerling DJ et al. (1994). Fallopia japonica (Houtt.) Ronse Decraene. Journal of Ecology 82, 959-979.

Brunerova, A et al. (2017) Potential of wild growing japanese knotweed (Reynoutria japonica) for briquette production.

Drazan D et al. (2021) History of knotweed (Fallopia spp.) invasiveness. Weed Science 69, 617–623.

Hollingsworth ML and Bailey JP (2000). Evidence for massive clonal growth in the invasive weed Fallopia japonica (Japanese Knotweed). Botanical Journal of the Linnean Society 133, 463-472.

Lavoie C (2017) The impact of invasive knotweed species (Reynoutria spp.)
on the environment: review and research perspectives. Biological Invasions 19, 2319–2337.

©John Grace

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