K ALMIA X R HODODENDRON D EBUNKED

M IKE G RANT, NICOLA H. TOOMEY & ALASTAIR C ULHAM

Introduction 1996 it was noted that it fell foul of Article Even if they have not grown it or seen it in the 17.9 of the International Code of flesh, many rhododendron growers have Nomenclature for Cultivated Plants (Trehane probably heard of a reputed hybrid between et al 1995) because it is styled in Latin. Kalmia and Rhododendron. If such a thing Therefore a new name, ‘Everlasting’, was could be confirmed it would represent the coined by Pierce (Murray 1996 in litt.). It is most genetically distant cross recorded in the quite widely cultivated by enthusiasts in Ericaceae. This article is a summary of an north-west USA (Halligan 1994) and is investigation (Grant et al 2004) conducted by favored for its long flowering season, perhaps the authors into the status of putative Kalmia caused by its apparent sterility, and its x Rhododendron hybrids. seemingly curious origin. ‘Everlasting’ is widely regarded as being an intergeneric History of Kalmia x Rhododendron The first reputed cross between Kalmia and Rhododendron (see right) was made in the 1950s by Halfdan Lem (1885–1969) in the USA, allegedly between K. latifolia and R. williamsianum (Pierce 1974). The hybrid was reported as having ‘pinkish white, cup- like flowers of all kalmias, but with a larger leaf resembling the male parent, R. williamsianum.’ (Pierce 1974). Note how this statement implies that Kalmia was the female parent. The one plant resulting from the hybridisation was named ‘No Suchianum’ (sometimes styled as ‘Nosuchianum’) in jest by Warren Berg in 1973. Unfortunately, when the cultivar name was brought to the Rhododendron ‘Everlasting’ at RHS Wisley, attention of the International Cultivar previously thought to be a hybrid between Registration Authority for Rhododendron in Kalmia and Rhododendron

25 Rhododendrons with Camellias and Magnolias 2005

Dendrogram resulting from analysis of chloroplast DNA trnL-F sequence data

26 Kalmia x Rhododendron Debunked hybrid on the basis of its claimed parentage, shoots of the current season. However, in 2004 apparent sterility and the seemingly Kalmia- this plant flowered at Arboretum like characters of a shallow, saucer-shaped Trompenburg and a photograph has kindly corolla and widely spreading stamens. Despite been supplied by Dick van Hoey Smith (see the corolla and stamen characters there do not p.31). appear to be any other features of the flower that suggest an affinity with Kalmia. Other intergeneric hybrids in Ericaceae ‘Everlasting’ was reputedly crossed with Of other claimed intergeneric hybrids in the R. arboreum subsp. delavayi resulting in a Ericaceae, the only ones that stand up to cultivar called ‘Brilliant Abbé’ (Anon 1985). rigorous inspection are within tribe The latter reference erroneously lists one Phyllodoceae. These are X Kalmiothamnus, parent as ‘Brilliancy’ (Murray 1996 in litt). X Phylliopsis and X Phyllothamnus and they We were not able to obtain material of represent crosses between Kalmiopsis, ‘Brilliant Abbé’. Phyllodoce and Rhodothamnus. This The next reputed cross between Kalmia interfertility reflects the close relationship and Rhododendron is represented by a plant between these three genera. The only cross in (hereafter referred to as the Hillier taxon) in this group that has not yet been performed the collection at the Sir Harold Hillier successfully is that between Kalmiopsis and Gardens and Arboretum in England. Harold Phyllodoce (Starling 1982). G. Hillier obtained it from the US National Intergeneric crosses in other sections of Arboretum (where it may have originated) in the Ericaceae have all proved to be less 1961 and it is referred to as growing there in resilient to taxonomic re-alignments. Jaynes (1997). It is also grown at Arboretum X Gaulnettya resides in Gaultheria now that Trompenburg in the Netherlands where it is Pernettya is sunk, X Ledodendron resides in listed as an intergeneric hybrid (van Hoey Rhododendron now that Ledum is sunk and Smith 2001). It was thought to represent a Bruckenthalia x Erica resides in Erica now hybrid between K. latifolia and R. maximum that Bruckenthalia is sunk. X Ericalluna (Hillier & Sons 1981) but subsequent editions merely proved to be an abnormal Erica, not a (e.g. Hillier & Coombes 2002) have been hybrid with Calluna. This leaves us with the more cautious and listed it under R. reputed Kalmia and Rhododendron hybrids maximum. Likewise, Jaynes (1997) has mentioned above. The former belongs to questioned its intergeneric status. The tribe Phyllodoceae and the latter to tribe reported evidence for hybridity is on the basis Rhodoreae, but Kron et al (2002) show that of the tendency of its flower buds to abort these two tribes are quite distantly related. (Coombes, pers. comm.; Fortgens, pers. comm.) and its long petiole relative to length of leaf The investigation lamina; the latter character is reminiscent of K. Claims for intergeneric status of various latifolia. It has been suggested that its hybrids are made on a fairly regular basis by reluctance to flower is because Rhododendron horticulturists. Such pronouncements are flower buds develop on shoots of the previous sometimes worth investigating, not least season whereas those of Kalmia develop on because of what they may tell us about the

