Rhododendron calendulaceum X Rhododendron molle, which was received by the Arnold Arboretum in 1921 from Hunnewell Pinetum, has been determined to be a tetraploid 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 calendulaceum is a tetraploid species. Rhododendron molle is a diploid species.
Tetraploid deciduous azaleas normally reject pollen from diploid deciduous azaleas. Diploid / tetraploid interaction in deciduous azaleas normally results in triploid offspring.
Listing the parentage of a cross alphabetically is not uncommon especially prior to 1930. In the literature, Rhododendron molle, which is a species, is frequently confused with Mollis which is a reference to deciduous azaleas hybrids containing Rhododendron molle.
Summary of Ploidy Levels
Diploid Rhododendron molle,
Tetraploid Rhododendron calendulaceum X Rhododendron molle,
Tetraploid Rhododendron calendulaceum
Source: Arnold Arboretum Accession Number: 10768
John and Sally Perkins
