Casual Gardener: Be alert for aliens

Ireland's Atlantic seaboard highlights the ecological threat posed by invasive alien plants...

Crocosmia's attractive orange flowers belie what is actually a vigorous, invasive plant
Crocosmia's attractive orange flowers belie what is actually a vigorous, invasive plant

EARLIER this month, during what has been for decades an annual foray into the West, we visited Achill Island. It's a place that's quintessentially Irish, meaning it has much in the landscape and among its people that's alluring, yet it's also haunted by ghosts of a troubled past and bears the scars of a modern, laissez-faire approach to development.

It also happens to be Ireland's capital for invasive plant species. South American Gunnera tinctoria, pampas grass, Japanese knotweed, Himalayan balsam, monbretia, giant hogweed, fuchsia and Rhododendron ponticum – it's got them all, in significant numbers, warmed by the Gulf Stream and rooted in the moist, peaty earth characteristic of Co Mayo's Atlantic seaboard. On Clare Island, a couple of miles south out in Clew Bay, it's the same story, and on swathes of the mainland too.

Over the centuries, the Irish have generally been tolerant of overseas visitors, bar those whose purpose was to subjugate. Our gardeners too have been enthusiastic in welcoming plants from all over the world. Many of these plants have settled benignly, naturalised among the native flora, while those mentioned above have been branded "thugs" and "bullies", suppressing indigenous plantlife often with dense, vigorous canopies.

Irish-born gardening legend, William Robinson, must bear some of the responsibility for this. The Victorian landscaping doyen and naturalistic planting pioneer, along with his many acolytes, embraced many of these large, exotic perennials because when planted en masse their scale would imbue a previously tamed garden or park with a wild, elemental feel.

Somewhat counterintuitively, imported plants tend to thrive in our gardens, primarily because native predators and pathogens leave them alone. Some, such as fuchsia – West Cork's adopted flower – are great for pollinators but others, such as the giant rhubarb, create an ecological desert, a monoculture that's the antithesis of biodiversity.

It's alarming to think these irreversible changes to ecosystems and habitats, which can include the invasive species' so-called non-biotic effects, such as reducing or impeding water flow or completely transforming the pH of the soil, came about inadvertently from something regarded, at the time at least, as benign.

There are at least three dozen plants included on the 'EU Regulation on Invasive Alien Species' lists, which still applies on both sides of the border and makes it an offence to help them spread into the wild. Three-cornered garlic (Allium triquetrum), a handful of cotoneasters, Spanish bluebells (Hyacinthoides hispanica) and a range of vigorous aquatics are among those listed.

The RHS says: "These plants should not be planted or caused to grow in the wild but in addition are banned from sale and gardeners possessing them should undertake measures to control them."

My advice to fellow gardeners, all of whom love the idea of something for nothing, is simple – be cautious and wary of Greeks bearing gifts, which is often nature itself. In my novice gardening days, I looked on Crocosmia x crocosmiiflora (montbretia) as a godsend, replanting this strappy-leaved, southern African native liberally.

I can at least argue that it had already colonised my corner of Lecale before I began propagating it but it's a poor defence. Now it has spread throughout the garden and I'd like to eradicate it completely, leaving only the non-invasive Crocosmia 'Lucifer'.

Accepting a friend's ostensibly generous offer of surplus plants may seem like a good thing, but as Achill's tainted landscape is testament, it's a gesture that can backfire on a grand scale.