Invasive species week: Tuesday 16th May

Invasive species mythbusters: Debunking common Misconceptions

Which invasive species makes a nice crumble and which one will kill you? Which species cannot be killed? Can knotweed break through concrete? We unwrap some common myths.

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You can eat knotweed

True: knotweed can be cooked and eaten – but there are caveats!

There are a few restaurants already serving knotweed in various recipes, most popular is the English classic desert – Crumble. Apparently it cooks and tastes very similar to rhubarb.


There’s a big problem with cooking and eating knotweed, and that’s the issue with knotweed waste. Regardless of what you may read in the press, knotweed waste has to be removed to a landfill site licenced to handle noxious waste. This means you simply cannot throw any unused knotweed into the bin, or a compost, or worse – into the countryside.

Invasive weeds aren’t poisonous

Not true: Hemlock water dropwort is the most toxic plant in the UK

It isn’t called ‘Dead Man’s Fingers” for nothing – and yet it is very easy to mistake it for the edible herb Chervil. This mistake has taken lives as foragers believe they are adding a harmless and tasty addition to their salads or sandwiches. It is also a danger to livestock who see the roots that may be exposed on river banks (mostly in drought season) as a source of food. The consequences of ingesting this water species are undoubtedly serious, and that’s apart form it being an invasive species that must be controlled or eradicated.


Japanese knotweed can be killed

Not true: bleach, petrol, fire, boiling water will not kill knotweed

Even though in this picture the knotweed looks dead, these are the dormant canes that turn brittle over winter – the plant is alive underground. Knotweed grows from underground rhizomes (roots) which if not completely removed will grow new plants. Even a fragment of rhizome will allow new growth. Academic studies have proven that the most effective method of control (other than specialist excavation) is the correct use and application of Glyphosate based herbicides. Applied at the right times of year these herbicides can control the growth of knotweed, stopping it from spreading and ultimately putting it into a dormant state. So the only effective options are chemical treatment with a suitable herbicide (Glyphosate) or excavation to completely remove all soils rhizome and surrounding soils that may contain fragments. A treatment programme carried out over several years is an effective long-term solution providing the ground is not intended to be disturbed by gardening or building works. Excavation is the only option for complete eradication.


Herbicides will become ineffective

Possibly: certain invasive weeds are becoming resistant to some herbicides

It could be predicted that the continued use of a single herbicide would eventually result in the evolution of resistant weeds, but the reality is that most weeds that do develop a resistance have no growth or fitness advantage, than those who do not. The key is to use herbicide intelligently, and with a more controlled strategy that may involve other non-chemical methods. This reduces build up that may kick-start a resistance adaptation process.

Water Soldier, Stratiotes aloides

Bamboo can be killed by cutting it down 

Not true: leaving the root system underground enables it to grow again

Like Japanese knotweed, Bamboo plants grow from an underground rhizone (root) system. Bamboo comes in two varieties; Running and Clumping. Both are invasive if not controlled but it’s the Running variety that causes encroachment issues as the roots simply run underneath fences and even walls to encroach on neighbouring properties. A ‘stump treatment’ programme, where the stems are injected with herbicide that is absorbed into the root system will kill the rhizomes, but simply cutting the stems down will not kill Bamboo.

knotweed can break through concrete and brick

Not true (subtle difference): Knotweed plants take advantage of any crack or crevice in structures

Contrary to popular opinion, knotweed shoots can’t  break through solid concrete. Left uncontrolled knotweed can form a strong enough mass to exploit weaknesses or extrapolate existing flaws in paving, brickwork, and concrete (as shown in the image). However, it’s presence on a site and the restricted use (loss of amenity use) impact it poses until controlled or removed can be seen as bigger nuisance to that of occasional structural damage.

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