Rhododendron "(flammeum X calendulaceum) not", which is a John Thornton hybrid, has been determined to be a diploid using flow cytometry by
Dr. João Loureiro, Dr. Silvia Castro, José Cerca Oliveira, 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.
Ploidy testing indicates that Rhododendron calendulaceum may not be the pollen parent of this hybrid.
Summary of Ploidy Levels
Diploid Rhododendron flammeum,
Diploid "(flammeum X calendulaceum) not",
Tetraploid Rhododendron calendulaceum
Source: John Thronton
John and Sally Perkins