JARS v47n3 - Fragrance in Rhododendron Species

Fragrance in Rhododendron Species
Peter Cameron
Dunedin, New Zealand
Reprinted from the Dunedin Rhododendron Group Bulletin, No. 17, 1989

The genus Rhododendron comprises one of the largest and most diverse groups of ornamental plants in cultivation, ranging from alpine cushions through evergreen and deciduous shrubs to spectacular trees; it even includes tropical epiphytes. Rhododendrons have a number of desirable attributes: attractive bushes with handsome leaves of interesting shape, patterning or indumentum; colourful young leaf shoots or buds; and above all beautiful flowers, which in some varieties may be sweetly scented. In discussing such characteristics, John Bond, Keeper of the Gardens at Windsor Great Park, made the point that for ornamental plants to be really garden-worthy, they should not only be attractive year round, but should also possess two or even more special qualities.
I used to think that the ideal was Rhododendron yakushimanum , which appeared to have everything. Peter Cox (1) depicted it as "having near perfect habit, foliage and flowers"; while Greer (2) described it as "a hardy plant with excellent foliage and fabulous indumentum...the pink buds open to snow-white flowers of breathtaking beauty," and gave it a merit rating of 5,5,4 (on a 5 point scale) for flower, plant and performance - the highest rating for any species or hybrid in his book.
But I have altered my view somewhat lately, for the flowers of R. yakushimanum are not scented. If flower perfume were to be high on the list of desirable attributes, one might select Rhododendron edgeworthii , which Cox (3) described as "one of my favourites of all species and worth every trouble to grow well...it has amongst the most interesting foliage, finest flowers and sweetest scents of all species."

Fragrance Defined
Beauty of foliage, flower, and fragrance are all important qualities, but here I shall be discussing fragrance in rhododendron species. A comprehensive treatment of this topic does not appear to be in the rhododendron literature, nor are the various terms for "smell" adequately defined. The Shorter Oxford Dictionary uses "smell" and "odour" as general terms. "Scent" is defined as meaning a distinctive odour, now applied to agreeable odours, such as those of flowers. "Perfume" denotes the volatile particles emitted by any sweet-smelling substance; while "fragrance" refers to sweetness of smell or a pleasing scent. So "scent" or "fragrance" seem to be the best words for describing the sweet smells of flowers.
There are other terms too: "aroma" relates to the distinctive fragrance of spice or plant. This would apply to an agreeable odour of leaves, such as the honey-like aroma of leaves in Rhododendron glaucophyllum . "Stench" and "stink" refer to offensive smells, but we are not aware of any stinking rhododendrons - certainly the specific epithets "pungens" or "foetidissimum" do not appear in any species lists, despite the fact that some rhododendron foliage is found to be toxic.
Scent can be variable with different plants even of the same species, and of course each of us has our own peculiarly individual perception of smell. Weather affects the extent to which odours pervade the air: in the case of Dunedin, a relatively breezy place, much perfume may be dissipated by the wind. Scent is generally strongest during damp mild weather, and most noticeable in the early morning, or evening.

Fragrance in Rhododendron Species
Generally speaking, the highly scented blooms do not occur in early flowering species, but rather in the mid-season to late flowering species, many of which are on the tender side. Also, the fragrant species are among the larger rhododendrons (except perhaps R. edgeworthii and some of the smaller plants of the Maddenia subsection, such as R. ciliatum ).
Rhododendron species with scented flowers occur in four sections of the genus:
I.  Among our familiar evergreen temperate rhododendrons, in the Hymenanthes section, fragrance occurs in three subsections: Fortunea, e.g., R. fortunei , Auriculata, e.g., R. auriculatum , and Choniastrum section, e.g., R. stamineum .
II.  In the Rhododendron section, two subsections are scented: Edgeworthia, e.g., R. edgeworthii , and Maddenia, e.g., R. maddenii and about a dozen other species.
III.  In the Pentanthera subgenus (formerly known as the Azalea Series) there are at least eight scented species.
IV.  In the Vireya section, at least 12 species of tropical rhododendrons are scented.
Taking each of these sections in turn, let us look in some detail at species of fragrant rhododendrons most of which are well known here, and which have been available on our local plant lists.

