Tucked away on a steep slope above the village of Cove on the Rosneath peninsula in Argyll, surrounded by Loch Long, Gare Loch and the Holy Loch, is arguably Scotland’s most extraordinary garden.

Like a mini-rainforest, teeming with exotic plants from Peru, China and the Himalayas, Linn Botanic Gardens is a magical feast for all five senses – a garden you can taste, smell and hear as well as see and touch. Many of the plants are rarely seen in the wild, let alone in cultivation, while the 40 species of bamboo, rare rhododendrons, Japanese acers, magnolias, ferns and roses produce a riot of colour.

Sitting at the heart of this horticultural wonder, certainly one of the most biodiverse places in Scotland, is Linn Villa, the dilapidated Victorian house that is being slowly consumed by the spread of greenery around it. Nature takes precedence over everything else; here, you can feel it.

I’m being shown around by the man who bought the house and started working on the garden in 1971, and has nurtured it for much of the last 44 years, botanist and political activist Dr Jim Taggart. Aged 83 – but dressed like a student in skinny jeans, a hooded top and trainers – Taggart makes for a fascinating if frail host.

It’s fair to say the tour, like the garden itself, is unusual. As well as touching on family history and ecology, Taggart, an Oxford-educated theologian as well as a botanist, focuses on mathematics and philosophy as he talks through the reasons for choosing certain plants, shrubs and trees, certain designs, and how these decisions impact on and are impacted by the beautiful landscape all around.

He describes how he has transformed the garden from a typically genteel Edwardian space, complete with tennis court, herbaceous borders and croquet lawn, into the lush, exotic space we see today.

In between, he talks about his experiences as an anti-nuclear campaigner over the last four decades – he is a friend of UK Labour leader Jeremy Corbyn – a cause perhaps made all the more pressing by the garden’s close proximity to Faslane, the naval base which houses Britain’s nuclear weapons, just a few miles across the loch. He also talks about the financial toll of keeping up such a house and the garden.

Walking through the garden with Taggart is a strangely overwhelming experience. But it becomes it becomes even more so – almost unbearably so – when the tragedy that now haunts and hangs over the place is mentioned by Taggart, out of the blue.

“Jamie developed the garden massively,” he says, looking out over the waterfall. “He went off to China and Vietnam to source plants. We’d had just 20 months of financial security when he went off to Vietnam that second time. It doesn’t seem fair.”

The Jamie he speaks of is his younger son, who took over the running of the garden in 1997, giving it a whole new vision and lease of life.

Jamie Taggart, also a talented botanist, disappeared aged 41 in November 2013 during a plant-finding trip to Vietnam. Despite a number of searches in the country by the authorities, local people and his family, and a high-profile media campaign, he has never been found. His passport was found at a guesthouse in the Sa Pa district, where he had left an official route to go looking for rare orchids and rhododendrons.

As Taggart points out the plants his son brought in and tended, the huge sections of the garden he redesigned, the cottage he lived in, Jamie’s presence becomes palpable. One wonders what it must be like for an elderly man not to know what has happened to the son he worked with day in, day out, side by side, for 16 years.

I ask Taggart whether he gets comfort from the garden.

“This is Jamie’s memorial, obviously,” he says quietly. He starts to say something else then closes his mouth and walks on. He turns back to look at me. “We still have an outside hope that Jamie may turn up. You do sometimes read of these things – some mysteries do end up having a solution.” He smiles wanly. It is yet another unbearably sad moment in what must have been a horrendous two years for the Taggart family, and the close-knit community of Cove, where Jamie was also a firefighter.

Taggart and his wife Jill Mary divorced in the 1980s, and he now lives in Linn Villa with his elderly Irish setter. The couple’s elder son, Peter, and daughter Janet both live in England.

Unsurprisingly, the future of the garden is now in doubt. Taggart, say those who know him, is becoming more frail as the months go on, and as well as trying to cope with the disappearance of his son, is now struggling to keep the dense three-acre garden.

Janet comes to help when she can, says Taggart, as do other relatives and local friends. But without funding, a vision and some leadership, one wonders how much longer the garden can survive in its present form.

