LIKE sunshine on a cloudy day, the bright yellow blooms of Lysichiton americanus are a cheering sight on a dank, late April afternoon in Brodick Castle's woodland garden. But it's their scent that hits you first: heady, powerful, it's actually rather revolting, unless you are a creepy-crawly on the hunt for a bit of rotting flesh. For when these flowers emerge on the frozen reaches of their native north-western America and Canada – having used their own heat-generating capacity to melt through snow – that fetid, sulphurous odour attracts the carrion-loving beetles and flies that are the only available pollinators at this time of year.

Imported to the UK early last century, the malodorous skunk cabbage – to give the plant its common name – is highly prized by Scottish gardeners though as National Trust for Scotland (NTS) horticulturalist Tim Keyworth explains, this decorative bog plant can no longer be sold, having recently been officially classed as an invasive species thanks to its tendency to infiltrate waterways and crowd out native flora.

Here in their Isle of Arran setting, clustered around a muddy woodland pond, those yellow beauties seem remarkably well-behaved although Keyworth does point out a few escapees that have seeded themselves along the burn.

Keyworth, Brodick Castle's former head gardener and now NTS's designed landscape manager for Ayrshire and Arran, appears familiar with this garden's every leaf and shoot. Many of these plants, he explains, were sourced in the early 20th century by famous Scottish plant-hunters such as George Forrest, who ventured into remote parts of the Far East to bring back the exotic specimens then coveted by wealthy Edwardian landowners such as Brodick Estate's Hamilton family.

A few hundred yards from the skunk cabbage pond, Keyworth introduces me to his favourite plant, Rhododendron Fortune – a magnificent century-old giant of a shrub with huge, creamy-yellow blooms and a string of horticultural prizes to its name. Like many of Brodick Castle's rhododendrons, magnolias and camellias, Fortune blooms its heart out every spring without causing problems. But you needn't look far to find evidence of the carnage wreaked by its delinquent cousin, the notorious Rhododendron ponticum. A native of southern Europe and southwest Asia, ponticum was introduced to the UK in the 18th century and was widely planted as game bird cover on shooting estates. Once established in the country's damp, acidic soils, it grew vigorously, reaching heights of up to 8m and creating a thick tangle of roots that harbour phytophthora ramorum, a disease that kills oak, larch and other native trees. By the late 20th century, the plant was out of control and today, vast tracts of Wales's Snowdonia National Park have been swallowed by the shrub while in Ireland, mountain rescuers recently struggled to locate two tourists who had become lost amid the dense rhododendron forest of Killarney National Park.

Here, Forestry Commission Scotland describes Rhododendron ponticum as the nation's “most threatening invasive non-native plant” which, left unchallenged, “will eventually dominate the habitat to the virtual exclusion of all other plant life”. In 2010, it predicted that clearing the 50,000 hectares of rhoddy-infested national woodland would take 10 years and cost £15.5m. “Since then,” says its website, “we’ve been using chainsaws, herbicides, heavy machinery and considerable human muscle power in the battle against this unwelcome alien.”

On Arran, conservationists began tackling the problem in the 1970s but as Brodick Estate's head ranger, Kate Sampson, explains, the plants continue to pose a serious threat to native woodlands: “The problem with ponticum is it grows thick and lush and it's evergreen. It out-shades all the native flora and fauna so as mature trees die, there are no saplings to replace them because they can't grow through the dark, impenetrable mass of rhododendrons.”

Each plant produces around a million seeds, which blow into gardens, fields, hedgerows and onto hills. In places they grow so thickly that the battle against those purple-flowered incomers appears lost. But Kate Sampson isn't giving up and nor are the armies of willing volunteers, including school children, who are periodically deployed to chop down the plants. Chemicals are later applied to the cut stems to prevent grow-back but since light-starved wildflowers such as bluebells and red campion have often been obliterated, Sampson's team then reseed the land in order to kick-start woodland regeneration.

Native flora are not the only re-colonisers, however and in a patch of woodland that was recently cleared of ponticum following an outbreak of phytophthora disease, Tim Keyworth draws my attention to some sprigs of Salmonberry - a pink-flowered North American shrub that's spreading rampantly across parts of Scotland - and several vigorous-looking shoots of the New Zealand evergreen griselinia, whose roots can reach 10 inches thick and whose seeds are currently germinating with abandon along Arran's east coast.

But if controlling those horticultural thugs is a Sisyphean task, Kate Sampson appears undaunted. Further out on Brodick estate, she shows me a gully that was once infested by ponticum but where birch saplings are now resprouting lustily and where wood warblers, which had deserted the area along with their arboreal habitat, have returned in full song.

