Photographs: Charles Francis
November 2008: Masses of yellow and crimson flowers, from a succulent known affectionately in Cornwall as “Sally-me-ansum”, stage a spectacular springtime show when they carpet the cliffs close to Lizard Point. And at the height of summer, clumps of bright orange montbretia bring a welcome touch of seasonal colour to many a drab roadside verge throughout the county.
But these two plants — known formally as Carpobrotus edulis and Crocosmia x crocosmiiflora — share something besides their undoubted beauty: they are both spreading so rapidly that they threaten to smother wipe native plants, with devastating results for Cornwall’s natural environment.
Cornwall is the adopted home of many Invasive plants on horticulturalists’ hit-lists. Many were brought to the county from Africa, South America and the Far East by Victorian plant-hunters. ”Cornwall was a trial ground for a number of species, because of our agreeable climate,” says Cornwall County Council’s vegetation advisor James Macfarlane. “They came from a very different environment — and they came without the plants which evolved with them in that environment, and which controlled their spread.”
This, he adds, makes them far more difficult to deal with than native pests, like common ragwort, which is poisonous to grazing livestock but can be controlled by pulling up and burning. “Invasive plants began to become a big issue post Second World War. That’s when we began to move plant material around the country — and if you do that, you get more contaminants moving around.”
Montbretia has squashed less showy hedgerow species, and is now spreading into heathland, particularly around St Agnes. “Sally-me-ansum” — a name derived from the plant’s previous classification as a mesembryanthemum — is at home in Cape Cornwall and the Newquay area, as well as on the Lizard. “It looks very attractive, but only for a short period of the year. “It makes the soil very acid, and nothing else then grows there,” says James. “You need a variety of plants to colonise an area. With a monoculture, a sudden frost can leave you with nothing.”
Rachel Holder, a National Trust warden in south-west Cornwall, explains that Hottentot fig — yet another name for “Sally-me-ansum” — has been a feature of the cliffs at the Lizard for half a century, but it has only been recognised as a problem in the last 15 to 20 years.
“It is a particularly difficult species to eradicate, and quite a sensitive issue,” she says. “We’ve carried out a local of local consultation. Some people wanted it to be left alone, while others wanted to see it completely removed. We have to take some action, as it is threatening rare native species, such as long-headed clover and wild asparagus.
“On the less steep slopes, we pull it up by hand with the help of volunteers, but on the steepest sites, we have to use specialist contractors with ropes. It’s a question of achieving a balance between landscape value and nature conservation. We have agreed that we will not remove it from Lizard Point itself, where it cascades down the cliff.”
A notorious foreign invader is Japanese knotweed (Fallopia japonica). Spread entirely by vegetative means, it is extremely difficult to control. New plants can grow from the nodes of green stems, and even from fragments of rhizome smaller than a fingernail. “It can take over large areas quite rapidly, and native species can’t co-exist with it,” says James Macfarlane.
Japanese knotweed was introduced to Britain as an ornamental plant in the early 1800s, but by the end of the 19th century its escape from gardens into open countryside was noted with concern. In the 1907 edition of his book, The English Flower Garden, William Robinson commented on its vigour, describing it as “easier to plant than to get rid of”.
When Cornwall County Council first began to tackle the knotweed problem, it had an estimated 1,800 sites to deal with. A partnership with the Environment Agency and English Nature led to the creation in 1997 of the Cornwall Knotweed Forum, which has become an international blueprint for the co-ordinated control of non-native plants.
The forum offers information on methods of dealing with invasive species, which are being refined continuously in the light of new research. Advice varies according to the identity and location of the problem plant.
In the case of knotweed, powerful herbicides which are only available to people qualified in pesticide use can be effective, but are not permitted where they might make contact with watercourses. Small areas can be tackled with pesticides approved for use by amateurs, by burning, or through placing plants in secure polythene sacks and taking them to a civic amenity site with prior agreement. Knotweed must never be put in with green waste for composting.
The forum is also keeping an eye on plants which are wreaking havoc elsewhere in Europe. Gunnera tinctoria, a giant South American plant which thrives in damp areas, has a vast spread, and is choking high watercourses. Pampas is much admired for its pretty feathery plumes, but in France, where it was first cultivated from seeds collected in Ecaudor, it is rampaging through coast and countryside. Pampas dislikes frost, but can withstand drought, and James believes it could become a serious problem as climate change gathers pace.
Already starting to be a nuisance in Cornwall is bear’s breeches (Acanthus mollis), a fast-growing perennial with eye-catching stalks of pale lilac and white flowers in early summer, and lush green leaves nearly all year round. It thrives on the edges of woods, is frost-hardy, and is spreading rapidly in some areas in the west of the county.
James adds: “The increasing popularity of aquatic plants has provided opportunities for parrot’s feather, for which methods of control are very limited.
“Buddleia is a lovely plant, but if it seeds, it becomes a huge problem, particularly when it colonises old mine sites.”
Himalayan balsam (Impatiens grandulifera), brought from Asia as an ornamental plant in the early Victorian era, decorates riverbanks with its pretty pink blooms. But it is a prolific source of nectar, and diverts the attention of bees from native plants. Despite measures to stop its spread, it has been known to grow back within weeks of treatment.
James despairs when gardening magazines and television programmes recommend plants which the council is doing battle with. One TV gardener advocated the use of mind-your-own-business, a mat-forming herbaceous perennial with a mass of thin, sprawling stems covered with tiny leaves. Although it can be raked out of lawns, large areas often need to be dug out and re-turfed. “At the Royal Cornwall Show, where I had a display of invasive plants, more people asked me how they could get rid of mind-your-own-business than any other plant,” comments James.
“Another programme featured a garden when a giant hogweed was a special feature. “They can be wonderfully architectural — but if you get it in contact with your skin and then combine it with sun, you can get huge blistering problems.” And each hogweed plant can produce 50,000 seeds, which can remain in the soil for up to 15 years. However, digging out the root crown can be an effective method of control, and the plant is now in retreat in Cornwall. It is now primarily a problem in the Tamar Valley.
To James’ amazement, one magazine recently chose winter heliotrope (Petasites fragrans) as its plant of the month. A native of North Africa, it is particularly noticeable alongside the A30 between St Erth and Crowlas in west Cornwall. With pleasant mauve flowers early in the spring, it offers reliable and attractive ground cover – and then starts to smother everything in its path.
Native bluebells are facing a two-pronged attack, both from the three-cornered leek (Allium triquetrum), whose white flowers are often mistaken for wild garlic, and from their domestic Spanish cousins. “Areas like the Helford, the Carrick Roads and Cardinham Woods have some of the best English bluebells in the country, but Spanish bluebells can easily hybridise with them. You often see them planted in gardens in villages near woodlands, and spreading along the road. The English bluebell is far more delicate, and I would say it’s a better colour. The Spanish one is more vigorous and in-your- face.”
James emphasises that he is not trying to demonise exotic and attractive plants — just advising people to use them with care. “Some of these plants have an immediate impact — but we need to think about whether they are desirable in the wider environment. Our human perspective is very short. If something looks nice, we don’t look at the long-term effects.
“Then there is the gardener’s desire for novelty. But instead of bringing in things from somewhere else which have a transitory appeal, we can rediscover our native plants. It’s a question of engaging people’s hearts and minds.”