I. Subsections Fortunea and Auriculata; Section Choniastrum
Species in the subsection Fortunea have been well represented in our recent plant lists, and notably R. fortunei itself, which Greer (2) ranks in merit second only to R. yakushimanum . "The deliciously fragrant flowers are often more than four inches in diameter, and are a blush pink fading white...beautifully formed, falling and brimming over the entire plant."
All the Fortunea species described here grow into trees 6 to 10 metres high (20-35 feet); so that they will need plenty of room. Rhododendron fortunei ssp. discolor has been regularly on our plant lists, is similar to R. fortunei , but is faster growing and takes longer to flower. It blooms in December'.
Two lovely white-flowered species have also been on our lists. Rhododendron decorum has a growth habit that is upright yet fairly compact. The oblong leaves are handsome year round; the exceptionally abundant large flowers are white or pale rose, sweetly scented, and often with frilly edges. Rhododendron diaprepes , the Gargantua form has large white blooms with a greenish-yellow throat, and is also sweetly scented. Both these species, being late flowering (November to December) are a real asset in the larger garden.
Another choice species (available recently on Pukeiti's list) is R. griffithianum , which has big white bell-shaped flowers, with a saucer-shaped green or pink-tinged calyx. One form has semi-double flowers which are also fragrant. It may be worth seeking out.
Subsection Auriculata has only one species in cultivation: R. auriculatum , a slow-growing tree which will reach 9 metres (30 feet), and is also slow to flower, taking at least 10 years. It has showy white or rosy-pink flowers, and is quite fragrant. Its great virtue is its late flowering, usually in January, but it does need shelter for its large 30 cm leaves. We had one for 12 years; in 1988 it produced its first flowers - they turned out to be rather small, white, and hardly scented at all - while its leaves were shredded by the winds. So we dug it out. At least the trunk and branches made excellent firewood!
The section Choniastrum (the old Stamineum Series) has several species which are fragrant. Rhododendron stamineum and R. westlandii are strongly scented; R. stenaulum and R. oxyphyllum have a jasmine or heliotrope-like perfume; while R. wilsoniae has a delightful smell like the azalea R. luteum . A recent introduction from Thailand by botanist Peter Valder of Australia is R. moulmainense with a scent like jonquils. As yet these have not been on our plant lists and do not seem to be available locally, but they are species worth watching for. Cox (4) writes: "This much neglected section deserves much more horticultural attention. These plants should flourish in climates like the milder parts of New Zealand."