Another creature that depends on Scotland's iconic woodland is the red squirrel, which is thriving here on Arran where the American greys – imported in the 1870s and blamed for the reds' decline – have never established a foothold. Yet according to Sampson, red squirrels are not actually native to the island. “They were introduced by the Duchess of Montrose [Mary Louise Douglas Hamilton] in the 1950s,” she tells me. “Maybe these days, we wouldn't try and introduce a species. However she did it and now Arran is a haven for red squirrels. In the future it might be one of the last populations we have in Scotland.”

Few could deny the creature's tufty-eared appeal and elsewhere in Scotland, conservationists are attempting to safeguard its future by hunting down the bigger, brawnier greys, which are accused of colonising the reds' habitat and infecting them with a virus to which they themselves are immune.

Is this fair? Campaigning organisations Peta and Animal Aid don't think so. In a joint letter to UK environment secretary Michael Gove earlier this year, they denounced the practice as scientifically and ethically unjustifiable, adding: “Culling may actually lead to an increase in grey-squirrel populations because recolonisation occurs rapidly when food and habitat are easily available. Furthermore, anyone who cares about one type of squirrel should extend that compassion to all squirrels – who feel pain, enjoy raising families, and want to live – regardless of their colour.”

That last phrase is a reminder that the ethics of tackling “alien”, “non-native” or “invasive” species are not clear-cut, and Black Environment Network president Jenny Ling Wong recently warned against creating the “illusion” that certain characteristics – such as aggressiveness or a tendency to “take over” - belong only to plants brought in from overseas. Speaking on BBC Radio 4's Nature's Great Invaders programme, she said: “Sometimes you almost think – are they really talking about plants?” Allied with emotive reports about the “pressure of immigration”, such talk “does have an effect and it can upset a lot of people”, she added.

In the same programme, ecologist Dr Ken Thompson pointed out that some native UK plants can be every bit as “thuggish” as imported species. “If bracken were an alien,” he said, “it would be a national emergency, because it's just as bad as Japanese knotweed and 1000 times more common – you see whole hillsides completely covered by it.”

So what exactly do we mean by those terms? Scottish Natural Heritage (SNH) defines non-native species as “those that occur outside their natural range due to direct of non-direct introduction by humans” and describes them as “invasive” when they “spread and cause damage to the environment, the economy, our health and the way we live”.

The problem is thought to cost Scotland's economy at least £250m a year and in a bid to tackle it, SNH has secured £1.55 million from the Heritage Lottery Fund for its new Scottish Invasive Species Initiative (SISI). Supported by fisheries boards and Aberdeen University, it aims to engage volunteers and community organisations in tackling some of the most troublesome species affecting rivers and lochs across northern Scotland.

Targeted plantlife includes notorious pests such as giant hogweed, Japanese knotweed and Himalayan balsam as well as white butterbur, coneflowers and those lovely skunk cabbage plants. Then there is the American mink, a carnivorous mammal that was imported by fur coat producers during the 1920s and began breeding in the wilds in the 1950s after escaping or been released from fur farms. The sole animal species on the hit-list, it is, says SISI project manager Callum Sinclair, “a hugely impressive coloniser which, because it is both aquatic and land-based, can predate on all sorts of species that may have been protected from other animals”.

A threat to ground-nesting birds such as moorhen, the mink has also driven the water vole – Scotland's fastest declining mammal - to near extinction. But the incomer's fortunes could be about to turn as SISI prepares to send volunteer nature detectives along riverbanks armed with special clay-bottomed rafts that will act as a kind of aquatic fingerprinting system. “When the minks leave their footprints in the clay we know that they are around,” explains Sinclair, “and only at that point do we deploy traps to try and catch them.”

And then? “They are humanely dispatched,” says Sinclair delicately, making it clear that he doesn't relish this aspect of the process but pointing out that systems are in place to ensure the traps are checked daily, and that the deed is done only by trained, certified individuals using air pistols, which is “recognised as the most humane method”.

“We are very conscious that this is maybe seen as a contentious issue,” he adds, “but thus far people seem to be supportive and recognise the damage that American mink cause.”

Plant control is perhaps less emotive, particularly where a noxious weed like giant hogweed is concerned, and SISI plans to tackle its growth along the Rivers Spey and Deveron through a combination of professional contractors and volunteers – including a flock of Blackfaced sheep, which apparently can munch the young shoots to no ill effect.

Targeting selected areas across one-third of Scotland's landmass, SISI is the most ambitious invasive species programme ever launched here. Could it spell the end for serious pests like Japanese knotweed, once hailed as an ornamental, medicinal and soil-binding wonder-plant but now reviled for its destructive ability to damage roads, drains and buildings?

