atlanticum X caledulaceum is a Tetraploid

[Rhododendron atlanticum X Rhododendron calendulaceum] "Broken Arrow", which is a Dick Jaynes hybrid, has been determined to be a tetraploid 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.

Summary of Ploidy Levels

Tetraploid Rhododendron atlanticum,
Tetraploid Rhododendron calendulaceum,
Tetraploid Rhododendron atlanticum X Rhododendron calendulaceum "Broken Arrow",
Tetraploid Rhododendron atlanticum X Rhododendron calendulaceum orange,
Tetraploid Rhododendron atlanticum X Rhododendron calendulaceum pink

Source: Arnold Arboretum Accession Number: 410-2002

John and Sally Perkins