II. Subsections Edgeworthia and Maddenia
The subsection Edgeworthia contains three species, only two of which are common in cultivation. Rhododendron pendulum is not perfumed, but R. edgeworthii is sweetly scented, and is the parent of many well-known fragrant hybrids. This lovely species needs perfect drainage to grow well. Being epiphytic in the wild, it can thrive if planted on an old tree stump.
The Maddenia subsection is the largest and best known group of species with sweet-scented flowers. In the Maddenii Alliance, R. maddenii and R. crassum are typical examples. These two have the extra attributes of handsome glossy leaves year round, and large white flowers coming out late in the season: November to December for R. crassum and December to January for R. maddenii . Rhododendron polyandrum , though now merged with R. maddenii , is much earlier, its rose-pink fragrant flowers coming out in mid-October.
The Cilicalyx Alliance has many well-known and much loved species in our gardens: nearly all have some fragrance, though the smaller varieties R. fletcherianum , R. valentinianum , R. ciliatum and R. burmanicum are not highly scented. However, R. formosum , which blooms prolifically for us every October, has fragrant flowers of white tinged pink, while both R. johnstoneanum and R. dendricola have slightly scented flowers of white tinged yellow. In South Island gardens there is a beautiful and sweetly scented plant grown as R. ciliicalyx . The validity of this name is questionable as Cox (4) maintains that the true R. ciliicalyx is probably not in cultivation.
The five species in the Dalhousiae Alliance: R. lindleyi , R. nuttallii , R. taggianum , R. dalhousiae , and R. dalhousiae var. rhabdotum are giants of the Maddenia subsection. Each is outstanding in its own right, but they all have certain qualities in common. As Cox (4) states: "The flowers are often magnificent, like large white to yellow trumpet-shaped lilies, and the scent is frequently superb." Distinctive hollows or "marks of the potter's thumb" at the base of the corolla enhance the shape of the flowers. Rhododendron dalhousiae is a tree with unusual greenish-yellow blooms; while the smaller R. dalhousiae var. rhabdotum . flowering very late in the season (January to February) has spectacular trumpets of creamy-white with bold red stripes down the outside of each lobe. Neither of these species has a strong perfume; but R. lindleyi , R. nuttallii and R. taggianum are all highly scented, and are included in Cox's list of scented species.

Rhododendron lindleyi , central to the Dalhousiae Alliance, has been collected in the wild many times, and as a result it varies quite widely - the Ludlow and Sherriff 6562 form being considered superior. The large funnel-shaped flowers are white with a yellow-orange base, sometimes tinged pink; they are sweetly scented.
Rhododendron nuttallii , for height of plant and size of flower and leaves, is a grand specimen. The heavily bullate reticulate leaves are handsome, up to 20 cm (8 inches) long, and the young leaf shoots can be an attractive purple-red. Cox (4) states that this is perhaps the most magnificent rhododendron of all. Gorer (5) described R. nuttallii and R. sinonuttallii as "the busty pinups of the floral world." The flowers can be huge, up to 13 cm (5 inches) long: white, cream or creamy-yellow, suffused yellow or orange in the throat, with the lobes sometimes tinged pink. The strong scent varies, and if grown indoors can be quite pungent. (Cox recalls that a lady with a sensitive nose likened it to horse-piss). Growing well in full sun, R. nuttallii may be quite a compact bush, but in sheltered woodland it can reach up to 9 metres (30 feet).
Rhododendron taggianum is a smaller upright leggy shrub. In the best forms the buds are light amber to cream, opening to pure white with a yellow throat. For quality of flower, this is a superb plant. According to Cox (4), "The texture of the flowers looks like fine porcelain, and (they) are often frilled." They are sweetly scented: as Leach (6) put it, "deliciously fragrant."
One of our favourite species, R. megacalyx , now the sole member of the Megacalyx Alliance of the Maddenia subsection, is a leggy woodland shrub, having attractive leaves, long obovate-elliptic in shape, with prominent sunken veins. The flowers, although not prolific, are outstanding. They are flushed purple in the bud, opening to pure white with a greenish-yellow base. The lax petals have a satiny texture. These tubular funnel-shaped flowers are up to 9 cm (4 inches) long. The consistently large calyx of 2.5 cm (1 inch) of green or pinkish hue gives the species its name. The aroma is distinctive, of a strong nutmeg flavour.

III. Azaleas
Those of us who have been in the Upper Garden in Dunedin in late October or early November when the azaleas are in full bloom are well aware of their strong perfume. The Pentanthera subgenus (the old Azalea Series) has many perfumed species, notably the common R. luteum from Europe, and mollis ( R. molle ) from China.
Flowers from some azalea species have unusual scents: R. arborescens smells of heliotrope; R. atlanticum has a heavy almost pungent odour; R. canescens and R. japonicum are sweetly scented; while R. prinophyllum has a clove-like aroma, and R. viscosum has a sweet spicy scent.
But the one most familiar to us, having been on our plant list for many years, is R. occidentale , native to the Californian Pacific Coast. The form often grown here is 'Delicatissima'. Rhododendron occidentale is a deciduous shrub, up to 3 metres (10 feet); it smothers itself with bold white to creamy-yellow to apricot flowers having a yellow to red flare; and they are sweetly fragrant. They open in early November to put on a magnificent display. It is quite hardy, but it is a pity that the bush is not handsome for the remainder of the year, although in frosty areas the leaves may change to scarlet in the autumn.

