Rhododendron 'English Roseum' X [Rhododendron diaprepes X Rhododendron hyperthrum], which is a Thornton hybrid, has been determined to be a diploid using flow cytometry by
Dr. João Loureiro, Dr. Silvia Castro, José Cerca, and Mariana Castro
Plant Ecology and Evolution Group,
Centre for Functional Ecology,
Department of Life Sciences,
Faculty of Science and Technology,
University of Coimbra, Portugal.
Rhododendron diaprepes 'Gargantua', which is a tetraploid, may have been one of the parents in the cross of Rhododendron diaprepes X Rhododendron hyperthrum. Neotetraploids and triploids can produce diploid offspring.
Rhododendron diaprepes is most likely a diploid species.
Summary of Ploidy Levels
Diploid Rhododendron 'English Roseum' X [Rhododendron diaprepes X Rhododendron hyperthrum],
Tetraploid Rhododendron diaprepes 'Gargantua'
Source: John Thornton
John and Sally Perkins