JARS v43n4 - Rhododendron leucogigas 'Hunstein's Secret'

Rhododendron leucogigas 'Hunstein's Secret'
L. A. Craven
Canberra, Australia
J. L. Rouse
Victoria, Australia

Of all the species of vireya rhododendron cultivated in Australia, none has generated as much discussion about its identity as the plant now known as R. leucogigas 'Hunstein's Secret'. The saga began in August, 1966, when one of us (Craven) discovered a large-leaved rhododendron at about 1200 m on Mt. Hunstein in the Sepik District, Papua New Guinea. Only a single individual was found, a small seedling growing on a branch fallen from the forest canopy. It was so small that only a single scion was available. It is doubtful that the plant would have survived for long on the forest floor as the light levels there were much less than the plant would have been receiving high in the canopy. The scion was sent to Melbourne, Australia, and there successfully propagated as an own-root cutting.
The first identification of the plant, tentative since it was based on leaf material only, was R. schlechteri . When it flowered several years later a firm identification was made, R. aff. gardenia , which is the botanist's shorthand for " Rhododendron species with affinity to R. gardenia ." Rhododendron gardenia is classified in subsection Phaeovireya which includes species such as R. konori and R. phaeopeplum . The Mt. Hunstein plant has flowers which are large and white, and with a form generally similar to that of R. konori (Figs. 1, 2). Each flower is about 12 cm long (the tube length, measured from corolla base to the limb) and about 12 cm wide (the limb diameter).

'Hunstein's Secret' in bud
Fig. 1  'Hunstein's Secret' in bud, April 1987. The dark red
perulae (the protective scales of buds), a striking feature of
this species, can be seen below the buds.
Photo by J. L. Rouse
'Hunstein's Secret' in full flower
Fig. 2  'Hunstein's Secret' in full flower, April 1987.
Photo by R. Moodycliffe

Not surprisingly, this species has become of great interest to Australian vireya growers and its pollen has been widely used in hybridising (by Rouse in particular). A great deal of hybrid seed has been distributed, with this plant recorded in the genealogy (usually given as " R. gardenia Aff.") and flowers of one of the hybrids is shown in Fig. 3.

R. konori x 'Hunstein's Secret'
Fig. 3 R. konori (a West Irian form of this species) x 'Hunstein's
Secret'. See also color photo on cover of this issue.
Photo by J. L. Rouse

In addition to being used in hybridising, 'Hunstein's Secret' has been selfed (ripe, dehiscing capsules from selfed flowers are shown in Fig. 4). Plants of R. leucogigas are relatively slow in making growth and therefore propagation by cuttings is not a feasible technique for increasing stocks; propagation by seed currently is the most suitable method. Rhododendron leucogigas is one species for which propagation by tissue culture may have advantages.

Initial phases of dehiscence in 'Hunstein's 
Secret' capsules
Fig. 4  Initial phases of dehiscence in capsules from selfed flowers of 'Hunstein's Secret', December
1987. Left: the epicarp and mesocarp layers have ruptured at the base of the style and are beginning
to peel away from the endocarp. Right: the epicarp and mesocarp layers have ruptured centrally on
the capsule and are peeling away in both directions. This mode of dehiscence occurs much less
frequently than that shown in left photo.
Photos by J. L. Rouse

The similarity of the plant to cultivated plants of R. leucogigas caused Craven to question the accuracy of the determination R. aff. gardenia . Study of the leaf scales of the Mt. Hunstein plant showed that these were stellate and mostly sessile, although dendroid scales were by no means uncommon (Fig. 5). Using the identification keys in the treatment of Rhododendron in Flora Malesiana (Sleumer, 1966) the plant readily keys to R. leucogigas , a species then known only from the Cycloop Mts. in Irian Jaya and which is classified in subsection Euvireya series Javanica. Comparison with the description given of this species in Flora Malesiana , with herbarium specimens held in the Australian National Herbarium, and with live plants known to be named reliably, has proved to our satisfaction that the Mt. Hunstein plant represents R. leucogigas . Thus Mt. Hunstein is apparently the second reported locality for this species.

Leaf scales of 'Hunstein's Secret'
Fig. 5  Leaf scales of 'Hunstein's Secret'. Left: a portion of the marginal zone of the adaxial
("upper") surface of a leaf. Right: a portion of the marginal zone of the abaxial ("lower")
surface of the leaf shown on the left. The bar represents a length of 0.5 mm.
Photos by J. L. Rouse

The Mt. Hunstein form of R. leucogigas has been given the cultivar name 'Hunstein's Secret' in allusion to the fact that for so many years this species had been growing, unknown to botanists and rhododendronophiles, in the virgin rainforests of Mt. Hunstein. Indeed, it is likely that relatively few humans have ever seen it there. Until the 1960's this region had been occupied by scattered, very small groups of nomads who travelled through the forest finding their food as they went, in sharp distinction to the majority of Melanesians who grow the bulk of their food requirements in gardens and live in more or less permanent villages. Now these nomads have been aggregated along the rivers, the communication routes in this part of New Guinea, and they rarely travel through the lands that once had been their home.
A secondary source of confusion over the name R. gardenia has arisen as a result of vireya growers in the Wollongong district, New South Wales, cultivating an entirely different plant under the name R. gardenia . This plant was introduced from Holland by W.F. Mearns (Mearns, personal communication). The name was derived from a perceived similarity in flower colour to Gardenia globosa (correctly Rothmannia globosa ) by the original Dutch grower. It seems that the plant is of hybrid origin and that the name is to be regarded as a cultivar name rather than a scientific name, in which case it should be written as R. 'Gardenia' or R. cv. Gardenia rather than as R. gardenia .
The species described by Schlechter as R. gardenia (Schlechter, 1918) is a very different plant. Rhododendron 'Gardenia' keys to R. konori in Sleumer (1966) but it differs from this species, inter alia , in the shape and colour of the corolla. The parentage of this very attractive cultivar presumably includes R. konori or a closely related species such as R. phaeopeplum . It has not been ascertained as to whether or not the name R. 'Gardenia' in fact has been registered with the international registration authority for Rhododendron cultivars but it would appear that under the International Code of Nomenclature for Cultivated Plants (Brickell etal., 1980) the epithet 'Gardenia' should be regarded as invalid [cf. Articles 27(a) and 31(a)].

The following people have assisted us with the provision of plant material and/or information on the source of different identifications; to them we are indebted: T. Lelliott, W.F. Mearns, J.C. Smith, G. and W. Snell, and R.M. Withers. R. Moodycliffe provided the magnificent photograph of 'Hunstein's Secret' used as Fig. 2.

Brickell, CD., etal. eds. 1980. International Code of Nomenclature for Cultivated Plants 1980. [Regnum vegetabile. vol. 104] (Bohn, Scheltema & Holkema: The Hague).
Schlechter, R. 1917-1918. Die Ericaceen von Deutsch-New-Guinea. Bot. Jahrb. Syst.55; 137-194.
Sleumer, H. 1966. Rhododendron In Flora Malesiana ser. I, 6; 474-668.

L.A. Craven is with the Australian National Herbarium, CSIRO Division of Plant Industry, Canberra, ' Australia.
J.L. Rouse is associated with the School of Physics, The University of Melbourne, Parkville, Victoria, Australia. He was co-author of "A Vireya Azaleodendron In Flower", ARS Journal, Vol. 42:3, Summer 1988.