Some alien species in Ireland, such as Rhododendron, could be classified as monsters
THINK OF THE plants that stand out most in the Irish landscape, especially in the west: hedgerows of hot-pink fuchsia, ditches filled with ember-glowing montbretia, hillsides washed with soft mauve Rhododendron ponticum. These are the images that send our foreign visitors' cameras into a frenzy of clickety-click activity.
All these Irish picture-postcard plants have something in common - besides their prevalence in the landscape. They're not Irish at all, but are alien species. Yet, like those other famous foreign invaders, the Normans, they have become "more Irish than the Irish themselves". The fuchsia flower, for instance, has become the logo for a west Cork food and tourism brand.
Non-native plants that have made themselves at home comprise about half of our floral species. According to Sylvia CP Reynolds' A Catalogue of Alien Plants in Ireland (published in 2002 by the National Botanic Gardens), we have about 920 alien plants, most of which have been introduced since the beginning of the 19th century. These share our island with 900 native plant species, about 200 hybrids, and another 80 whose status is uncertain.
Many of our new Irish plants are benign: fuchsia, which originated in South America, doesn't spread much beyond where it was planted in the hedgerows, and montbretia (from South Africa via France), which is a "garden escape", seems to coexist happily in the ditches and banks with true natives, such as meadowsweet and hogweed. Buddleja davidii, the Chinese butterfly bush, is a common sight in built-up areas, where it favours stony ground, and crevices in wall and rooftops. Its nectar-rich, purple florets bring bees and butterflies flocking, so it helps maintain urban biodiversity. It's more of an amiable opportunist rather than an irritating pest.
Rhododendron ponticum (which sent my last batch of American visitors into ecstasies) is another story altogether. The waxy-leaved evergreen is a native of the Mediterranean, introduced into England from southern Spain in 1763, and some years later into Ireland. Apparently, when it first arrived on these islands, it wasn't very popular, because it was inordinately expensive, and mauve was unfashionable at the time (although you might have thought that with so few plant introductions they would have been grateful for anything exotic). But all of that changed in the next century, when seedlings became cheaper to buy, and when its colour suddenly became voguish. It was planted widely on estates as cover for game, and as a windbreak plant.
Now, however, this rhododendron has become one of the worst possible blights on the Irish landscape. It spreads relentlessly by seed and by layering, excluding light from the ground, and suffocating all other growth in its path. Its roots exude allelopathic chemicals, which prevent other plants from germinating or growing. Its leaves, moreover, are toxic, so nothing eats it. Truly, it is a monster.
We've plenty of other monsters that we might mention while we're considering this Pandora's box of plants. One of the worst scourges for gardeners is the Japanese knotweed (Fallopia japonica). Its rhizomes can travel seven metres from the parent plant, and to a depth of three metres (which makes digging it out well-nigh impossible). And although all the plants in Britain and Ireland are female, which means they are unable to reproduce sexually (that is, by seed), that doesn't stop their pink-and-green, spotty-stemmed gallop. This Asian bully can regenerate from a morsel of rhizome weighing less than a gramme - and can thrust through tarmac, paving and house foundations. It was introduced as a garden plant in the 19th century; William Robinson praises it in his English Flower Garden (fifth edition, 1896): "of fine and graceful habit . . . creamy-white flowers borne in great profusion . . . very handsome". He adds a rider: "Unfortunately it is weedy, and in light soils springs up everywhere".
Japanese knotweed was often planted along rivers and streams by Victorians keen to create naturalistic and wild-looking water gardens.
But water is one of the most efficient modes of transport for plants that are eager to colonise. With knotweed, bits of rhizome and stem are carried downstream.
With other plants, such as giant hogweed, another alien monstrosity favoured by Victorians (its sap can cause skin lesions when exposed to sunlight), it is the seeds that float down river until they find a new bit of bank, where they sprout and put down roots.
But it is the plants that actually live in the water that are the most damaging invaders from abroad: seven of the 13 most significant alien plant species (see below) are aquatic.
But let me end on a positive note: not all the foreign plants that have made themselves at home are brutal thugs. Most of these new Irish have brightened our floral palate. Life would be so much duller if we didn't have candy-coloured valerian lining our railways, old man's beard fuzzing the hedgerows and telegraph poles with silky whiskers, and surprise snapdragons popping out of garden walls.
Top 13 aliens
These aliens have been deemed as the most significant invasive plant species in Ireland by the National Plant Conservation Strategy: New Zealand water fern (Azolla filiculoides) Hottentot fig (Carpobrotus edulis) Large-flowered waterweed (Egeria densa) Canadian waterweed (Elodea canadensis) Nuttall's waterweed (Elodea nuttallii) Japanese knotweed (Fallopia japonica) Giant rhubarb (Gunnera tinctoria) Giant hogweed (Heracleum mantegazzianum) Floating pennywort (Hydrocotyle ranunculoides) Indian balsam (Impatiens glandulifera) Curly leaved waterweed (Lagarosiphon major) Parrot's feather (Myriophyllum aquaticum) Common cord grass (Spartina anglica).