Japanese knotweed (Fallopia japonica) is a plant originally native to East Asia. And not to be confused with Dyer's knotweed/japanese indigo. Perhaps the name already makes you nervous? For many people, knotweed is probably THE plant species they think of as an 'invasive neophytes'. It was deliberately introduced to Europe in the 19th century though, as an ornamental and fast-growing crop plant, not such a good idea from today's perspective...
Japanese knotweed is now well established in Germany. It can be found in many places, often along riverbanks, and it is probably also benefiting from increasing temperatures in the climate crisis - so we will probably have to come to terms with it somehow. The deciduous plant survives our winters and often forms large stands. It grows very quickly and can even reach a height of 3 to 4 meters under favourable conditions. The stems become woody over time, are green and red, the leaves are bright green and slightly leathery to the touch.
The species is related to Dyer's knotweed (and you can read about how to get indigo blue from Dyer's knotweed here and here). Japanese knotweed can't dye blue, and it's hard to confuse it with the much smaller annual plant... You shouldn't plant Japanese knotweed in your dye garden on purpose, because it spreads a lot - we shouldn't help it any further, should we?
In Germany, it is on the so-called "black management list" (as is a hybrid of the species, bastard knotweed, Fallopia bohemicain other words, "the threat to biodiversity is proven". The plant's populations endanger native plants and insects by changing vegetation structures i.e. taking over areas by growing faster and higher than pretty much any other plant.
So it is important to pay close attention to knotweed. At the same time, it can also be used in a variety of ways - perhaps this could be combined with containing the populations?
But before we get into the creative use of this plant, here is some important information.
Responsible harvesting: How knotweed reproduces
Although Japanese knotweed also forms seeds - the species is dioecious, meaning that there are both male and female plants - it mainly spreads underground via rhizomes. This often results in large formations in which all the plants are actually clones. There can be only male plants over long stretches, and yet the population continues to spread. They shed their leaves over the winter, but sprout fresh and vigorous again the following spring.
So what should you look out for when collecting for the dye pot? Under no circumstances should you 'scatter' parts of the plants, but rather collect everything you remove. This is because not only pieces of the rhizomes can sprout again and thus populate new locations, the stems are also capable of doing this - supposedly even the leaves! (Perhaps you have already taken advantage of this eagerness to form new roots with the annual Dyer's knotweed? An indigo patch can be quickly replenished with newly rooted cuttings).
This characteristic gives Japanese knotweed an advantage in places where it was not originally native - it takes root practically everywhere and then grows quickly and tall.
Ignorance can therefore quickly lead to the further spread of the plant: if soil is moved in which pieces of rhizome are stuck, stem sections end up in rivers or streams, or the plant is spread as green waste - or if the rhizome is conveniently transported around with heavy equipment and the soles of shoes. If the plant is removed (and it does not end up in the dye pot) it should therefore not be put in the compost, but in the residual waste.
So, stems and leaves belong in your collection container and nowhere else. I used leaves and the thinner twigs for coloring. There are more suggested uses below if you're looking for more inspiration!
Dyeing with Japanese knotweed
This Instagram post talking about ways of using the plant, made me think of dyeing with knotweed.
So I went out and picked a large bag of dyer's knotweed in a park on the way to my studio.
When I arrived, I immediately made a dye bath with the fresh leaves (and thin stems). Instead of the expected bright yellow, the first tones were very pale - in this situation I usually measure the pH value first.
And unsurprisingly (as the plant is rich in oxalic acid), the pH value was in the acidic range. Many dyes cannot be absorbed by fibers in these conditions.
So the next thing I did was add a very small amount of washing soda to balance it out. You could certainly try potash or calcium carbonate instead if that's what you have on hand. And if your results are different - as always, the water can play a big role as well as the pH value.
The first small samples, which I dyed with a corrected pH value, were a bright warm yellow.
The next tests then resulted in beautiful rust tones.
The Japanese knotweed was also very generous, I infused the material a second time and the second dye extraction was still very rich. We tested it right away in the next 1-day workshop and we loved the tones on the different fabrics and yarns of the participants. And afterwards I even dyed another batch with the dye bath myself.
Out of curiosity, I later prepared small dye baths with either only dried leaves or only crushed young stems. Here the stem-only dye was much more reddish. I also used some of the dye bath to make a color paste to paint directly onto fabric - and got a warm rust tone.
And since the dye bath was still not exhausted, I made lake pigments from it to test out all the possibilities.
Japanese knotweed is undeniably a problematic plant outside its original habitat - and unfortunately we have not (yet?) found a simple strategy to reliably contain its further spread.
On the contrary, I recently noticed on a walk that careless work with heavy machinery had spread Japanese knotweed enormously. During work in a wood plantation, the ground was churned up over a large area, all the existing plants were removed and knotweed, which was previously only found in a small area at the side of the path, is now sprouting everywhere like a plantation. That sight did make me a little sad.
Although I was already writing this article, in which I didn't just want to show the plant in a negative light.
Because apart from dyeing, there are many other ways to use knotweed.
First of all, in culinary form: the young shoots can be steamed as a vegetable, fermented or used as a sweet rhubarb substitute in desserts. However, it is best not to eat them daily and in large quantities because of the oxalic acid they contain.
The young stems can also be used to make cords, while woody stems are presumably suitable for making paper instead. I haven't made cords from them yet, but I have sharpened a few of the stems as drawing pens and left them to dry - so that I can then draw with ink from the leaves. Which works well! Thicker, hollow stems are suitable for filling with paint to make your own colored pencils (I saw this in the book ‚The organic artist for kids‘* - but haven't tested it yet).
Japanese knotweed in the gardenn
This is probably already obvious, but once again: Do not plant as a fast-growing tall hedge! The german Federal Agency for Nature Conservation recommends: "This species should be avoided. It was removed from the assortment by many nurseries years ago. It must not be planted in the open landscape, especially not near natural watercourses."
If the plant is perhaps already in the garden, or nearby, you can use it for gardening in a completely different way: as plant manure. I haven't tried that yet, but as it's growing over from our neighbor's garden, I'll probably give it a go.
Also good to know: Knotweed can be used to detoxify soil, as it absorbs harmful substances. So if you gather knotweed for consumption, you should pay particular attention to the location.
And finally: Various strategies for controlling the plant are listed here (as of July 2023) - mechanical removal is very time-consuming and can easily contribute to its spread unintentionally, and I very much doubt that widespread use of herbicides is a holistic, long-term solution. So maybe we should eat it and make dye and paper out of it...
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