IV. The Vireya Section
Many of these Malesian rhododendron are perfumed. These include the well known R. herzogii , R. laetum , R. orbiculatum , R. phaeopeplum , R. superbum , and R. zoelleri . Rhododendron jasminiflorum is very fragrant, as are the sweet scented R. multinervium , R. retivenium , R. rugosum and R. suaveolens . Perfume resembling the scent of carnations is exuded by R. konori , R. leucogigas , and R. majus ; while R. brookeanum is lemon-scented.
In summary, fragrance is found in rhododendron species which tend to grow large or tall, are medium to late in flowering season, and are rather tender, with large flowers often with lily-shaped trumpets. The medium to large size of fragrant species may place some limits on their extensive use in small town plots or rock gardens - and even more so in tubs or window boxes!
Over recent years, hybridizers have been working to produce dwarf crosses with scented flowers which open early in the season. The only successful example we know is 'September Snow' ( R. edgeworthii x R. leucaspis ) raised by the late Bruce Campbell of Dunedin. Fragrance in hybrid rhododendrons will be the topic of another article.
If one possesses only a tiny garden and wishes to have some fragrant rhododendron species, one might include R. edgeworthii , and maybe a few of the Maddenii or Ciliicalyx Alliance. From the Dalhousiae Alliance, the beautiful R taggianum could possibly be slim enough to be accommodated. Some deciduous azaleas might be favoured: three examples available in New Zealand are R. occidentale , up to 3 metres high, but fairly upright; R. atlanticum , up to 1 meter high, from eastern USA; and R. luteum , up to 3 metres high, from Europe. All are strongly fragrant.
For those with a penchant for flamboyant flowers, there are the Vireyas. Tropical species sometimes available include the very fragrant R. jasminiflorum , and also R. konori , R. laetum , R. orbiculatum , R. superbum , and R. zoelleri . If the garden is warm and sheltered, sunny slopes with an overhead canopy of trees may be used to provide a frost-free environment. Or maybe a glasshouse or shade house boosted by extra warmth in winter could be used to enhance the microclimate.
A fragrant plant with its appeal to the sense of smell must add a further dimension to the pleasures derived from a garden. No matter how small or how windswept the garden is, it is always worth the effort of finding a place for a sweet-scented rhododendron.

References
1.  Cox, Peter. Dwarf Rhododendrons . London, Batsford. 1973.
2.  Greer, Harold. Greer's Guidebook to Available Rhododendron Species and Hybrids . Eugene, Oregon. Offshoot Publications. 1982.
3.  Cox, Peter. The Smaller Rhododendrons . London, Batsford. 1985.
4.  Cox, Peter. The Larger Species of Rhododendron . London, Batsford. 1979.
5.  Gorer, G. "Some Notes on the Maddenii Series and Their Hybrids as Garden and Greenhouse Plants." RHS, The Rhododendron and Camellia Yearbook . London. 1969.
6.  Leach, David G. Rhododendrons of the World . London, George Allen & Unwin Ltd. 1962.

Peter Cameron is Past-President and Life Member of the Dunedin Rhododendron Group and currently serves as President of the New Zealand Rhododendron Association and board member of the Pukeiti Rhododendron Trust.

Editor's Note: Dunedin, New Zealand, a Southern Hemisphere city, has an average maximum temperature of 58.8°F in January, its warmest month, and an average minimum temperature of 43.6°F in July, its coldest month.