27 Rhododendrons with Camellias and Magnolias 2005

Dendrogram resulting from analysis of nuclear DNA ITS sequence data

28 Kalmia x Rhododendron Debunked relatedness of the genera in question. The Horticultural Society’s gardens at Wisley and two Kalmia x Rhododendron cases demand Rosemoor (England), and kindly supplied by investigation because, if their parental status the Sir Harold Hillier Gardens and could be proved, they would represent the Arboretum (England) and Glendoick most distant intergeneric hybrids in Gardens (Scotland) (see table below). DNA Ericaceae and this would have important was extracted from the leaves and two gene taxonomic implications. Perhaps more regions were amplified and sequenced. One important is the potential for ornamental region is known as trnL-F and is located on plant breeding! As International Cultivar chloroplast DNA, the other region is known Registration Authority for the genus as ITS and is found on nuclear DNA. As Rhododendron, the Royal Horticultural chloroplasts tend to be maternally inherited, Society has an interest in putative hybrids sequencing of chloroplast DNA would give an between Rhododendron and other genera. indication of the female parent if the We therefore submitted material of the two intergeneric nature were proved. available cultivars and related species to We also obtained numerous additional DNA sequencing. This would hopefully DNA sequences (including some from reveal any evidence for hybridisation and the Bejaria, Cassiope and Gaultheria) from the direction of the cross should one be evident. EMBL/Genbank/DDBJ database, in order to put our sequences into context. All the Materials and methods sequences were then aligned and analysed and Plant material was obtained from the Royal presented in the form of dendrograms (see pp

Name Source Origin Accession no. R. ‘Everlasting’ a & b RHS Garden, Cult. W20012690 Wisley Hillier taxon a & b Hillier Gardens Cult. H19841501 & Arboretum K. latifolia a RHS Garden, Cult. W951916 Wisley K. latifolia b RHS Garden, Cult. W960618 Wisley R. williamsianum a RHS Garden, Cult. R960393 Rosemoor R. williamsianum b RHS Garden, Cult. R888391 Rosemoor R. williamsianum c Glendoick Gardens Wild None R. maximum Glendoick Gardens Wild None Table: Accession data for plant material used in DNA sequencing

