JARS v48n1 - Fragrance in Rhododendron Hybrids

Fragrance in Rhododendron Hybrids
Peter Cameron
Dunedin, New Zealand
Reprinted from the Dunedin Rhododendron Group Bulletin, No. 19, 1991.

The companion article "Fragrance in Rhododendron Species" by Mr. Cameron appeared in the Summer 1993 issue of the ARS Journal.

Writing about fragrant rhododendron hybrids is no easy task. There are some dozen scented rhododendron species, but there are hundreds of scented hybrids; and hybridizers have been working in this field for nearly two centuries.

The hybridizer's main purpose is to try to combine desirable features of species (and/or hybrids) and hopefully enhance these features in the resulting hybrid. According to Peter Cox, a first cross between suitable species can usually be relied upon to give a fairly high percentage of good offspring, and crosses between hybrids are more uncertain in their result. So, greater success seems to have been achieved in breeding fragrant hybrids by crossing fragrant species, rather than hybrids with hybrids.

As most rhododendron species are temperate or even tropical plants - this applies particularly to fragrant species - one important quality to incorporate in hybridizing would be to induce greater hardiness. Much work has been done to this end in Britain, Europe, USA, and Canada (see "The Hardiest Rhododendrons," Dunedin Rhododendron Group Bulletin , No. 18, 1990, p. 84). To us in Australasia, with our milder temperate climate, the need for hardiness does not seem very pressing. Other qualities to foster might include:

•  In plant performance: toughness, toleration of sun and heat, wind tolerance, disease resistance;

•  In foliage: larger or more attractive leaves, very good indumentum, compactness of foliage and plant;

• In flowers: colour, size, texture, lasting quality, and fragrance.

Considerations in Producing Fragrant Hybrids

Success in hybridizing for fragrance seems more likely by crossing fragrant species. As Judith Beresford (2, p. 141) says: "The best results come from mating species that are near to each other in character and habit." So, crossing large scented species, such as those in the Fortunea or the Auriculata subsections, might produce large fragrant hybrids. And this has been the case in practice. Over a score of fragrant hybrids have as a parent fortunei ssp. fortunei or fortunei ssp. discolor ; while decorum , diaprepes , and auriculatum have each been a parent of several fragrant hybrids.

Crossing medium sized species, such as those in the Maddenia or Edgeworthia subsections, might produce medium sized fragrant hybrids. Again, in practice, edgeworthii has been a parent of at least six fragrant hybrids.

Unfortunately there are no truly dwarf fragrant temperate rhododendron species. The semi-dwarf ciliatum can be slightly fragrant, and edgeworthii , which is sweetly scented, despite being included by Peter Cox in The Smaller Rhododendrons , can grow quite leggy - as tall as three metres.

To create a choice rhododendron hybrid which is fragrant, the grower, then, has a few problems. First, a scented species or hybrid will be needed as a parent. Most existing fragrant temperate rhododendrons are medium to tall, and often straggly in growth; this severely limits the chances of producing a fragrant dwarf hybrid. Second, most species with sweetly scented flowers are rather tender, although that is less of a problem for us in New Zealand than in those northern hemisphere countries mentioned earlier.

Third, fragrant temperate species tend to bloom only later in the season; this makes it difficult to produce an early scented hybrid. Fourth, rhododendron flowers which are scented tend to be pale colours: white, cream, yellow or pink. So if the hybridizer wants to produce a red-flowered or blue-flowered hybrid which is fragrant, he is not likely to succeed.

So what is a typical fragrant rhododendron hybrid? Perhaps the best known is 'Fragrantissimum' ( edgeworthii x formosum ). Salley and Greer (3, p.111) describe it as "a rather leggy tender plant that is one of the most fragrant of all shrubs." Cox and Cox (4, p.119) say "it is famous for its deplorably straggly arching habit, and it looks best on a wall, even as a fan or espalier, or trained around stakes." This is hardly window box or tub material! But its flowers are big and showy, white tinged pink, and with a strong sweet scent, akin to a nutmeg flavour. We see many in local gardens, often sprawling above fences, blooming in early November.

