Imagine a summer stroll along a local riverbank or footbath. With councils cutting back on maintenance, things are lusher and wilder than ever: you might spot oversized leaves topped with plumes of creamy blooms, intricate purple blossom with an appley scent, or giant towering lacy white flowers. Idyllic, no? Well, that’s Japanese knotweed, Himalayan balsam and giant hogweed, three of the UK’s worst invasive species. Every summer brings fresh horror stories about these biodiversity-bashing bullies, especially knotweed: “I don’t feel very safe near it,” a Croydon woman whose garden is overrun said recently; homeowners in Llanelli are watching, helpless, as a forest of knotweed creeps ever closer.
How worried should we be? From the tens of thousands of non-native species grown in British gardens, only a tiny proportion escape to become invasive. Yet the expression “Invasive non-natives” has a sort of… Farage-y quality. Alien plants, coming over here, taking our nutrients, confusing our pollinators and destroying our biodiversity? It’s the stuff of nightmares: time to take back control. The truth, predictably, is less alarmist. Alastair Fitter, plant ecologist and author of The Wild Flowers of Britain and Ireland, explains, “Any time you bring in a new garden plant, you’re running a roughly 1/1000 chance of it becoming a pest.” Moreover, most pollinators don’t care what they pollinate, and non-natives add colour and beauty to our environment: “There are lots of non-native plants people admire: we rather like having meadows of fritillaries.”
“Native” doesn’t necessarily mean beneficial either. “Lots of things that are native are very invasive,” says Fitter. “Like bracken: it’s taken over most of the uplands. Or you can have monocultures of heather as far as the eye can see, entirely artificially maintained by grouse moor owners, but people say how beautiful it is. There’s no logic to this.”
There are, however, real issues when a plant, alien or otherwise, is too successful: “Anything really abundant creates the seeds of instability,” says Fitter. The non-native nasties opposite are terrifyingly effective in outcompeting native flora – most are destructive, and one is harmful to humans. We take them seriously: the EU list of banned species – plants it is illegal even to sell – is part of UK law; other plants are banned from sale here and many more fall under Schedule 9 of the Wildlife and Countryside Act: “You’re not allowed to plant, grow or allow them to spread into the wild,” explains John David, head of horticultural taxonomy at the RHS.
If you have a potentially invasive species in your garden, what should you do? For a start, don’t share it around. “If it’s doing very well, don’t give it to other people – there’s a tradition among gardeners to share plants, but these are things you really shouldn’t be sharing,” says David. Be careful, too, how you dispose of it: fly-tipping is a huge problem, leading to uncontrolled spread. The Environment Agency prefers they aren’t moved at all, so burning or composting is best: “If they’re properly composted and the centre of the compost heap gets to the right temperature, it will kill off the plants,” says David.
The other thing gardeners can do is watch out for future triffids: they are the people best placed to know when a new non-native might be becoming a problem. That’s the principle behind the citizen science project Plant Alert, where gardeners can register garden plants that show signs of getting out of control. “If you have something in your garden that is ornamental and spreads and annoys you, report it to Plant Alert,” urges Katharina Dehnen-Schmutz, Plant Alert coordinator. Simply upload a picture of the offender – no need to know its name.
Dehnen-Schmutz’s initial analysis of Plant Alert data in 2018 allowed her to identify several possible future culprits, including tetrapanax, the attractive, jungly looking rice paper plant: it might be minding its own business in my borders, but is now found in the wild and heading out of control in Cornwall and near the Regent’s Canal in London. “It was really bad how it had spread.”
Can we beat invasives? According to Fitter, “Sooner or later their natural enemies catch up with them: nature abhors a monoculture.” The question is whether the level of harm they cause before that means we need to intervene before their predators do. Apart from regular management, one avenue of research is introducing a natural predator. “Biological control” research with fungi and various insects is ongoing, with varying degrees of success.
Then of course there’s the big planetary unknown: “We don’t know the impact climate change might have,” says John David, a thought echoed by all the botanists I speak to. “Plants might perform differently as a result of warmer winters.” While we wait to discover what our future leafy overlords may have in store, here are some of the worst bullies to keep an eye out for now.
