This name is a synonym of Rumex occidentalis var. procerus (Greene) J.T. Howell.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 50079693) with original publication details: Publ. Field Mus. Nat. Hist., Bot. Ser. 17(1): 114 1937.
Full publication details for this name can be found in IPNI: http://ipni.org/urn:lsid:ipni.org:names:224428-2.
This name was in version 1 of The Plant List, record tro-50079693, and has not been changed.