'Tang' is a Diploid

Rhododendron 'Tang', which is a 1952 David Leach hybrid documented as Rhododendron cumberlandense 'Scarlet Salute' x 'Red King' 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.

Rhododendron 'Red King' is documented to be a mollis hybrid.

Summary of Ploidy Levels

Diploid Rhododendron cumberlandense,
Diploid Rhododendron molle,
Diploid 'Tang'

Source: Barlett Arboretum

John and Sally Perkins

Rhododendron groenlandicum is a Tetraploid

Rhododendron groenlandicum 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.

Rhododendron groenlandicum was formerly classified as Ledum groenlandicum.

Summary of Ploidy Levels

Diploid Rhododendron columbianum,
Diploid Rhododendron subarticum,
Diploid Rhododendron subulatum China,
Tetraploid Rhododendron diversipilosum,
Tetraploid Rhododendron diversipilosum 'Milky Way',
Tetraploid Rhododendron groenlandicum,
Tetraploid Rhododendron groenlandicum Saco Bog, Maine,
Tetraploid Rhododendron hypoleucum,
Tetraploid Rhododendron neoglandulosum,
Tetraploid Rhododendron tolmachevii,
Tetraploid Rhododendron tomentosum Turku Archipelago

Source: Bartlett Arboretum
Source: Saco Bog, Maine (John and Sally Perkins)
Source: Kristian Theqvist

John and Sally Perkins