Research

Mike Grant, Nicola Toomey & Alastair Culham

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. If proved to be genuine they would represent a distant cross within the Ericaceae which would have taxonomic implications. Their existence would also have important implications for ornamental plant breeding.

The RHS is International Cultivar Registration Authority for Rhododendron and breeders look to us for an authoritative view on such hybrids. 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.

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.

Publications resulting from this work:

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

Grant, M.L., Toomey, N.H. & Culham, A.C. (in press) Kalmia x Rhododendron de-bunked. RHS Rhododendron, Camellia & Magnolia Yearbook 2005

Armitage, J.D. (2004) Bigeneric hybrids in the RHS Plant Finder. RHS Plant Finder News 5: 3