Authors: Roxanne I. Hastings & Dr. Henk C. Greven

Plants in dense to loose patches, pale green to red-brown. Stems 1.5--4 (--5) cm. Stem leaves narrowly ligulate from an ovate base, 1.5--2.5 × 0.5--0.7 mm, both margins incurved, intermarginal bands absent, often sheathing, muticous, cucullate, obtuse-rounded, costa narrow proximally; distal laminal cells 2--3-stratose, rounded, thick-walled; medial laminal cells rounded to quadrate, straight, thick-walled; basal juxtacostal laminal cells short-rectangular, straight, thick lateral walled, pale yellow; basal marginal laminal cells short-rectangular, straight, thick lateral walled, pale yellow, hyaline. Perichaetial leaves enlarged. Seta straight to slightly sigmoid, 2--4 mm. Capsule occasionally present, exserted, brown, oblong-ovoid, exothecial cells short-rectangular, thin-walled, stomates present, annulus of 2--3 rows, rectangular, thick-walled, operculum long-rostrate, peristome perforate and split in distal half. Calyptra mitrate.

Cracks of wet acidic, siliceous rocks especially along streams or splash zones of lake shores; 225--2000 m; Greenland; Ont., Que., Nfld.; Alaska, Calif., Maine, Mich., Minn., N.Y., Vt.; Eurasia; Africa (Ethiopia).

In Canada, Grimmia unicolor is found predominantly around the Great Lakes of southern Ontario reaching eastwards into southwestern Quebec, with a disjunct site in Newfoundland. In the United States, it is also commonly found in the Great Lakes region extending eastwards into southwestern Maine. The disjunct site on the Seward Peninsula of Alaska may be a Beringial link with Asia, where this species is widely distributed across that continent and into northern Europe. Grimmia unicolor is often found in the splash zone of rocky shorelines, especially cliffs, and along rivers. Except for Grimmia olneyi, this habitat is rarely occupied by other species of the genus. Grimmia unicolor is readily distinguished from other species in the subgenera by its leaf morphology. Its leaves are oblong lanceolate to lingulate, with an obtuse, rounded apex which is muticous. Grimmia olneyi has an acute leaf apex with long awns, and its basal marginal cells are quadrate, contrasting with the rectangular cells of G. unicolor. Sporophytically, G. unicolor typically has a straight seta while that of G. olneyi is sigmoid. In contrast with G. laevigata, Grimmia unicolor has a much narrower costa proximally and lacks an awn, in marked contrast to the broad costa and robust awn of G. laevigata. The shape of the leaves of these two species are distinct, with G. unicolor having a pronounced ovate base and narrow leaves tending towards being lingulate, while G. laevigata has broadly oblong-ovate leaves without a shoulder separating the distal and proximal lamina.