Hybridizing Fragrant Rhododendrons Over the Last Two Centuries

The hybridizing of rhododendrons began early last century. The first recorded by the RHS had the apt name of 'Hybridum' [ viscosum (a deciduous azalea) x maximum (of the pontica subsection)] by Dean William Herbert in Britain, 1817. This azaleodendron had pink-edged flowers flecked with yellow, and was scented.

The first hybrid using only evergreen rhododendron parentage was 'Altaclarense' [( catawbiense x ponticum ) x arboreum ] by J. R. Gowen, in 1831; but of course the red flowers on this large tree were not fragrant.

Dean Herbert and Waterer were both active hybridizers in the 1830s and 1840s, but it was not until 1862 that the first fragrant rhododendron hybrids were introduced: 'Countess of Haddington' ( ciliatum x dalhousiae ) by Parker; and 'Princess Alice' ( edgeworthii x ciliatum ) by Veitch. In 1868 'Fragrantissimum' ( edgeworthii x formosum ) was introduced by Rollison, and received an FCC in the same year.

In the 1860s, we find the first vireya hybrids, each with some fragrance. 'Princess Royal' ( jasminiflorum x javanicum ) by Henderson in 1863; and 'Princess Alexandra' ( jasminiflorum x 'Princess Royal') by Veitch in 1865; were followed by 'Taylori' ( brookeanum var. gracile x 'Princess Alexandra') by Byls in 1874 and exhibited by Veitch in 1891.

It is worth noting that the catalogue issued about 1880 by William Martin, nurseryman of Fairfield, Dunedin, listed 'Countess of Haddington', 'Princess Alice', 'Fragrantissima' (sic), and 'Taylori', as well as species javanicum . So, over a century ago, people in Dunedin could obtain fragrant hybrids and vireyas from a local nursery.

Other 19th century fragrant rhododendrons included: 'Victorianum' ( dalhousiae x nuttallii ) by Cuvelier in 1879, and 'Kewense' ( griffithianum x fortunei ) by Kew Gardens, 1888. This particular cross had an important place in the development of large hybrids with scented flowers.

Perhaps we should go back to about 1850, when Joseph Hooker, exploring in the Sikkim-Himalaya, discovered griffithianum , which he called aucklandii . It had large white or pink flowers with a green base, and they were often scented. Then, in 1856, Robert Fortune, from his plant-hunting expeditions in China, introduced fortunei , with large shell-pink to rosy-pink blooms. The first cross of these two species, 'Kewense', did not have the quality that its auspicious parentage should have ensured. About 1900, Sir Edmund Loder, at Leonardslee, West Sussex, crossed griffithianum with a particularly fine large-flowered sweet-scented clone of fortunei to produce 'Loderi' which was registered in 1901. There are now well over 30 named forms of 'Loderi'; all have enormous trusses of very large trumpet-shaped flowers of white to cream to soft pink, which are richly scented. Several varieties of 'Loderi' are well know to us in New Zealand:

'Loderi King George' (1901, FCC 1970): large white flowers;

'Loderi Pink Diamond' (1901, FCC 1914): rose-pink flowers;

'Loderi Venus' (1901, AGM 1968): pale pink very fragrant blooms;

'Loderi Irene Stead' (Edgar Stead, 1947): deep lilac-pink flowers;

'Ilam Cream' ('Loderi' x unknown) Stead, AM 1985. Award of Distinction (New Zealand): rich cream blooms, deep pink edges;

'Lalique' ('Loderi' or griffithianum seedling) Holmes, 1979; Award of Distinction (New Zealand): huge trusses of pale rose fading to white;

'Mrs. Percy McLaren' ('Loderi' x pink seedling unknown) (Medlicott, Dunedin, 1979): shell-pink flowers with frilled lobes.

The Loderi hybrids grow into substantial trees, and to walk in a grove of them when they are in full flower can be a heady experience. As Judith Beresford (2, p. 47) puts it: "The cool delicious scent of 'Loderi' floats on the air."

