This name is a synonym of Rhododendron maximum f. purpureum (Pursh) Fernald.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 50120883) with original publication details: Fl. Amer. Sept. 1: 297 1814.
This name was in version 1 of The Plant List, record tro-50120883.