‘Knot’ a weed? Triffids, Knotweed and Rhododendron


If weed is a plant in the wrong place does that by definition mean that your prized peony becomes a weed when it’s outgrown its allotted space? No. In that situation it needs dividing not composting. But a plant can be a cultivated garden inhabitant and still become a weed; or worse…

I may have mentioned Triffids in passing last post, but whilst they may be fictional, two of the most notorious examples of invasive plants species that are posing a real threat are Japanese knotweed and Rhododendron ponticum.

The rhododendron was planted in the 19th century as cover for game birds on shooting estates. It thrives in dense shade and outcompetes most of our native species; this has become a major problem as it spread into the wild. Not only does it form dense thickets and smother other species that way, but Rhododendron ponticum also sends out phenolic chemicals into the soil which suppress the ability of other species to grow nearby. This shrub is highly shade tolerant and can grow in only 2% light; most plants need around 12% or more. If you remember your biology lessons from school, light (sunlight or artificial) is one of a plant’s requirements in order to photosynthesise and thereby produce its food (in the form of sugars). To be able to feed yourself in 2% light is therefore a very useful trait.

However, the rhododendron poses less of a problem in most garden situations, mainly because a less invasive variety has generally been planted; although we have come across some communal gardens where it was most definitely a weed and creating a problem.

Japanese knotweed seems to have the title of the most invasive plant in the UK. Does it deserve it? Probably, yes; it even got a mention at the Earth Summit in 1992. Japanese knotweed is an extremely vigorous rhizomatous (underground root system) perennial, originally introduced as a decorative and fodder plant in the early 19th century.

It reproduces by vegetative means rather than by seed; even very small stem fragments are enough. It is thought that most of the plants currently in Britain are actually clones of the original plant, which was a female. Luckily no males were introduced or we would have had the knotweed reproducing by pollination/ seed as well as by root and rhizome fragments. As Japanese knotweed is already distributed throughout the British Isles (the Orkney Islands are the least affected area) a somewhat nightmarish scenario comes to mind…

There may be hope though, a tiny sap sucking insect – Aphalara itadori – is the knotweed’s native predator in Japan and it may just chomp its way through the British population. There are still some concerns with regard to some of our native species also being tasty. And of course, biological control brings its own issues – in order to have a sufficient predator population you do need to have something for them to eat.

For the foreseeable future, Japanese knotweed remains public enemy number one and its removal and disposal is covered by various laws; for example, under the Environmental Protection Act 1990, Japanese knotweed is classed as ‘controlled waste’. If you are not sure whether you have knotweed in your garden do call us as we can advise you on the legal and suggest weed control options.

Having said that and given that there were concerns over the asparagus crop this year, perhaps we should be giving some thought to eating Japanese knotweed; the young shoots, although mainly eaten by sheep, are supposed to taste very like asparagus. Anyone fancy trying it?

Radical solutions aside, eradication is a long term process for Rhododendron ponticum, Japanese knotweed and all those other invasive species. Definitely in the wrong place; definitely weeds.

Just for your delectation here’s a link to a BBC time delay film of Japanese knotweed growing to 1 metre tall in 3 weeks… don’t blink…

If you would like some advice, perhaps a consultation visit on how to de-weed and de-pest your garden in an easy, environmental way  giving yourself more time just to sit and enjoy the sunshine, get in touch.


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