Japanese knotweed
The plant bogeyman everyone fears, Japanese knotweed can invalidate your mortgage offer and grow through concrete, years after you think it has been beaten. The visible part – bamboo-like stems, feathery cream flowers – grows to 2-3m, but more importantly so does the root system, which can remain dormant for up to 20 years before bursting into life again, possibly through your foundations. The asbestos of the plant world, it must be disposed of as hazardous waste (“This applies to most species that are deemed invasive,” John David says). But how much of a problem is it really? “I think it’s grossly overstated,” says Alastair Fitter. Given where it grows – derelict sites, mainly – Japanese knotweed is rarely a risk to biodiversity: it’s an economic, not an ecological problem. Removing it from the Olympic park in Stratford cost £70m, but Fitter says: “All you need to do is cut it every year for five years and it’ll eventually die.”
Triffid fact: Gardeners in Oldham in 1887 were bemoaning how it “kept appearing in nearly every piece of cultivated ground”; in 1899 Gertrude Jekyll warned it should be “planted with caution”.
Giant hogweed
You know cow parsley, beloved of Instagram posters and Etsy crafters? Well imagine that but absolutely gigantic (up to 5m in height), hugely invasive and very bad for you. The sap of giant hogweed contains a compound – furocoumarin – which makes skin intensely photosensitive, leading to vicious burns and blisters when someone who has touched it is exposed to sunlight. The effects on skin can recur over months, or even years. Giant hogweed originates in the Caucuses and was introduced in the UK in the mid-19th century; we’ve been trying to stuff the genie back in the bottle for decades. “It’s a really nasty plant and it absolutely needs to be got rid of,” says Fitter, unequivocally.
Triffid fact: A giant hogweed seedhead can produce up to 50,000 seeds, which can remain viable for up to 15 years.
Himalayan balsam
Big, bright, beautiful and the scourge of riverbanks: Himalayan balsam “grows very quickly, doesn’t need much in the way of water and nutrients and it can get up above all our native plants and shade them out,” says Jonathan Dent, who is responsible for balsam eradication for York nature reserve, St Nicks. Like most problematic invasives, Himalayan balsam creates monocultures, outcompeting other plants. It also dies back to nothing yearly, “leaving bare soil along the riverbanks that can get eroded away”. Clearance is tricky: people are aware it’s bad news, and want to pull it up, but could be doing more harm than good. From May to the start of July cutting or uprooting is usually safe, but once seed heads are ripe and ready to disperse, it’s a job best left to the experts. “If the roots haven’t been broken up and are just sitting on the soil, it can reroot itself,” Jonathan Dent also warns.
Triffid fact: Himalayan balsam seed pods pop, dispersing seed for many metres: to prevent that, Dent uses a bomb disposal-style controlled explosion, putting the seed heads in a plastic bag.
Rhododendron
What is it about pretty purple flowers? Rhododendron is our second purple offender: this 18th-century import thrives in ecologically delicate and valuable environments that support many species, such as oak woods and dunes. It’s quick spreading, growing to 8m in height, shading out and smothering other low-lying vegetation. It’s an interesting one, John David says, because it’s illustrative of the ambivalence we feel toward some invasives: “People say it’s really nice when it’s in flower.” It is: you might have visited plantations (I certainly did as a bored child), before we became aware of quite how problematic it is.
Triffid fact: Rhododendron flowers, leaves and pollen contain toxins (grayanotoxins) making them unpalatable to herbivores, and honey produced from the flowers can cause “Mad honey disease” (short-lived cardiac symptoms, nausea and vomiting), though “mad honey” is also made deliberately from rhododendron for its apparent hallucinogenic effects.
New Zealand pigmyweed
Aquatic plants present a particularly knotty set of challenges when they get invasive: you can’t use herbicides, they fragment and multiply as shoots break off, grow new plants and form a dense carpet excluding other plants and animals and damaging habitat. This is why the five plants banned from sale under UK legislation are all aquatics (the others are water fern, parrot’s feather, floating pennywort and water primrose). All of this is true of New Zealand pigmyweed: it’s “very problematic” according to John David; “A real pain” says Alastair Fitter. “It looks nice in people’s ponds, but they get fed up, toss it into the local stream and it just takes over.”
Triffid fact: A single 10mm stem fragment is enough to produce a whole new pigmyweed plant.
