Plants small, to ca. 1 cm, forming dull green tufts. Stems 0.4-1.2 cm, simple. Leaves to ca. 3.5 mm, erect-patent, weakly secund, lanceolate to linear-lanceolate, gradually acuminate to a channelled subula, acute, lamina 1-stratose except on margins; margins bistratose, entire or serrulate near the apex, irregularly thickened and recurved at least in distal half; costa stout, percurrent to excurrent, in section with abaxial stereid band; proximal lamina cells elongate-rectangular, distal lamina cells subquadrate to short-rectangular or somewhat irregular in outline, ± incrassate. Specialized asexual reproduction by rounded to pyriform or irregularly-shaped, yellow to orange rhizoidal tubers, to 150 µm. Sexual condition dioicous; perichaetial leaves similar to stem leaves but with a somewhat longer subula narrowing more abruptly from the base. Seta becoming reddish with age, to 1.5 cm, erect. Capsule erect, dark brown to reddish brown, ovoid to cylindrical, symmetric, 1-2 mm, smooth to ± furrowed when dry; operculum obliquely conic-rostrate, blunt, 0.3-0.7 mm; peristome split nearly to the base into 2 segments, brown, densely and finely obliquely-ridged and lightly papillose, 200-250 µm. Spores 11-20 µm, appearing ± smooth.
Capsules mature spring-summer (Apr-Jun). Bare, disturbed calcium-free clay, sandy or gravely soil banks, disturbed habitats, especially along roads and trails, and sometimes in cliff crevices; low to moderate elevations; B.C., N.B., Nfld and Labr., N.S., Ont., P.E.I.; Ala., Ark., Fla., Ga., Ill., Ind., Iowa, Ky., La., Maine, Mass., Mich., Minn., Mo., Nebr., N.Y., Tenn., Tex., Va., Wis.; Europe; Asia; Africa; Atlantic Islands (Iceland).
When sterile, Ditrichum pusillum is morphologically very similar to D. ambiguum and D. tortuloides. When sporophytes are present, the dense, fine, obliquely striate peristome teeth with weak papillae immediately distinguish D. pusillum from those two species, which have spiculose peristome teeth. The spores of D. pusillum (10-19 µm) are also larger compared to those of D. ambiguum (6-11 µm) and D. tortuloides (9-13 µm). A discussion of the relationships of D. pusillum with both D. ambiguum and D. tortuloides was given by R. R. Ireland and H. Robinson (2001).