Mentzelia rhizomata, a new species of sect. Bartonia, is described from the Roan Plateau area of Garfield Co., Colorado. It may be readily distinguished from other taxa of the section by its sparsely branched, horizontally spreading, elongate, woody, deeply seated underground caudex system. From its nearest relative, M. goodrichii of northeastern Utah, the Roan Cliffs blazingstar differs in being a sprawling plant with five petals, generally shorter pedicels and sepals, an urceolate fruit, and numerous, long, twisted and often tangled, exceedingly fragile and easily detached caudex branches. The plant has been reported in the Colorado literature under the name Nuttallia argillosa.

Communicating Editor: Matt Lavin