Fragrant Rhododendrons

As gardening as a passion took me in its hold, one particular group of plants began to capture my imagination, the cool climate evergreen rhododendrons. Living at the time in southern Tasmania, I was naturally drawn to the big, colourful trusses of the popular hybrids. Each spring I spent many an hour at the botanical gardens in Hobart watching and waiting as one by one these splendid plants unveiled their delights for a few short weeks before rain would come to wilt their petals.

Rhododendron ciliicalyx	'Fragrantissimum'	Rhododendron nuttallii

Soon I realised there was much more to the vast and diverse Rhododendron genus than the showstopping garden shrubs found in every corner nursery. There are tiny, ground-hugging prostrate plants, with spiky leaves and trusses of flowers no larger than a five cent piece, that have adapted to surviving heavy frosts and winter snows of windswept rocky outcrops in the Himalayas. There are forests of trees with gigantic, leathery leaves as much as a metre in length. And there are epiphytes perching precariously in the forks of trees in the misty, mountain jungles of southeast Asia and New Guinea.

The rich variety of rhododendron foliage can equal the delights of their blooms, especially the young spring shoots which rise optimistically like candlesticks once the flowers are spent. And an added bonus that is often overlooked in a number of species and hybrids is fragrance to rival the sweetest rose or magnolia.

Fragrance in flowers is just one of several devices evolved by plants to attract birds or insects as pollinators and a number of species have survived by perfecting this trait. Fragrance in the rhododendron is genetically linked to pale flower colours, so that fragrance becomes an unfortunate trade-off with colour. Any strongly coloured rhododendron can be guaranteed not be fragrant.
About the only relief from white in fragrant rhododendrons one can hope for are pale creams or lighter shades of yellow, or perhaps a pink or purplish flush to the flower buds which inevitably fades to white as the flowers open.

R. nuttallii, from the Himalayas, is a tough, generous plant whose strongly scented, bold white flowers positively glow from a vibrant yellow throat. It thrives in quite open positions, and its large leaves are like sumptuous, deeply grained green leather. R. lindleyi, also from the Himalays, has a more open habit and less interesting foliage, but crosses between the two have resulted in some very beautiful, highly fragrant plants.

The heat tolerance and suberb fragrance of R. maddenii make it a good choice for Australian gardens. It is widely variable in form and found across a broad area of southeast Asia and the Himalayas. There are some pale yellow forms of R. johnstoneanum from northern India, but as always the rule is that the deeper the shade the weaker the fragrance will be.

The cream flowered R. dalhousiae is another evergreen species that is closely related to R. maddenii and worth seeking for its powerful fragrance. Its variety 'Rhabdotum' offers the bonus of a bold red stripe along the side of each flower.
Fragrance is generally passed on to the hybrids of species rhododendrons, and perhaps the best known of the fragrant hybrids is the aptly named R. 'Fragrantissimum'. Its Latin name suggests that it is a species but it comes from the early days of hybridizing, when Latin names were still permitted for hybrids.

'Fragrantissimum' is one of the edgeworthii hybrids, a cross between the richly fragrant R. edgeworthii and the non-fragrant R. formosum, both white flowering species from the Himalayas. But one wonders why the uninspiring 'Fragrantissimum' ­ lanky and fairly sparse of leaf ­ remains so popular, when even the parent R. edgeworthii develops a far superior garden habit. Its straggly, open form does, however, lend the plant to training as an espaliere against a wall or trellis.

Other sweetly fragrant plants with R. edgeworthii in their parentage include 'Princess Alice' and 'Countess of Sefton'. The more compact 'Suave' and slightly larger 'Daviesii' have very similar, large simple white flowers tinged with pink, and a fragrance to rival that of 'Fragrantissimum', but they are far better behaved, compact shrubs.

The Loderi hybrids were bred by Sir Edmund Loder around the early 1900s, and are crosses which include two white flowering fragrant species in their parentage, the Himalayan R. griffithianum and R. fortunei from eastern China. Probably the best known of these hardy, vigorous plants are 'Loderi King George', with its enormous trusses of white funnel-shaped flowers, and the more sun-tolerant and compact 'Loder's White'.

The island of New Guinea is the domain of the vireya rhododendron and is home to over two hundred species with certainly more treasures awaiting discovery. Others are found scattered across much of southeast Asia, with two very similar red-flowering species, R. lochiae and R. notiale, native to the far northeast of Queensland.

Recent years have seen the release of many new vireya hybrids with forms superior to the often leggy species, as these versatile plants rapidly gain popularity. So long as sufficient sunlight and some protection from frost can be provided, vireyas may be grown in gardens as far south as Hobart.

Many of the vireya rhododendrons are strongly scented, probably the best known examples being R. konori from New Guinea and R. jasminiflorum from Malaysia, Sumatra and the Philippines. The unique R. tuba, also from New Guinea, is an unforgettable sight in flower, massed with trusses of dangling white tubular flowers with a heady perfume.