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26 and 28) showing the relationships between for the corolla lobe to tube ratio. The the species and hybrids. morphology of this plant fits the placement of the cultivar into subsection Pontica very well Results and this matches the molecular evidence. The The analysis of chloroplast DNA trnL-F (see open corolla tube is probably the result of a p. 26) shows that Rhododendron ‘Everlasting’ is mutation in the genes controlling floral very close in sequence to R. brachycarpum, and development. the Hillier taxon is identical in sequence to R. The Hillier taxon also shows no sign of Kalmia maximum. Neither shows any closeness to in either DNA sequence and a close match to Kalmia. Rhododendron maximum in both sequences. The The analysis of nuclear DNA ITS (see p. flowers produced in 2004 suggest a strong affinity 28) shows that Rhododendron ‘Everlasting’ has with R. maximum. The evidence suggests that this, no exact match (note that a R. brachycarpum like R. ‘Everlasting’, is a chance mutant of a sequence was not available for this analysis) but rhododendron. groups with R. williamsianum and its close relatives. The Hillier taxon again shows an A new cultivar name exact match to R. maximum. Neither shows The plant that we have referred to as the any closeness to Kalmia. Hillier taxon clearly needs a cultivar name. Although not the intention of this study, We therefore provide a name here and give a note how the trnL-F analysis shows that validating description. The name has been Rhododendron albrechtii, R. camtschaticum and registered with the International Cultivar R. redowskianum are quite distantly related to Registration Authority for Rhododendron. A the rest of Rhododendron, while Menziesia and nomenclatural standard herbarium specimen Ledum fall within the genus. Of these, only R. has been deposited in the RHS Herbarium at camtschaticum was available for ITS analysis but Wisley (WSY) and a duplicate at the Harold shows a similar placement. Hillier Herbarium (provisional code HILL). The name we have chosen reflects its Discussion historical, but erroneous, connection with The claimed origin of Rhododendron Kalmia. ‘Everlasting’ as a hybrid between Kalmia latifolia and R. williamsianum is refuted by the Rhododendron ‘Kalamity’ DNA sequence evidence. The combination of An elepidote Rhododendron cultivar both chloroplast and nuclear DNA evidence derived from R. maximum, recognised by indicates that Kalmia has not contributed to its abnormally long petiole relative to leaf the genetic makeup of ‘Everlasting’ but that length and flower buds that often abort, the cultivar is closely related to R. propagated vegetatively. Leaves brachycarpum or the R. williamsianum group. evergreen, c.100–120 x 20–30mm; leaf When regarded as a rhododendron, blade elliptic, apex mucronate, underside ‘Everlasting’ can be keyed out to subsection with thin, scattered, non-persistent, 1 Pontica in section Ponticum in Chamberlain’s brown indumentum; petiole c. ⁄2 as long key (1982) to subgenus Hymenanthes, except as total leaf. Flowers as in R. maximum.

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HILLIER & SONS (1981) Hillier’s Manual of Trees and Shrubs. David & Charles, Devon. JAYNES, R.A. (1997) Kalmia – Mountain Laurel and Related Species. Timber Press, Oregon. KRON, K.S., JUDD, W.S., STEVENS, P.F., CRAYN, D.M., ANDERBERG, A.A., GADEK, P.A., QUINN, C.J. & LUTEYN, J.L. (2002) Phylogenetic classification of Ericaceae: molecular and morphological evidence. Bot. Rev. (Lancaster) 68(3): 335-423. PIERCE, L.J. (1974) An unusual intergeneric Rhododendron ‘Kalamity’, previously cross. Quart. Bull. Amer. Rhododendron Soc. thought to be a hybrid between Kalmia and Rhododendron 28(1): 45. STARLING, B.N. (1982) Bigeneric hybrids Nomenclatural standard (designated within Ericaceae. The Plantsman 4(2): 91-99. here): Sir Harold Hillier Gardens and TREHANE, P., BRICKELL, C.D., BAUM, B.R., Arboretum, accession no. 1984.1501, HETTERSCHEID, W.L.A., LESLIE, A.C., herbarium barcode WSY0046145, 1 January MCNEILL, J., SPONGBERG, S.A. & VRUGTMAN, 1996, Cult. (WSY; duplicate at HILL). F. (1995) International Code of Nomenclature for Cultivated Plants – 1995. Quarterjack References Publishing, Wimborne. ANON. (1985) Additions to the International VAN HOEY SMITH, J.R.P. (2001) Arboretum Rhododendron Register 1984/5. Trompenburg. Strichting Bevordering, Rhododendrons 1985/86 with Magnolias and Holland. Camellias: 96-112. CHAMBERLAIN, D.F. (1982) A revision of Rhododendron. II. Subgenus Hymenanthes. Notes Formerly a Senior Botanist at RHS Garden Roy. Bot. Gard. Edinburgh 39(2): 209-486. Wisley, Mike Grant is now Deputy Editor of GRANT, M.L., TOOMEY, N.H. & CULHAM, A. The Plantsman, based in Peterborough. (2004) Is there such a thing as Kalmia x Rhododendron? J. Amer. Soc. Hort. Sci. 129(4): Nicola Toomey is a Research Assistant and 517-522. Alastair Culham is a Lecturer in Plant HALLIGAN, P. (1994) Northwesterners rate Taxonomy, both in the School of Plant their rhodies. J. Amer. Rhododendron Soc. Sciences, University of Reading. 48(4): 182-185. HILLIER, J. & COOMBES, A. (eds.) (2002) The Hillier Manual of Trees & Shrubs. David & Charles, Devon.

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