Claimed hybrids between Kalmia and Rhododendron

RHS project team
Mike Grant, RHS
Dr Nicola Toomey and Dr Alastair Culham (Reading University)
Start date
01/02/2003 08:49:45
End date
01/02/2004 08:49:49

Chloroplast DNA , nuclear DNA, ITS,  trnL-F, cpDNA,  nrDNA,  phylogeny, Ericaceae, molecular marker, DNA sequence, Kalmia hybrid, Rhododendron hybrid.

The problem

Two putative hybrids between Kalmia and Rhododendron are cultivated by enthusiasts and their status has always been the subject of debate. Morphological assessments have not settled the arguments and one taxon has never flowered.


The two cultivars and their suspected progenitor species and related taxa were submitted to DNA sequencing of cpDNA trnL-F and nrDNA ITS regions in order to test whether there was DNA sequence evidence both for hybridization per se and for the direction of the cross, should one be evident.

Benefits to gardeners

The putative hybrids represent a distant cross within the Ericaceae, which has taxonomic implications and important consequences for ornamental plant breeding.

Summary of results
Comparison of eight DNA sequences from these putative hybrids with Rhododendron and Kalmia species showed clear evidence of origin within Rhododendron. No evidence of Kalmia DNA was detected. These putative intergeneric hybrids appear to be mutants of Rhododendron and not of hybrid origin.
Further information

Grant M L (2006). Kalmia x Rhododendron is a myth. International Dendrology Society Yearbook 2005.
Download the pdf (137kB)

Grant M L, Toomey N H & Culham A C (2005). Kalmia x Rhododendron de-bunked. RHS Rhododendron, Camellia & Magnolia Yearbook 2005. RHS, London
Download the pdf (143kB)

Grant M L, Toomey N H & Culham A C (2004). Is there such a thing as Kalmia x Rhododendron? Journal of the American Society for Horticultural Science 129(4):517-522

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