"Oh my word, just look at this: a native laurel! This is just absolutely magical!" A quivering peak of botanical ecstasy had been noisily attained by one of our party. As so often with such experiences, though, anticlimax soon followed. My companion lost his footing and took an inelegant tumble on the sloping path. No harm was done but had the damage been worse I imagined composing the letter to his wife: "He died with a smile on his face. He'd just seen an indigenous Azorean tree, it's how he would have wanted to go."
We are on the island of Sao Miguel in the Azores, 50 metres along a rough track that cuts steeply along the inside of the rim of the dormant Agua de Pau volcano, and my head is spinning with the spectacular beauty unfolding around me. I can see why my plant-loving companions are behaving like five-year-olds on Christmas morning. We are overlooking Lagoa do Fogo, 1,000m above sea level, and far below the rain-water has transformed the bottom of the caldera into a serene lake, edged with sheltered coves and intriguingly half-submerged caves.
The caldera's flanks are cloaked in trees of so many different species and greens that they resemble paint drying at different speeds. A mist seems to have drifted in from a Scottish glen and, as the exultation of my party suggests, the flora is pretty special with native heather, holly and mosses grown so big that they look like bonsai Christmas trees.
Plants, shrubs and flowers are featuring prominently with good reason. My tour of the Azores has been organised by the Royal Horticultural Society, which has recently branched out into overseas holidays. It reasons that British plant-lovers are curious types who enjoy growing exotics in their own gardens and so will appreciate the chance to nose around foreign flora hotspots in the company of like-minded souls and expert guides.
If you find the idea of a hard-core plant-focused tour a little daunting, not to mention the spectre of an oncoming herd of azalea-clutching horticulturalists from deepest middle England, fear not: my tour was more nuanced than that and catered both for the traveller with a passing interest of flora as well as those who might choose camelias for their specialist subject, should they appear on Mastermind.
The Azores is an archipelago of nine islands positioned in the mid-Atlantic, 1,500km from Portugal and 3,400km from the United States. They are composed of the exposed peaks of dormant or extinct volcanoes and positioned on the mid-Atlantic ridge, the oceanic backbone that curls from Iceland down to Ascension, St Helena and Tristan da Cunha.
The archipelago is primarily known as one of the world's best destinations for whale-watching, but this volcanic history also underpins the richness of the Azores' vegetation, as the rich piebald-coloured basalt soil makes a fertile compost base for plants and animals.
The "Azores High", which we associate with good weather, means something a little different here and the mix of warm air, temperate weather – usually between 8C and 25C – and high rainfall, means conditions are primed for nature to put on a show.
Sao Miguel, the focus for our tour, is the largest island in the Azores and stretches 65km from east to west and 16km north to south, cramming in an extraordinary variety of landscapes. I clamber over giant roots of fig and rubber trees and dip under knotted vines straight out of Angkor Wat, while those volcanoes suggested something from out of the South Pacific. Squat palm trees have been shaped and pruned so that they resemble giant pineapples or amorphous chess pieces and there were manicured gardens seemingly parcelled up and posted from the Home Counties.
More often than not, I just find myself gazing at all the greenery, usually against a hauntingly beautiful backdrop of coast, mountain ridges and sweeping valleys. Rolling, lumpy folds of land tumble to the sea and call Devon to mind. All over the island you will find quintas, enclosed areas that are somewhere between an allotment and a private garden, where people grow taro and other food crops. It's all extremely easy on the eye.
Lagoa do Fogo is just one of several volcanic lakes. In fact it's a mere toddler compared to the vast double-lake caldera of Sete Cidades far to the west of the island. These two lakes contrast in colour, the deeper one blue, the shallower one green, and are framed by an enormous volcanic crater four kilometres in diameter and 12km in circumference. We track down the small road to the shore edge where children fish for carp and common terns hang jerkily in the air.
The sea lies just over the lip of the Sete Cidades volcano, and for the bold there is a narrow, kilometre-long path through the mountainside alongside a canalised river that links the two. An elegiac collection of houses situated by the water's edge makes up the sleepy hamlet of Mosteiros.
We look down at the village from the mirador at Ponta do Escalvado. Mosteiros is the home village of one of our guides, Manny Paolo. "It hasn't changed a bit since I grew up there," he says. "They've had plumbing, but that's pretty much it. It's a slow pace of life, very different to what visitors come from. It feels like old Europe."
The Azores also have that distinctive light that comes to small islands miles from continental influence, a 360-degree reflection of sea that seems to throw up all kinds of colours. This may be an RHS trip but it is no blinkered plants-only tour: it is also about their setting.
By the end of my visit, I have settled on two favourite locations. There is Terra Nostra Park, positioned deep on the valley floor inside yet another vast caldera and something of a show-stopper.
The setting is jaw-dropping and wherever you look you see a razor-edge of a volcanic lip encircling the gardens. It's a miniature version of Yosemite, where smouldering fumaroles break and burp through the surface; you can even order lunch cooked in one of the scalding hot wells. And it's time to break another stereotype: in the UK, gardening can often be portrayed as a rather formal practice of creating neat lines, but for Azoreans the term "English garden" has a more raffish connotation. Terra Nostra is gorgeously eccentric and eclectic – oaks, monkey puzzles, and enormous 40-metre-high Norfolk pines stand cheek by jowl – the result, according to our guide, Carina, of English influence. "It's free, wild and asymmetrical," Carina says with a smile. An avenue of harmonised ginkgo trees overhangs a moss- covered path that shines and glitters like green tinsel. There's a geothermal lake for swimming, giant lilac water lilies from South America, serpentine ornamental lakes. It's like Giverny, on steroids.
The other treat is the public garden at Ponta do Sossego on the distant, wilder north-east of the island. We visit towards the end of the tour and by now I've come to realise that breathtaking gardens and views are two-a-penny on the Azores; the locals have so much choice they can afford to turn some of them into barbecue venues. But the brick cooking bases are discretely trucked away behind the agapanthus, leaving me to concentrate on the stirring coastal views that rise above pink watsonias. Ponta do Sossego is positioned high on a promontory, its flanks tumbling away to the sea.
Our trip ends in Ribeira Grande, the most charming of Sao Miguel's small towns. More than anywhere else, Ribeira Grande reminds me of a dozy South Seas settlement. It's the only place where decent waves crash on to a beach, while a river has sawn a gorge dramatically through the heart of the town, leaving its central park all but fronting a high cliff.
Men play chess in this park while families sit on steps or in the shade of the New Zealand Christmas trees, whose huge aerial roots have coalesced and droop down, like the ears on a beagle puppy. Yellow wagtails peck among the crumbs and dust and are more intensely coloured than back home, as though they've been daubed with a highlighter pen.
I could happily have strung up a hammock between two of these trees, sipped the local blackcurrant liqueur and idly batted those leafy ears back and forwards, and still be there now.
A nine-day holiday created in partnership with the Royal Horticultural Society and Collette Worldwide Holidays (0800 023 8617; colletteworldwide.com) costs from £1,999 per person, with flights, four nights' B&B in Lisbon, four in Sao Miguel, three lunches and four dinners. This price also includes transfers to and from the airport in the UK up to a distance of 100 miles.
Bradt Guide to the Azores by David Sayers (bradtguides.com).