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Can you speak knotweed?

Exploring the scientific nomenclature for the Japanese knotweed s.l. taxa

This article explores the accepted naming conventions of Japanese knotweed and is meant as guide for anyone unclear about the subtle differences in how knotweed is written and spoken about across scientific and commercial literature.

One iron law of working alongside academics is that everyone seems to prefer long words over short ones. Academics tend to find the longest way of saying something, falling foul of Shakespeare's ironic charge of being sesquipedalian.

We’re the first to admit our scientific paper is a mouthful; 'Optimising Physiochemical Control of Invasive Japanese Knotweed' hardly flows.

Besides writing scientific papers we also talk about Japanese knotweed a lot in day to day conversation. Most of the time, out of convenience, we simply refer to it as knotweed. This isn’t strictly accurate in terms of nomenclature, but everyone generally understands what we mean and this verbal shortcut gets to the point.

We don’t like to bore people with dense ecological classifications, unless they're also ecologists. However, details do matter. It's useful to know the nuances and reasoning behind the naming conventions for Japanese knotweed, even if there’s no need to use them all the time.

The main types of Japanese knotweed

In standard European usage Japanese knotweed is a collective term describing four distinct, but related plant species, all falling within the same taxa. It is customary to follow the English language name with it's latin counterpart in brackets, thus:

  • Japanese knotweed (Fallopia japonica var. japonica)
  • Dwarf knotweed (F. japonica var. compacta)
  • Giant knotweed (F. sachalinensis)

The ‘F.’ indicates Fallopia — the genus of between 12–15 flowering plants within the buckwheat family and paying homage to the Italian botanist Gabriello Fallopio, who discovered fallopian tubes, which knotweed stems are supposed to resemble.

Also included are the two hybrids between Japanese knotweed and Giant knotweed, and between Japanese knotweed and Dwarf knotweed, known collectively, and quite poetically, as;

  • Bohemian knotweed (F. × bohemica)

For completion, backcrosses between Bohemian knotweed and any of their parent plants are also covered under the Bohemian knotweed classification.

To account for all of the above, the collective of knotweed species is referred to scientifically as Japanese knotweed s.l. — with s.l. being the neat latin indication for sensu lato, which translates to ‘in the broader sense’.

Of the Japanese knotweed s.l. taxa, Japanese knotweed (F. japonica var. japonica) and Bohemian knotweed (F. × bohemica) are the most commonly encountered in the UK. There is conflicting anecdotal evidence to suggest that Bohemian knotweed is the most resilient to herbicide treatment; we're not so sure that it is!

Synonyms

Less common historical names for knotweed are the eponymous Polygonum sieboldii (1842) - after German physician Phillip Franz von Siebold (1796 -1866), infamous for introducing knotweed to Europe in the first place.

Even earlier, Reynoutria japonica was used to describe Japanese knotweed by Houttuyn in 1777, and there is now a modern debate about the returning the classification of Japanese knotweed from Polygonum to the Reynoutria genus.

In the US, Japanese knotweed is mainly known scientifically as Polygonum cuspidatum. Polyononum referring to the many ‘knees’ or joints of knotweed stems and cusipidatum describing the sharp, pointed tips of new knotweed shoots.

Itadori!

In contrast again to both the accepted European and US nomenclature, is the native Japanese name for knotweed, Itadori. Itadori hints at knotweed’s medicinal properties, in addition to it’s resilience, the literal translation being ‘take away pain’ and Itadori also being interpreted as ‘strong plant’ or 'tiger staff'.

Rarely discussed in the UK context, knotweed has a well regarded status in Japan as an edible wild-food. The stems are edible and are eaten like spears of asparagus. The plant can also be made into a tea or extract high in reservatrol and vitamin C.

There is a very small niche of ethno-botanists who also promote Japanese knotweed in recipes for desserts, jams and relishes and even as a cocktail flavouring much like sloes are used to flavour gin. This said, we can’t recommend any culinary exploration of knotweed in the UK; legal restrictions on transporting and disposing of the plant, and the possibility of contamination from chemical treatments and heavy metals in the post-industrial landscape of the UK make knotweed a potentially incriminating, and likely poisonous ingredient.

Itadori will be familiar to those dealing with Japanese knotweed commercially in Wales - Aphalara itadori is the name of the native Japanese psyllid, the aphid-like arthropod and natural, highly vaunted predator of Japanese knotweed. Aphalara itadori has been extensively trialled and now approved for biological knotweed control in the UK by Defra, the Welsh Government and CABI - the first such case in the EU.

Final thoughts

Whether Japanese knotweed, Japanese knotweed s.l. or Japanese knotweed (F. japonica var. japonica), Polygonum cuspidatum, Reynoutria japonica, Itadori or simply knotweed, as Japanese knotweed gains a larger public profile, more and more people will be expected talk about it fluently. We hope this helps provide a clearer route through language.

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