JAPANESE KNOTWEED : Fallopia japonica
Japanese knotweed was introduced from Japan in the 19th century. It has since become perhaps the most troublesome of the invasive non-native plants found here. it grows along many riverbanks, on waste ground and along roadside verges where it prevents native species from growing simply by its sheer density both above ground and below.
It is problematic across most of the UK. It proliferates wherever water courses spread it, but can also be found on almost any open ground, particularly that which has been disturbed or simply not maintained. Glasgow has been a hotspot and their Council has been running determined projects to eradicate it for years. See the Glasgow web link below.
In West Dunbartonshire it appears to be less prolific, but that seems to be more down to chance. Stands have been found in such places as the small road between Lomond Industrial Estate and India Street and in Renton near the footbridge.
Japanese knotweed is the most prolific and problematic of 4 knotweeds found in the UK; the others being Dwarf knotweed, Giant knotweed and Bohemian (hybrid) knotweed
The UK Government offers plenty of advice. Within Scotland we are directed to SEPA. It is useful to read both of these sources. See links below. Several commercial services also provide details for eradication.
The UK Government website covers :
- How to identify Japanese knotweed
- Where knotweed grows
- How knotweed spreads
- If you have knotweed on your land or property
- How to stop knotweed spreading
- How to dispose of Japanese knotweed off site
- Contact the Environment Agency
WHAT IT IS AND RECOGNISING IT.
Japanese knotweed is a strong-growing, clump-forming perennial, with tall, dense annual stems. Stem growth is renewed each year from the stout, deeply-penetrating rhizomes (creeping underground stems). [Wildlife Trusts].
Japanese knotweed can grow to be very tall, up to 3m, although when first noticed it may simply look like a floourishing green shrub with closely growing stalks. Its leaves are described as triangular or almost heart shaped. Its stems are hollow, a bit like bamboo. They are generally green but may have red or purple specks on them.
Another characteristic is the way the stems have distinct knots (as the name suggests) at each leaf of stalk branch and at each the stem kinks slightly. (Bamboo has similar junctions, but remains straight).
Small, white, tufty flowers appear in late summer and into autumn.
As temperatures start to plummet in winter, Japanese knotweed looks like it’s dying off. The vigorously growing bamboo-like canes turn brown and look like they have no life in them. The canes appear spindly and lose their leaves.
Don’t be fooled by its brown, brittle state. It remains very much alive and is just lying in wait for the spring when it will emerge bigger and stronger with fresh new shoots. The first signs of life will be red or purple shoots that look very much like asparagus. [Japanese Knotweed Specialists website]. This is not the time for complacency, but can be an opportunity to get in there and dig up its rhizomes.
On a positive note : It is available as a tincture. Extracts such as tinctures or essences offer a convenient alternative to herbal teas. Many are used in food, food supplements, cosmetics and as an ingredient by herbalists in bespoke formulas.
Ingredients: Fallopia japonica extracted in sugar beet ethanol (alcohol). The part used is the root. [Napiers website].
Not to be confused with dogwood, which until it bears flowers and fruits look much the same. Similar leaves. Similar red leaf stalks. index.asp?pageid=732904
This is probably how you will first see this plant. Lush green folliage. Distinctive leaves.
If you turn them over they are strongly veined.
The leaves and their stalks off the main stems are also distinctive. The leaves with their reddish stalks alternate and at each junction there is a slight kink in the stems.
The stem junctions are where the leaves and flowers emanate.
If you cut a knotweed you will find the stalks completely hollow.
This fragile plant seems to easy to remove by cutting. But that is an illusion as below ground there are unseen rhizomes pushing their way into new areas.
These examples above are on the small closed off road between Antartex and Instia Street and lean through the distillery fence. 2023.
The flowers develop through August. Some of the leaves are beginning to brown already on some plants, but the reproductive process for a new generation is well under way.
The example above is on the upper Leven towpath against the distillery wall. 2023.
Some of the plants are developing into sizeable shrubs by mid-August. Really quite beautiful. It is very obvious from this view why such plants were brought into Scotland. But if these get a stronghold they will exclude so many of our beautiful native plants.
These bushes are on the banks of the Leven near the bend opposite the Waterside House. 2023.
Tiny, but lovely cream flowers contrast with the red stalks in September. Almost spring in appearance just as other species are getting ready for autumn.
A common wasp does its thing to spread the pollen. Such a pity that japanese knotweed has gotten out of control. But then our problem invasive species first arrived here, because they are attractive.
To get the degree of dominance that this can achive in perspective, let's look across the Leven where it bends opposite Alexandra Road / Indis Street. This is a surprisingly large area of land currently cut off from most of us and presumably underveloped because of the Dalmarnock Textile Works nearby - although Dalmarnock House used to be here and not the works itself. Anyway the point is that few people ever go here except for a few fishermen and maybe a dog walker. The photos below were taken in late June 2023.
This is a very large thicket of Japanese knotweed down at the river edge. Remember that this plant mainly uses water flow to disperse its seeds. Anywhere downstream from here, even the islands in the Firth of Forth, are liable to affected.
This is a path cut by locals through the Japanese knotweed to get to their favourite fishing point at the bend in the river. These plants are about twice the height of the average adult person. Nothing like the sweet little plants across the river. Most plants will typically reach about eye level, but if they take a permanent hold can reach as high as 3 metres.
In this view taken from the west side of the Leven you can see the same stand of Japanese knotweed to the right. The path cut through as shown in the picture before that is behind the old electrical building. To the left is what appears to be dead plants, also very tall, but which seem to have been treated with a herbicide. This has certainly worked, but the impact has been limited. New growth can be seen.
This is the same stand of Japanese knotweed as see from the west side of the river. Here there is a gap in the trees that is completely overtaken by this invader.
The application of a herbicide, if this is what it was, has certainly worked, but this view from across the river at low water shows new shoots rising up in defiance. The very contrasting water levels over the year are part of the problem. The seeds are spread mainly by water and this means higher up the river banks too.
Over the summer of 2023, other patches of Japanese knotweed have been noted around West Dunbartonshire. [Near the footbridge at Renton - small, but thriving; old small quarry on Renton Road near Dumbarton - previously treated with herbicide, but regrowing.
As with almost all invasive species, there is no need for panic. They are only a threat when other species of plant, specifically native, are detrimentally affected by being ousted. We need to be very aware and take action accordingly.
Japanese knotweed on the Leven towpath in November.
GLASGOW CITY COUNCIL : https://www.glasgow.gov.uk/index.aspx?articleid=19432
JAPANESE KNOTWEED SPECIALISTS website : https://www.japaneseknotweedspecialists.com/news/should-you-still-worry-about-japanese-knotweed-in-the-winter#:~:text=As%20temperatures%20start%20to%20plummet%2C%20in,appear%20spindly%20and%20lose%20their%20leaves.&text=As%20temperatures%20start%20to,and%20lose%20their%20leaves.&text=start%20to%20plummet%2C%20in,appear%20spindly%20and%20lose
GB NNSS - NON-NATIVE SPECIES SECRETARIAT : https://www.nonnativespecies.org/non-native-species/information-portal/view/1495#