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16 Sep 2021 < Back

Invasive plants: The Jekyll and Hyde conundrum

Non-native invasive weeds get a lot of bad press for obvious reasons. But are they ‘all bad’? Robert Louis Stephenson’s allegorical tale about good and evil provides a useful analogy here so lets see how it helps?

Most invasive weeds arrived in the UK by deliberate human intervention because they were desirable ‘specimens’ for the embellishment of gardens and parks.  This also tends to mean they have rather showy flowers (or fruit).  No wonder that on his recent walk, Andy (our man in Ayrshire) decided to take some pictures of Japanese knotweed and Himalayan balsam!

Of course, he knew it was wrong to ‘like’ these ‘monsters’ but will he, like the good Dr. Jekyll, be damned for it? Can we admire their ‘good side’ or is that never going to end well…?  Read on……

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Why did these plants catch Andy’s eye?

Well both species are BIG, here over 2.5m tall, and they love riparian habitats so, once established, easily dominate the native bankside vegetation. These characteristics are the basis of all non-native invasive plant classifications; alien invasives are the thugs of the botanical world. Very much Mr Hyde; not a bit like the nice Dr Jekyll – right?

Well….. lets look at the other side of the coin. The second reason Andy noticed these plants was that they were flowering when most native species have long-since finished for the year (providing both aesthetic beauty and food for insects etc.). So should we re-think our opinion of these thugs? Are they a bit Dr Jekyll after all? Don’t forget, BBC Springwatch earlier this year advocated planting buddleia ‘for butterflies’ for exactly this reason!

Is removal/management always necessary?

The law is the law so don’t take this the wrong way. I am not saying lets ignore Schedule 9 invasive species because they are “not all bad”! But it is worth stopping every now and then to think about the site specific impact of an individual invasive species; in short, is the removal or management of the plant always necessary or desirable?

Of course, we all know what happened to poor Dr Jekyll. He lost his battle with his alter-ego and surely this is always the inevitable conclusion with non-native invasive plants. Yes, they are sometimes doing positive things in a local environmental context, but given the imperative to prevent their spread ‘to the wild’ it is difficult to see how stopping to admire their positive-side is ever going to end well?

Day-to-day variables – budget v opportunity!

But let’s consider the ‘real-world’. The day-to-day variables we have to deal with are budget and opportunity. Given that many of the plants we want to control are already extremely widespread there is often a need, even the Environment Agency find themselves in this dilemma, to choose between the worst of two (or three or four) evils! In a river system (including ponds/lakes etc) where there are threats from Japanese knotweed, Himalayan balsam and water primrose… the water primrose would get priority for available budget spend (not yet widespread in the UK, hugely expensive to eradicate if it became established – currently on Defra’s ‘species alert list’).

What we are doing, sometimes sub-consciously, is a risk-assessment to establish priorities where we can’t achieve all desirable outcomes in all places at once. The Japanese knotweed and Himalayan balsam are (likely) already the dominant species and although the ultimate aim may be to restore the site to its original ecological state, meantime these plants are probably the only source of food for the local invertebrate population (in September anyway)!

Long-term strategy for management and control

What useful conclusion can we draw. Only this; that all invasive non-native plants should always have a long-term strategy for management and control but that sometimes the negative impact of short-term eradication could, overall, be detrimental to the habitat you are working in. Has a re-vegetation strategy been established (and budget allocated)? Is the client in for the long-term – re-colonisation from upstream sources or the seed bank will demand future site monitoring and follow-up works. Is the removal of the ‘target’ invasive species just going to provide an opportunity for a different one, or increase flood risk/bank erosion in the short-term!?

It isn’t easy to quantify all these things but it should always be the starting point when meeting the client. What are your priorities? What funding sources are available in the short, medium and long-term? Let’s tailor the programme to ensure what we achieve is sustainable, etc.,.

Dr Jekyll thought he could control the beast within, but once let-loose Mr Hyde had other ideas! We see this story played out all around us – they are called invasive weeds!

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