Recording the distributions of weeds both in Wellington and further afield is an ongoing interest for Te Papa’s Botany Curator Leon Perrie and Researcher Lara Shepherd. Together they have added hundreds of weed specimens to Te Papa’s herbarium since the Covid lockdown. One recent weed collecting trip to Upper Hutt demonstrated how the use of common names can lead to confusion.
Many plant species have at least two names: a scientific name and one or more common names. Within a given classification, each species can have only one scientific name, and there is a set of rules for the naming process. These names are Latinised and can be daunting.
In contrast, common names are typically easier to use – being shorter and free of Latin. However, plants often have multiple common names, which can vary by language, region, or country. Additionally, the same common name can refer to multiple, sometimes unrelated, species. For example, the te reo name mingimingi, meaning “twisted” is used for at least three unrelated native plant species.
This ‘double-dipping’ of common names can lead to confusion, as Leon and Lara recently experienced! On a recent trip, Lara, who was several hundred metres ahead on the Hutt River track, texted Leon to keep an eye out for the knotweed beside the track, as it is an uncommon weed in the Wellington region. Leon replied that he had already collected it.
It was only several days later that we realised that we were talking about completely different species! Leon had seen pink knotweed (Persicaria maculosa, also known as willow weed), a reasonably common weed of waterways. But Lara had seen Asiatic knotweed (Reynoutria japonica, also known as Japanese knotweed and a host of other common names), a much more concerning weed.
Pink knotweed and Asiatic knotweed are in different genera but are both in the knotweed (Polygonaceae) family. Luckily, we were able to return to the site later and make a collection of the Asiatic knotweed. The Greater Wellington Regional Council has also been notified about this weed. Its location on the bank of the Hutt River and ability to resprout from small fragments of stem or root means it could easily spread down the river.
Both knotweed specimens have now been deposited in the herbarium, imaged and are available online.
Asiatic knotweed – one of the world’s worst weeds
But back to knotweeds and why this record of Asiatic knotweed is cause for concern. Although this species has been recorded in Wellington City, this is the first collection from the Hutt Valley.
Asiatic knotweed is listed as one of the world’s worst invasive species by the World Conservation Union and is a major weed in Europe and North America. Once established, it is very difficult to eradicate as it can resprout from even a tiny fragment. It forms dense colonies that smother native vegetation and outcompete other plants.
In urban areas, its extensive root system can invade cracks in paths, buildings, retaining walls and drainage systems, causing considerable structural damage. In the United Kingdom, it is a prosecutable offence to “plant or otherwise cause to grow in the wild”. Homeowners and landlords who fail to control the plant on their property can face fines or prosecution. In some cases, the presence of Asiatic knotweed has prevented homeowners from securing a mortgage or selling their property.
However, eradication is not only very difficult but also costly. For example, it took four years and £70 million to remove knotweed from just 10 acres of land prior to the construction of the London 2012 Olympic Games velodrome and aquatic centre.
Asiatic knotweed in Aotearoa
Asiatic knotweed was first recorded naturalised in New Zealand in 1935, and it has now been found from Auckland to Dunedin. The species is listed in the 2020 National Pest Plant Accord, meaning it is illegal to sell, propagate, or otherwise spread it. Violations may result in fines.
If you have it on your property, Greater Wellington Regional Council has information about how to control it here:
Asiatic knotweed – Greater Wellington — Pest and Weed Central
If you find it on public land, please report the find to your local council. It is also useful to record observations on iNaturalist so that its spread can be tracked and hopefully controlled.
Further reading about Asiatic knotweeds in New Zealand
S.D. Desjardins, C.H. Pashley, J.P. Bailey (2023) A taxonomic, cytological and genetic survey of Japanese knotweed s.l. in New Zealand indicates multiple secondary introductions from Europe and a direct introduction from Japan. New Zealand Journal of Botany
Horrifying to learn that knotweed is in NZ. I battled it for years when we lived in Connecticut – terrible stuff and basically unkillable.
Good story – well written and illustrated. I will look out for this weed which I was not previously aware of.
Y’use/ name PHARLAP, here is a un-neccessary DISTRACTION I argue>>> Syd Moore