There is, however, a shard of light on the horizon. A beautiful new book and exhibition on Linn, put together by the English abstract artist Alison Turnbull and award-winning writer Philip Hoare, has sparked interest in the garden and its future.

Four years in the making, the book, called Another Green World, was conceived and written before Jamie went missing, while Turnbull was undertaking a residency at Cove Park, the internationally renowned artists’ community just three miles from the garden. A place where artists from all over the world and all disciplines come to work, Cove Park is funded by Creative Scotland, charitable foundations and private donations, and counts writer Margaret Atwood and Turner Prize-winning artist Simon Starling among its alumni.

According to Turnbull, the independent spirit of the garden and its intriguing father and son drew her back again and again after she first went there in 2011.

“Everybody at Cove knows about the garden,” she tells me from her home in London. “I went there a few times to walk and it was just such a magical place. On the second or third visit I met Jim, then Jamie, and the story of the garden started to unfold.

“They’re a wonderful double act – Jim has a very developed sense of aesthetics, mathematics and natural world, while Jamie is much more pragmatic, more concerned with the plant collection.

“I would say Jim and Jamie are as compelling as anything in the garden.”

She says such a garden could never exist in England as it would have been “colonised by the media” and ruined by too many visitors.

“Every time I visit the garden I feel like the world outside disappears,” she adds. “Jim and Jamie have treated the garden as a work in progress with the landscape beyond being a part of it. The place speaks to me – it really does.”

The book is a unique representation of the garden and its place in the landscape, and is unchanged from its original conception, except for a few short lines that refer to Jamie’s disappearance. A work of art in itself, the volume has extensive written pieces by Turnbull and Hoare, and is illuminated by the beautiful, often haunting pictures of local photographer Ruth Clark. It also includes a full plant index for the garden, compiled by Jamie before his disappearance.

Turnbull describes working on the book as an intense experience. But she says, ultimately, it is a celebration of the garden and its creators. She hopes it will help lead to more support for the garden – and more help for Jim Taggart.

“It is just so sad that the story of the garden took such a tragic turn,” she says. “While in Vietnam Jamie threw away clothes from his bags so he could get as many plants in as possible. The trips were all about the garden in Scotland – they were his lifeblood.

“Jim needs someone who understands the collection, the relationships between one species and another. It will need to be someone with significant leadership skills, and that person hasn’t been found yet. But I’m hopeful that the book will draw attention to Linn.”

Gardening aficionados and botanists from around the world know how special Linn is, as do the locals in this close-knit community. In 2013, Historic Scotland added the garden to its national inventory, highlighting its “outstanding national significance”. Despite this, nobody knows for sure what will happen to the garden when Jim Taggart is no longer able to look after it.

Back in the garden, we step into Linn Villa for a cup of tea. The house is cold and damp, smells a bit musty and hasn’t been updated for many years. Once grand furniture and paintings have fallen into disrepair. Clearly it is the garden rather than the house that has consumed its current occupant.

On the mantel in the sitting room is a picture dating from the late 1990s of Jamie Taggart’s graduation day at Glasgow University. His father tells me proudly that his youngest son took to gardening as a small child and was a talented teenage horticulturist.

He says it was Jamie who, through hard work, eventually put the house and garden on a more firm financial footing. He also talks about his son’s love of long-distance running.

In photographs, Jamie often looks serious and intense. But there is a softness in his eyes that resemble his father’s.

I ask the elder Taggart about his hopes for the garden. He talks about the possibility that his two other children may one day return to Linn. He then pauses and speaks with calm resolution.

“I want it continue as a place for people to enjoy,” he says. “I don’t see it as an attraction with a tearoom or that sort of thing. But it should be an educational resource. Ideally I’d like it to be an outpost of perhaps a university or a botanical garden.

“Jamie was a very good firefighter and a great runner, but the garden was always his passion. He’s done as much in the last 15 years as I ever did all these years.

“This is Jamie’s life work.”

Another Green World: Linn Botanic Gardens is out now, published by Art/Books, in association with Cove Park, priced £18.99. An exhibition to accompany the book runs at the Royal Botanic Garden in Edinburgh until January 2016. Linn Botanic Garden operates seasonal opening hours. Phone 01436 842242 for details.