Sinclair makes no such claim and accepts that “there are places where coverage is so extensive and dense that with available resources, complete removal is difficult at best if not impossible”. With determination and careful targeting, however, the hope is that problematic species can be brought under control allowing important natural habitats to recover. Crucially, community involvement is expected to help sustain those improvements well beyond the project's four-year life span. “The majority of the spend on this project is not on contractors,” says Sinclair. “It's on support, encouragement and engagement with volunteers – practical support that hopefully will help people to feel valued and valuable. That's important to us.”

Back on Arran, rhododendron wood cut down by volunteers has been stacked in neat piles and Kate Sampson shows me the huge metal kiln in which it will be incinerated and turned into charcoal, to be sold locally for barbecues. Like Sinclair, she sees local people as key to conserving Scotland's natural heritage for the future. “We're only here as custodians for a short time,” she says pragmatically. “But if you talk to the next generation, involve them and encourage them, then maybe you'll plant the seed of someone taking over. That's why education, and involving as many people as possible, is important.”

With 2,500 hectares of mountain and moorland in its trust – including the iconic Goat Fell – the NTS has ambitions to reverse some of the damage done by centuries of grazing by sheep and deer. “The bluebells, wood sage and dog's mercury that can be found in Glen Rosa are evidence of ancient woodland, but there haven't been trees there for centuries,” says Sampson. “This winter we hope to start planting trees there, such as oak, aspen and the island's own Arran whitebeam.”

Left undisturbed, what would Rhododendron ponticum do to the island's landscape? “We would certainly lose a lot of our beautiful mature native woodlands,” says Sampson. Could we lose the heather cover on the hills? “Yes, absolutely. It may take a long time but eventually you'd just get a desert of rhododendron, and you'd lose all your native plants. Instead of walking across heathery moorlands, you would be faced with a thicket of dense rhoddy, which is impenetrable.”

And with the heather and the woodlands would go all manner of wildlife. Tree pipits, redstarts and wood warblers would fall silent as the landscape of Scotland changed forever. I wonder what George Forrest and those other intrepid plant hunters would have had to say about that?

Six of the worst invasive species:

Japanese Knotweed

Latin name: Fallopia japonica

Native to: East Asia (Japan, China, Korea)

Introduced to UK: 1850

The problem: This rampant perennial, which produces tall bamboo-like canes up to 7m tall, can sprout from tiny sections of rhizome and grows so vigorously it can damage drains, house foundations and roads so must be removed before construction works begin. Threatens wildlife habitats and biodiversity. Eradication is notoriously difficult, costly and must be done by professionals.

Himalayan balsam

Latin name: Impatiens glandulifera

Native to: Himalayan foothills, India and Pakistan

Introduced to UK: 1839

The problem: Despite its pretty pink flowers, this fast-spreading relative of the Busy Lizzie is one of the UK's most invasive weeds, taking over river banks – where it clogs drains and creates a flood risk – smothering other vegetation, causing erosion during winter die-back and threatening biodiversity.

Giant hogweed

Latin name: Heracleum mantegazzianum

Native to: Caucasus mountains, Southern Russia and Georgia

Introduced to UK: 1817

The problem: Related to cow parsley, this huge plant produces white umbels up to 3m tall. As well as being invasive and out-competing native plants, it produces a toxic sap that can cause skin blistering and scarring. Poses flood and erosion risks during winter dieback, and can block access to rights of way.

American mink

Latin name: Mustela vison

Native to: North America

Introduced to UK: 1929

The problem: Imported to cater for the fashion for fur coats, this carnivorous mammal began breeding in the wild in the 1950s and spread rapidly. Preys on frogs, fish and other aquatic wildlife, reputedly driving the water vole towards extinction. They also eat the eggs, chicks and adult nesting birds and have been blamed for depleting moorhen populations.

Rhododendron ponticum

Latin name: Rhododendron ponticum

Native to: Southern Europe and southwest Asia

Introduced to UK: 1760s

The problem: This dense, evergreen, lilac-flowered suckering shrub, planted on Victorian shooting estates as ground cover for game birds, spreads rapidly, producing up to a million seeds per plant, and can quickly cover whole hillsides, shading out native plants and trees thus threatening the wildlife that depends on them. Having naturalised happily in Scotland's damp, acid soils, it poses a grave risk to the countryside according to ecologists.


Latin name: Rubus spectabilis

Native to: North America

Introduced to UK: 1827

The problem: The orange, raspberry-like fruits of this attractive pink-flowered shrub are loved by birds, which then seed the plants widely. It also spread rapidly by suckering and while valued as a garden and game cover plant, its spread is becoming problematic in parts of Scotland, where it shades out native plants and prevents woodland regeneration.