Vireya Rhododendron tuba	Deciduous Rhododendron occidentale	'Wedding Gown'

Few of the evergreen azaleas offer fragrance, although there are a few notable exceptions. 'Alba Magnifica', also known as 'Alba Magna', is an enduring, vigorous Indica hybrid from the 1850s, whose tolerance to sunlight makes it an ideal choice for planting in massed beds in an open setting, concentrating its subtle scent.

A rare exception to the pale colour rule is the mildly perfumed, pale mauve hybrid 'Schryderii Mauve', a sport of the equally fragrant white 'Schryderii'. Both plants have sticky foliage which potentially harmful insects avoid.
Many deciduous azaleas can be delightfully fragrant. R. occidentale, the western azalea, grows wild in temperate zones west of the Rocky Mountains. It is a particularly captivating plant whose white flowers bear a distinctive golden yellow flare, a characteristic passed on to other occidentale hybrids, such as 'Exquisita' and 'Delicatissima'.

The massed flowers of many deciduous azaleas burst open dramatically on bare branches before the foliage emerges, providing a striking contrast in an otherwise often bleak early spring landscape. The eastern European species R. luteum, which has a very strong fragrance resembling honeysuckle, is one such plant. A parent of many hybrids, its yellow flowers are among the strongest in colour of the scented rhododendrons, and it is also noted for its dramatic autumn leaf colour.

It's not just the flowers of the rhododendron plant that can be fragrant. A characteristic of the red to yellow flowering R. cinnabarinum, from the Himalayas, and one that is passed on to its many offspring, is the cinnamon-like aroma of its bluish green foliage when rubbed against or crushed.

Other plants with aromatic foliage include the species R. campylogynum, whose sulphur yellow flowers are also slightly fragrant, the pink or white R. glaucophyllum and and the purplish blue R. hippophaeoides, all Himalayan plants.
There are several forms of the heat-tolerant American hybrid 'P.J.M' in various shades of pink, and all with aromatic foliage. All of these plants demand siting alongside paths, where they will be brushed against, adding yet another dimension to the garden sensation.

A final word of caution. The flower parts of the rhododendron and even the honey from bees which have visited the plants can be highly toxic.

In the Western Highlands of New Guinea the flowers of the vireya R. macgregoriae are used in the preparation of baits for rats. Around 400 BC, a Greek army was stopped in its tracks after consuming the sweetly flavoured honey from bees that had visited the flowers of R. luteum. The poison is a narcotic which has been known to cause respiratory failure, so certainly sniff and enjoy the fragrance, but avoid touching or handling the flowers of any rhododendron.

Cultivation of rhododendrons

Success with all rhododendron types depends very much on providing the right soil conditions before planting and correct siting for each particular plant. Like other members of the erica family to which it belongs, rhododendrons enjoy a cool root run in constantly moist but freely draining and acidic soil, ideally with a pH level between 4.5 and 5.5.

They will bitterly resent a poorly draining soil which can often be overcome by mound planting in more friable soil raised above the surrounding ground level. Moisture levels can be maintained and soil temperatures moderated by maintaining a deep mulch of organic material and the use of drip irrigation in low rainfall areas.

Most, but certainly not all, varieties prefer protection from strong winds and some level of light or dappled shade, particularly from the afternoon sun in summer, although many varieties that are sun tolerant are now clearly labelled as such by nurseries.

No pruning is normally necessary, although 'dead-heading', or removing the spent flowers on younger plants will encourage bushier growth.

Few pests and diseases pose a serious threat to rhododendrons. Most problems that do arise result from poor growing conditions, such as lack of ventilation and insufficient or excessive soil moisture.

Feed rhododendrons with an application of slow-release fertiliser pellets at the standard rate or a combination of organic materials such as blood and bone and well decayed compost in late winter or early spring. The root systems of rhododendrons grow very close to the soil surface and into the mulch, so avoid any cultivating around established plants.

Some specialist rhododendron suppliers in Australia

• Camellia Grove Nursery, Cattai Ridge Road, Glenorie, NSW 2157. (02) 9652 1200.
• Dicksonia Rare Plants, 686 Mount Macedon Road, Mount Macedon, Vic. 3441. (03) 5426 3075.
• The National Rhododendron Gardens, The Georgian Road, Olinda, Vic. 3788. (03) 9751 1980.
• Vireya Valley Nursery, Woori-Yallock Road RSD, Cockatoo, Vic. 3781. (03) 5968 8676.

Article originally published in The Australian Gardener, September/October 1997.
Text, illustrations and photographs copyright Richard Francis, 1998.

Reference to and use of the material provided on these pages is acceptable, but please respect my rights when considering commercial use in return for my trust in offering the material for public access.

Revised 25 January 2006

© 1997-2006 Richard Francis