|Primula calderiana X Primula strumosa|
Subsequent to Richards publication, Polunin & Stainton in "Flowers of the Himalaya" (1984) and Gould in Hara, An enumeration of the flowering plants of Nepal" (1982) followed Richards without comment. The Flora of Bhutan (1999) and The Flora of China (1994) retained P. strumosa as a distinct species with comments justifying their positions. Mostly, this is based on notes by Ludlow that in the field, pure P. strumosa was found at a higher elevation than pure P. calderiana and the only hybrids were found at an intermediate elevation between the two pure populations. The recent book The Genus Primula L. in India goes one step further than Richards, in reducing P. strumosa to a variety of P. calderiana but they do not justify this position and do not acknowledge the altitudinal preferences of the two species.
Unless there is more convincing evidence, I am retaining P. strumosa as a distinct species.