Plants in hoary tufts, olive green to dark blue green. Stems 1--3 cm, central strand strong. Stem leaves ovate-lanceolate, 1.5--3 × 0.6--0.7 mm, keeled, one margin recurved proximally, sheathing, awn 0.5--1.5 mm, costal transverse section prominent, reniform to semiterate; distal laminal cells 2-stratose, not bulging, marginal cells 2-stratose, not bulging; medial laminal cells short-rectangular, sinuose, thick-walled; basal juxtacostal laminal cells long-rectangular to linear, sinuose, thick-walled; basal marginal laminal cells short-rectangular, straight, thick transverse and thin lateral walls, hyaline. Perichaetial leaves not enlarged. Sexual condition dioicous. Seta straight, 0.5--0.7 mm. Capsule occasionally present, immersed, yellow, oblong-ovoid, exothecial cells quadrate, thin-walled, stomates present in 2--3 rows, annulus of 2 rows, rectangular, thick-walled, operculum long-rostrate, peristome present, perforate in middle, split in distal half.

Exposed, dry, basalt and acidic granite, rarely on sandstone; 1800--2700 m; Ariz., Calif., Colo., Nev., N.Mex., Okla., Tex.; Mexico.

Grimmia arizonae is endemic to the American southwest and northern Mexico. It has a highly restricted distribution in North America, being found in mountainous areas in southeastern Arizona, through to the Rocky Mountain Front Ranges of western Texas, New Mexico and Colorado. It has disjunct sites in western Oklahoma and California.

As discussed above, Grimmia arizonae is part of a group that includes G. longirostris and G. pilifera. Its sheathing leaf bases, dioicous sexuality and immersed capsules will separate it from G. longirostris. The separation of Grimmia arizonae from Grimmia pilifera, however, has often proven problematic. Grimmia pilifera has been described as having strongly keeled leaves; margins 2--3-stratose, thicker than the medial lamina, and costa prominent (e.g. H. A. Crum 1994); and according to H. C. Greven (1999), usually with short awned, ovate-lanceolate leaves. Grimmia arizonae has less keeled leaves; margins 2-stratose, not thicker than lamina; and costa immersed. Greven notes its usually long awns and broadly lanceolate leaves. J. Muñoz (1999a), however, synonymized these species, attributing any differences as due to geographical variations and sexual development. He reports that in eastern North America, and in shade, G. pilifera has long, acuminate apices, distinct ovate bases, and costa prominent. In the west, and in sun, leaves are lanceolate without distinct bases; the costa is not distinct. To Muñoz, the characteristic absence of a central strand, is “too variable to be reliable;” fertile stems have a distinct strand, sterile stems have none. In contrast, in the present study, R. I. Hastings found the stem central strand reliable to separate these species. There is no correlation between sexual maturity and strand development; specimens of G. pilifera lack a central strand. Further, the stem epidermis is consistently twice as thick as it is in G. arizonae. R. I. Hastings found Western specimens, in full sun, that lack a central strand typical of Eastern specimens named G. pilifera. Many sterile specimens from Arizona have a central strand, but sterile or fertile, specimens in eastern North America do not have one. Based on these observations, G. arizonae and G. pilifera are not synonymous. Grimmia arizonae exists as an endemic to the Americas. It differs from G. pilifera by having a central strand, a thin epidermis, 2-stratose distal lamina with 2-stratose not-thickened margins, and usually a reniform costa in transverse section.