Large Rhododendron Hybrids with Fragrance

As well as the Loderis, there are many large hybrids with perfumed flowers. A few of the most outstanding are:

'Albatross' ('Loderi' x discolor ): pink buds, white blooms;

'Angelo' ( griffithianum x discolor ): huge trusses of white or pink flowers;

'Iceberg'* ('Loderi' x auriculatum ): white flowers with green centre. Late in season;

'Naomi' ('Aurora' x fortunei ) comprises a group of over a dozen named grexes; they have lax trusses in shades of smoky pink, yellow and white; they bloom in mid-season; are medium to tall; some slightly fragrant.

'Polar Bear' ( diaprepes x auriculatum ): large trusses of white flowers. Very late. Judith Beresford (2, p. 228) describes it thus: "The scent has great carrying qualities and will pervade a whole area of woodland."

Most of the fragrant hybrids with parents in the Fortunea or Auriculata subsections would be rather large for most town gardens, but room might be found for a few medium-tall (but not too broad) hybrids which have parents in the Dalhousiae Alliance of the Maddenia subsection, notably lindleyi , nuttallii and dalhousiae . These species have huge fragrant lily-like flowers. Some fine scented hybrids include:

'Tupare' ( nuttallii x lindleyi ) Edgar Stead, pre-1950: richly textured white blooms, yellow basal blotches.

'Mi Amor' ( lindleyi x nuttallii ) Sumner, 1962: white flowers, yellow throat. Can grow leggy unless shaped when young.

'Lady Dorothy Ella' ( lindleyi x nuttallii ) Duncan & Davies, 1986: flowers ivory-white, flushed pink, with basal golden flare in throat. Spectacular blooms.

'Tyermannii' ( nuttallii x formosum ) Tyerman, 1925: white blooms, yellow in throat, good texture; medium height.

'Michael's Pride' ( burmanicum x dalhousiae ) Michael, 1963: large waxy flowers, opening lime, fading to cream.

lindleyi x nuttallii x dalhousiae : this unregistered hybrid, affectionately known locally as "the three-way-cross," has, like lindleyi , a leggy habit and bullate leaves. The rather lovely white flowers are somewhat more demure, and are slightly scented.

Medium Sized Fragrant Hybrids

Crosses among species in the Maddenia subsection, and also with edgeworthii , are among the most popular and best known to us, perhaps because we can grow them so well in the Dunedin area. In this category, there are three Dunedin registered hybrids which are slightly fragrant: 'Maurice Skipworth' ( edgeworthii x burmanicum ); 'Rothesay' ( ciliicalyx seedling) and 'Ham Pearl' ( formosum x unknown).

Medium sized fragrant hybrids with Maddenia or Edgeworthia parentage include the well known 'Fragrantissimum' and 'Countess of Haddington', already discussed; but there are many other choice examples, and here are four real beauties:

'Princess Alice' ( edgeworthii x ciliatum ): deep pink buds opening to white flowers lined pink. Hardier and less straggly than 'Fragrantissimum'. Just as free-flowering, but scent not quite as strong.

'Else Frye' ( ciliicalyx x edgeworthii [?]) Bowman, 1963: loose trusses of 3-6 flowers of good substance, white edges flushed rose, and chrome yellow throat. According to Cox and Cox (4, p. 106), "One of the best and most popular of newer tender scented hybrids."

'Barbara Jury' ( maddenii x 'Sirius') R. M. Jury, 1982: funnel shaped flowers of Naples yellow, deepening on outside flushed pink. Orange-yellow inside. Dark glossy foliage on handsome vigorous shrub.

'Floral Dance' ( nuttallii x edgeworthii ) F. M. Jury, 1982: large funnel-shaped white flowers flushed camellia-rose with yellow blotch and frilled lobes. Fragrant. Handsome bullate leaves on an upright bush. Late.

We should not forget that a number of medium-sized registered in Dunedin usually have slight fragrance. They include 'Blue Mist', 'Ham Pearl', 'Maurice Skipworth', 'Rothesay', and 'Waireka'.

