Do you have aliens in your garden?

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This guest blog post is re-posted from the Sustainable Blewbury newsletter, and written by Jo Lakeland and Glen Meadows. Photos have been taken from the RHS website, or by Glen Meadows in the second half of the article.

More plants added to the EU Invasive Alien Species list

I (Jo Lakeland) had never heard of the Invasive Alien Species (IAS) list until I read a recent article in the Garden (the Royal Horticultural Society magazine) that said nine more plants had been added to the IAS list, and will be banned from sale from 2018 onwards.

I pictured Triffids, but these are not that exotic: invasive alien species are ‘non-native species that are deliberately or unintentionally introduced by human action outside their natural habitats, where they establish, proliferate and spread in ways that cause damage to biological diversity’. The article goes on to say that of the nine, six are widely grown or commonly found in UK gardens. These six are: Asclepias syriaca (silkweed), Elodea nuttallii (Nuttall’s waterweed), Gunnera tinctoria (Chilean rhubarb), Hercleaum mantegazzianum (giant hogweed), Impatiens glandulifera (Himalayan balsam) and Pennisetum setaceum (African fountain grass).

giant hogweed

When we first moved into our house in Blewbury 31 years ago we had no idea what had been planted before we arrived, so we were delighted to find a fantastic plant in the garden that looked like giant cow parsley. We didn’t disturb it because we didn’t want to lose it, but it dried beautifully in the autumn and we used it in Christmas decorations. But the next year we discovered its hidden menace. I was gardening in shorts, cutting back our spectacular free plant, but when I went inside for tea my leg started itching, and I later developed a patch of weeping blisters on my leg. And I still have the scar today! Giant hogweed is highly phototoxic: contact with its juice can cause severe burns upon exposure to sunlight. It’s highly invasive, out-competing native plants and smothering them, which can cause river bank erosion and flooding.


African fountain grass (below) is a recent arrival in our garden. I bought it, and it is still being sold by the RHS. Surely they would
not sell an invasive grass? The answer is that it is banned from sale from 2018 onwards, so it can still be sold this year.

African fountain grass

Nuttall’s waterweed is a close relation of Canadian pondweed, the UK pond owner’s favourite oxygenating plant, growing to 3 m long and also on the list. Pondweed illustrates the major problem of having an IAS in your garden. If you have it in a pond you’ll know just how prolific it is – it chokes my pond every year, and I have to take out 3–5 wheelbarrow loads every summer (It is a large deep pond).

The ban is not retrospective, so you do not have to get rid of any of the species you already have, but you must be able to control them on your own property, and not allow them to spread into the wider environment. I am still smarting from discovering that Blewbury’s ducks transported duckweed into both our ponds on their feet. Could they be spreading Canadian pondweed in the same way?

Himalayan Balsalm 2Finally, Impatiens glandulifera (Himalayan balsam). I am used to wild flowers arriving from neighbours’ gardens: I approve of the additional biodiversity, they attract insects, they can be interesting, and I can usually get rid of them if they prove to be a problem. So I was delighted when a very pretty pink flower arrived a few years ago. It had fleshy stems, so it seemed likely that it would be killed by the first frost – even better, an annual. I cleared away the dead stems and looked forward to a few pretty pink flowers next year. But it wasn’t just a few – we were invaded by thousands of tiny seedlings, which rapidly smothered everything around them. The more I learn the worse it gets: one plant can produce 800 seeds; its ripe seedpods shoot their seeds up to 7 m away; it can grow to 2–3 m tall. I have been fighting it ever since. Over to Glen Meadows now, who works for the Environment Agency and who encounters Invasive alien plants and the problems they cause at huge scale compared to my minor skirmishes!

As Jo has said, invasive alien species can cause catastrophic environmental and economic damage if not controlled. At the Environment Agency we come into contact with these species regularly, and over the years have formulated different types of responses to try and control them.

The first response is ‘eternal vigilance’. Knowing where species are located and what species are likely to invade helps us to control them before they become firmly established. You can help us – by using the plant tracker app on your phone you can tell us about invasive plants that you have seen. We have a national rapid response team – as soon as a new species arrives we can act quickly to eradicate it. This has been particularly successful in controlling water primrose (Ludwigia grandiflora) in the UK. This species, native to South and Central America, spreads rapidly through both stem propagation and seeds. It is a particular problem in France. In rivers and canals it completely chokes the watercourse, shading and out-competing native plants and making navigation very difficult.

Japanese knotweed (Fallopia japonica) is by far the most damaging of our invasive species. Its total cost to the UK economy was estimated in 2010 as £166 million. The rhizomes, which can spread 20 m underground, can penetrate concrete and the dense stands out-compete native plants. This plant can spread from the tiniest of stem fragments. The species is listed, so it is an offence to plant or spread it in the wild. The most successful method for control is cutting followed by applying glyphosate. Soil and other material containing the plant fragments are considered controlled waste under the Environmental Protection Act. Waste should be burnt on-site or buried five metres deep and covered with a root barrier membrane. This is tricky stuff to eliminate!

Another method of control for invasive species is biological control. In an invasive species’ country of origin it is usually not a problem. It has evolved in that environment and has predators that naturally control it. But removing the species from its native environment can remove its predators and it can spread unchecked. The hunt to find species-specific predators is a long and complicated process. Before introducing another, often non-native, species we must first check that it will only predate on the chosen species. After many years of research, the psyllid (small sap-sucking insect) Aphalara itadori was licenced to be introduced to the countryside. This creature only feeds on Japanese knotweed and can severely weaken the plant, so it is a useful weapon against these plants.

Another example of biological control is to use a weevil against the fairy water fern, Azolla filiculoides. Despite its pretty and delicate appearance, it smothers light from native plants and is a real pest, particularly near us on the Rivers Pang and Lambourn. However, the North American weevil Stenopelmus rufinasus loves feasting on this plant and is a highly effective control method.

The most common method of control is physical removal, often just hand pulling. As mentioned by Jo, Himalayan balsam is a problem, so every year I and a few colleagues go to sites along the Thames to go ‘balsam bashing’. The photo shows the results of one of our bashing sessions. It’s a really good way to get rid of some frustration! Balsam bashing days are often organised by local conservation groups, so you too can join in the fun.

Balsalm bashing
Pulling by hand can be effective in some circumstances, but when the problem covers a whole watercourse you are going to need more than a machete! The photo below shows floating pennywort choking the Foudry Brook in Reading. The bottom photo shows the sort of heavy machinery needed to remove it.

floating pennywort floating pennywort 2


Further reading
The GB Non-Native Species Secretariat (NNSS) at is a good source of information. Their species information page includes links to identification sheets for all the species, both animal and plant, and their EU IAS Regulation page includes a list of all the 13 new species; part way down there is a link to a brochure on the 49 species of EU concern (with a photograph and a brief description for each species). The RHS page on Invasive non-native species has a link to further guidance for gardeners. This guidance consists of ‘Be Plantwise’, which includes advice about control of aquatic plants, and a beautifully illustrated pdf guide: ‘Gardening without harmful invasiveplants’.