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Meet David Knott from Royal Botanic Garden Edinburgh

April 25th 2017

Discovering a fascination for rhododendrons

By David Knott

They say ‘The delicate mauve of a rhododendron flower cannot be imitated in any other flower, but in spite of their beauty, growing them can be challenging’.

Rhododendron is a large genus of some thousand known species. They form a large group of plants and come in all shapes and sizes from tall tree-like species over 20m high, with large leaves from the lush temperate forests of China, to very low growing species, also called tiny dwarfs, which are only a few cm in height and come from the high mountain passes of the Himalayas.

Although rhododendrons originally come from China, the Himalayas and North America, they are very much at home in Scotland and many species were first brought here by Scottish planthunters. Most flower in spring and the largest collection of species rhododendrons is held by the four Gardens of the Royal Botanic Garden Edinburgh (RBGE).

Rhododendrons are grown for their spectacular flowers and have been popular with Scottish gardeners since the 1800s, with good reason. Flower colour varies from white, pink, red and yellow through to the ubiquitous purple and mauve that many associate with the rhododendron. A very diverse group can also be found growing in the tropics of South East Asia, from hot steamy jungles to cooler isolated mountain tops. These mostly require frost-free glasshouse cultivation in the United Kingdom and they usually start flowering just as the light levels begin to improve in the early spring.

It was this diversity that first captured the attention of great plant hunters – such as George Forrest and Ernest Henry “Chinese” Wilson and continues to inspire gardeners today: from the range of sizes in their leaves to the variety of flower colours and forms and the length of flowering season with different species being in flower from early spring to almost mid-summer.

They do require a moist, well-drained, acidic soil and – until they are well established -appreciate regular rainfall. As this type of soil and –  normally – copious amounts of rainfall can be found across Scotland, this is perhaps why they grow so well and have become such a favourite here.

Challenges may be that varieties with large leaves, early growth or ones that are on the tender side for our climate require shelter from wind, particularly from south westerlies and north easterlies, and building additional shelter or placing them amongst surrounding shrubs for further protection, can make a big difference. The early-flowering species also prefer protection from the early morning sun particularly where and when late frosts are a problem to help ensure that the blooms are not lost.

As a guide, the larger leaved species and hybrids grow better in the milder, wetter west coast gardens and the smaller leaved species and hybrids grow better in the cooler, drier east coast gardens.

But these challenges should not deter. There are so many to choose from and the diversity of the temperate species has been used by hybridisers – significantly, in recent years, by the Cox family of Glendoick – to produce a range of dwarf rhododendron hybrids particularly suited to Scottish gardens and domestic settings. At Glendoick, a wide range of smaller growing rhododendrons, including azaleas, are bred that are very suitable for smaller gardens. Look out for rhododendrons named after birds or mammals to find the right type.

Despite rhododendrons being native to such a wide range of growing conditions they grow remarkably well throughout Scotland. So, don’t be put off by the reputation of a particular rogue that has tainted the good name of others. The delicate mauve of Rhododendron ponticum to be seen widely – particularly on the west coast – does indicate a problem! As such, many people associate this flower colour with the ‘wild’ rhododendron. This plant is not, in fact, native to Scotland and despite appearing on many picture postcards, it does create problems to wider biodiversity. It is in fact now illegal to plant this particular species.

The best way to do research is to get out and about and see what rhododendrons are growing well in your area. May is a particularly good time to visit some of your local gardens while the Scottish Rhododendron Festival is still in full swing.

The festival (1 April – 31 May) was launched by the Glorious Gardens of Argyll & Bute in 2015. After a successful first year, Discover Scottish Gardens extended the festival across the country and, this year, nearly 60 Scottish sites, including public and private gardens, are taking part. They offer horticultural events, guided tours and exclusive openings in celebration of this exotic Asian spring shrub that thrives so well here.

The long-held Scottish obsession with this genus seems to have reached every outpost, and specialist gardens nurtured by enthusiastic growers can be found all around us.

A full list of participating gardens in the Scottish Rhododendron Festival can be found at www.visitscotland.com/bloom.Pay particular attention to where rhododendrons are planted and what protection surrounding plants are giving. Then, visit your local garden centre and get started!

David Knott is the Curator of Living Collections at the Royal Botanic Garden Edinburgh, a founding member of Discover Scottish Gardens.