It’s humbling to think how many of the different varieties of plants growing in our gardens, allotments, parks and public outdoor spaces are the legacy of previous generations.
That much-loved variety of fruit tree? Very likely the work of plant breeders and nursery owners of the Victorian era. Your favourite rose? A thousand unknown hands and hundreds of years of expert plant selection almost certainly played a skilled role in bringing it into existence. Your favourite variety of daffodil, rhubarb, lilac, clematis, dahlia, beetroot or potato? The very same.
On the other hand, it’s chastening to consider how our insatiable hunger for new, exciting kinds of plants has inadvertently introduced species to parts of the world where some have subsequently become invasive, disrupting fragile, biodiverse ecosystems and threatening native plant habitats.
Here in Ireland, for example, visitors to our little island could easily assume that the ribbons of scarlet-flowered, shrubby fuchsia (Fuchsia magellanica) and orange-flowering montbretia (Crocosmia x crocsmiiflora) that gaily festoon the damp roadsides, ditches and hedgerows of mild coastal counties such as Kerry and west Cork are native, when in fact nothing could be further from the case.
Instead, the former is a deciduous shrubby species hailing from South America that first arrived in Ireland back in the 1820s, and which, while technically not formally classed as an invasive, is surely close to being so. The latter, most definitely invasive, is a vigorous and remarkably resilient perennial from South Africa. Montbretia’s super-tough underground corms allow it to withstand all but the most challenging growing conditions, making this naturalised hybrid perennial very difficult to control and almost impossible to eradicate once established.
These are just two of a growing number of non-native species whose exceptional vigour and ability to outcompete other plants threatens the delicate balance of vulnerable ecosystems in the wild Irish countryside. Others include: Japanese knotweed (Reynoutria japonica, formerly known as Fallopia japonica); giant hogweed (Heracleum montegazzianum); various non-clumping or “running” kinds of bamboo, including certain species of Sasa, Sasaella, Pseudosasa, and Phyllostachys; the sprawling evergreen periwinkle known as Vinca major; Rhododendron ponticum; and the appropriately named mile-a-minute vine (Fallopia baldschuanica).
Many of these names will be familiar to you already. Type the words Japanese knotweed into any search engine, for instance, and it’s certain to come up with an array of articles detailing how this stately perennial plant was introduced into Europe both as a much-admired ornamental plant and a fodder crop. You can read about how, in 1847, it was awarded a medal by the society of agriculture & horticulture at Utrecht for being “the most interesting new ornamental plant of the year”. And how William Robinson, the famous Irish man who looms large in the history of garden design, described it as “handsome in rough places”.
Dig a bit deeper and you can learn about how the Japanese knotweed plants that now populate large swathes of wasteland, roadside verges and river edges throughout Europe and the US are almost all clones, vegetatively propagated, of a single female parent. Sent to Kew Gardens by the German physician, plant hunter and explorer Philipp von Siebold, it quickly escaped into the greater London area, from where it leapfrogged its way across the city’s parks, gardens and wilder spaces into the wider world.
Such are its powers of vegetative reproduction (thankfully it doesn’t yet spread by seed), that just a single root fragment can grow into a sizeable plantation within a few years, with the power to potentially undermine roads, driveways and building foundations. It has, without overstatement, been described as the supreme weed, and yet astonishingly was still being sold in Irish garden centres as an ornamental plant right until the 1980s.
Many other similarly invasive species were introduced as ornamental plants for garden ponds, but now pose a serious threat to Ireland’s waterways. Examples include New Zealand pygmyweed (Crassula helmsii), whose dense, floating, carpeting growth habit is having a devastating effect upon native species by suppressing growth and germination, thus reducing the amount of oxygen in the water.
Another is the giant rhubarb, Gunnera tinctoria, whose ability to clog waterways and crowd out native species is notorious. So is giant hogweed, another “garden escape” that I wrote about in a recent column. First introduced into Ireland during the Victorian era, it can threaten the stability of river banks as well as human health. American skunk-cabbage, yet another once-highly prized garden ornamental perennial (Lysichiton americanus), which was adored for its statuesque growth habit and giant yellow flowers, has also become naturalised in some parts of our waterways. Such is its potential negative impact upon vulnerable ecosystems that the European Union has banned its sale and cultivation.
It’s easy to think of the story of invasive plants like these as a solely historic one belonging to the era of the great plant-hunters of the 18th, 19th and early 20th centuries. But that would be misguided. Gardeners’ curiosity and desire for novelty remain a constant and are now coupled with easy access to an often-bewildering range of species that can be purchased online as plants or seeds, sometimes from irresponsible nurseries or private gardeners blithely unaware of their potential dangers. Others are occasionally smuggled home in hand luggage from trips abroad.
While the vast majority won’t become invasive, there’s always the risk that a handful will. Assessing the true scale of that risk is challenging, especially in the light of climate change, where constantly evolving growing conditions can lead to certain species gaining a foothold. For example, it might be a surprise to many to discover that the popular ornamental deciduous species known as tree of heaven (Ailanthus altissima) is on the EU’s invasive alien species of union concern. This means that it can’t be imported, traded, or released to the wild, while measures must also be taken to prevent the spread of existing populations. While there are no records of it having naturalised in Ireland so far, this could change.
If we’re to avoid repeating the mistakes of the past, then it’s clear that gardeners have a key role to play. Practical measures include: sourcing plants and seeds responsibly; avoiding planting non-native species in the wild (this is illegal); avoiding accidentally allowing the seeds and root fragments of invasives to be transported outside our gardens and allotments; and taking decisive action when and if we find an invasive or potentially invasive plant species has inadvertently made its way into our garden or allotment. It’s by no means an easy task, admittedly, but an important one.
For a more detailed overview of invasive plant species, as well some excellent plant guides, see invasives.ie , biodiversityireland.ie , invasivespeciesireland.com and invasivespeciesni.co.uk
This week in the garden
Keep dahlias regularly deadheaded and liquid-fed, and make a point of regularly shaking any dropped petals off the foliage and stems to prevent disease setting in and spoiling the display.
Lavender bushes should be lightly sheared back at this time of year after the flowers have faded, removing all the flower stems along with roughly 2cm-3cm of the top growth. This will help to keep the plants bushy and encourage a great display of their fragrant blooms next summer. But always avoid cutting back hard into old wood which can shock and even sometimes kill the plants.
Dates for your diary
Farmleigh House Plant Fair Phoenix Park, Dublin; Sunday, August 3rd (10am-5pm). Stalls by many members of the Irish Specialist Nursery Association, and a craft and food market, admission free, see farmleigh.ie.
Wild Garden Adventures: Pond Dipping National Botanic Gardens, Glasnevin, Dublin 9, Wednesday, August 6th. A hands-on practical workshop exploring “the weird and wonderful diversity of insects and mini-beasts living in and around the ponds at the National Botanic Gardens”. Suitable for children aged eight and over, pre-booking essential, see botanicgardens.ie.
Irish National Vegetable Championships The Showgrounds, Moate, Co. Westmeath, Sunday, August 24th (10am-5pm). The annual championship takes place as part of the Moate Agricultural Show and includes a new junior category. See nvsuk.org.uk for schedule and entry form.