Smaller Fragrant Rhododendron Hybrids

As there are no truly dwarf fragrant species, and their time of blooming tends to be late in the season, it would seem difficult to create a dwarf scented hybrid, and well nigh impossible to achieve an early flowering dwarf hybrid with fragrance. Peter Cox (5, p. 44) writes: "I feel there is much work needed here for the hybridizer to produce dwarf growers with scented flowers, opening early in the season. My previous efforts at this have failed, but some promising seed has set this year. Few of the present scented species or hybrids can really be classed as dwarfs. Rhododendron edgeworthii , with its superb foliage, flowers and scent is one of the best." Peter Cox does not give away any secrets here; yet in Dunedin, a local nurseryman, the late Bruce Campbell, did succeed with one method. He crossed edgeworthii with an early truly dwarf species, leucaspis . And he made a breakthrough, producing 'September Snow', an early flowering semi-dwarf hybrid with fragrant flowers. It was introduced in 1972, and registered in 1981.

A full examination of all registered hybrids (Salley & Greer, 1986), shows only one instance of semi-dwarf hybrids with fragrance. A group of hybrids bred by C. Dexter (1862-1943) in Sandwich, Mass., now the Heritage Plantation, had unknown parentage and limited commercial availability. Two appeared to be fragrant: 'Dexter's Peppermint': pink flowers, green blotch, fragrant; and 'Dexter's Spice': white flowers, green spots, fragrant.** It seems that the search for fragrant dwarf hybrids is still at an early stage. Bruce Campbell also crossed 'Lovelock' with ciliatum to produce 'Waireka', which gains slight scent from ciliatum ; but 'Waireka' looks as if it will be medium sized rather than dwarf.

Why Grow Fragrant Rhododendron Hybrids?

There are those enthusiasts who favour rhododendron species only, believing that species have a purity and quality that cannot be matched by hybrids, many of which they consider to be rather coarse, even big and blowzy. From a connoisseur's or collector's point of view, this argument might have much to commend it, but there are sound reasons for growing hybrids as well as species. First, many species are difficult, and do not have "hybrid vigour." Hybrids usually grow more lustily, and are sometimes larger than one of the species' parents. Consider that most fragrant species edgeworthii , for example. Peter Cox (5, p. 91) writes about it: "While often seen in cultivation, few plants can be called a picture of health. It is not as easy to please as its hybrids such as 'Fragrantissimum' or 'Lady Alice Fitzwilliam'."

Again, if we compare edgeworthii with 'Floral Dance', we find the hybrid to be hardier and more vigorous, the flowers may be even larger and just as beautiful, the bullate leaves are bigger (though they do lack the lovely indumentum of edgeworthii foliage and stems). One could make similar comparisons between lindleyi and 'Tupare'. This hybrid has outstanding flowers and is a compact (less leggy) bush.

Second, the enhancing of some species' attributes by hybridization may produce spectacular results. "Big and blowsy" flowers of many hybrids are popular with many people, not only Americans. Regarding fragrance, the strong scent of 'Loderi' or 'Fragrantissimum' is hard to match in any species.

If variety is to be the spice of life in the rhododendron world, then there is a case for a greater diversity of hybrids to complement the species. This applies also to the search for spicy fragrant hybrids. In an age of diversity and variety, we are reaching a time when, for example, dwarf scented rhododendrons which bloom in early spring will become available. 'September Snow' is already with us. Others will surely follow.

References
1.  Cox, Peter. A. Dwarf Rhododendrons . Batsford, London, 1973.
2.  Beresford, Judith, Rhododendrons and Azaleas . Faber, London, 1973.
3.  Salley, H. E., and Greer, H. E. Rhododendron Hybrids: A Guide To Their Origins . Batsford, London, 1986.
4.  Cox, Peter A. and Cox, Kenneth N. E. Encyclopedia of Rhododendron Hybrids . Batsford, London, 1988.
5.  Cox, Peter A. The Smaller Rhododendrons . Batsford, London, 1985.

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: * 'Iceberg' is an illegitimate name for this hybrid; it is the registered name of a Kurume azalea. ** 'Dexter's Peppermint' and 'Dexter's Spice' can grow 5-6' in the US.