This name is a synonym of Rumex occidentalis S. Watson.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 50248175) with original publication details: Vasc. Pl. Wyoming 299 1988.
Full publication details for this name can be found in IPNI: http://ipni.org/urn:lsid:ipni.org:names:973484-1.
This name was in version 1 of The Plant List, record tro-50248175.