With its purple flowers and broad green leaves, it has gone from favourite plant of the Georgian era to unwelcome guest that just doesn’t seem to want to go away.

Rhododendron ponticum arrived from Gibraltar in 1763 and gradually crept north to be planted in parks, gardens and, particularly damaging, shooting estates and woodlands to provide cover for game.

However, the pesky plant didn’t just take root, it ran riot - and become so prevalent in some places that it has threatened native plants, upset woodlands and dominated landscapes.

Having handed landowners such as the National Trust of Scotland with the problem of how to get rid of it without destroying neighbouring plants – techniques include drilling holes in the stems and injecting the plant with poison – it is now being blamed for giving its fellow ‘rhodies’ a bad name, and casting shade on the struggle for survival of some of its rarest cousins.

According to a new book published by the Royal Botanic Garden Edinburgh (RBGE), the misery brought by wayward R.ponticum has smeared its entire family, with the rhododendron name now “unfairly vilified as an invasive weed that causes widespread ecological damage to woodlands and hillsides”.

Instead, it points out, rhododendrons come in countless varieties, with some so fragile that, unlike their apparently indestructible rival, they are now in the grip of a battle for survival.

And to claw back some respect for friendly rhododendrons, the book highlights their breath-taking beauty in a series of close-up images which reveals their palette of colours – from purest white to blood red, rosy pinks and deep purples – their distinctive features, and how delicately balanced their future may be.

According to Dr Alan Elliott, RBGE’s Biodiversity Conservation Network Manager for Major Floras, land clearance, livestock grazing, population growth and the climate emergency have put some rhododendron species at risk of extinction in their native landscapes.

“Out of 1100 species, R.ponticum is the one that misbehaves,” he says. “We brought them here, and shooting estates planted tens of thousands of them as game cover.

“One reason it does so well is because it originates from the Iberian Peninsula at the end of the last Ice Age.

“Since then, the climate in Spain and Portugal has become hot and dry, so numbers have shrunk to the point that their survival is threatened.

“Our climate is much like the Iberian Peninsula once was, so our environment is one they are incredibly suited to.”

There are more than 1,000 wild origin species of rhododendron stretching from America and Europe to the Himalaya, South East Asia and Australia, with new ones identified every year. There are a further 300 recognised subspecies and varieties, and over 20,000 registered cultivars in horticulture.

Nearly half of all threatened species are found in one or more of the four RGBE gardens - in Edinburgh, Benmore, Logan and Dawyck - where rhododendrons range from towering, large-leaf trees to creeping dwarf plants and delicate species from the tropics too tender to be grown outdoors.

As a result of the threat to certain varieties, experts from RBGE have joined others from around the world under the umbrella of Global Conservation Consortium Rhododendron (GCCR).

While to help ensure ‘at risk’ plants are correctly identified botanist and photographer David Purvis embarked on a labour of love spanning three years, capturing the intricate floral and leaf characteristics of the plants growing at RBGE gardens, at various stages of their growth.

A blend of taxonomy and photographic skill, the images echo the centuries old tradition of botanical hand illustration and the historic teaching diagrams often created from specimens brought to Scotland by daring plant explorers.

The RBGE’s links with rhododendrons date to at least 1775, when species from North America and Europe are on record as being cultivated at the Edinburgh garden.

Many of the plants which grow in its four gardens can be traced to specimens retrieved by late 19th and early 20th century ‘plant hunters’, such as Falkirk-born George Forrest.

Regarded as the ‘Indiana Jones’ of plant hunting, he battled through remote forests and mountains to gather around 31,000 plant specimens from Yunnan province, China.

In 1905 he was exploring the rhododendron forests bordering Tibet, unaware that hostilities had broken out with China. Caught in the chaos, he was helped to safety by locals who disguised him as a native before he endured a perilous six-months trek to safety.

“The large number of collections coming back to Edinburgh from the Himalaya and SW China gained recognition for RBGE as a world leader for rhododendron cultivation and a specialist taxonomic centre,” Dr Elliott adds.

The RBGE collection is one of the most diverse and best documented anywhere in the world, with over 7,300 plants cultivated from 525 species.

Many are now reaching their flowering peak: at Benmore, near Dunoon, the garden’s slopes burst into colour during spring and early summer, with large-leafed tree species of rhododendron from the Himalaya and China and wild azaleas from Japan.

More than 300 species thrive in the Argyll and Bute climate, including the coppery red blooms of R.viscidifolium - now endangered in its native setting - and a collection of wild azaleas from Japan.

At Dawyck, near Peebles, the garden features Himalayan rhododendrons which are more than a century old, and one variety with leathery, hairy leaves and a cocoa aroma.

The mild gulf stream climate in the south west has proved ideal for a variety of Vietnamese rhododendron which thrive at RBGE’s Logan garden but are too tender to be grown elsewhere, while Edinburgh’s botanic garden has a huge dozens of ‘rhodies’, from creeping dwarf plants to towering tree species.

However, in the wild, several species have been lost, and one-third are threatened with extinction.

“Some are very narrowly distributed and only occur on a single mountain or in isolated pockets. They are at risk from changing climate and disturbance to habitat from people,” adds Dr Elliott, who has travelled to Nepal to study rhododendrons in their native setting.

“For example, P.kanehirae became extinct in the wild in the 1980s when a reservoir was constructed, water levels rose and wiped them out.”

Seeds from the species, with its crimson to dark red petals, were shared with Edinburgh-based experts in a Taiwan-led project that has helped save it from being lost forever.

While the RBGE staff have been at the forefront of other projects designed to save rhododendrons from being lost forever.

Adds Dr Elliott: “RBGE is well placed to take a leading role in international conservation programmes through a combination of our long association with rhododendron, the diverse collection of living plants we maintain and our continued links and collaborative research with partner organisations.”

Rhododendron Dissected flora in close-up by David Purvis (£16.99) is available from rbgeshop.org