This species is similar to D. complanatum and D. digitatum, but the branches are somewhat squarish (1-2.2 mm wide) rather than broadly flattened, and the leaves of the underside of the branches are nearly the same size as the lateral and upper leaves (underside leaves are conspicuously smaller in the other two species). D. tristachyum is often conspicuously blue-green in color, especially the youngest growth, and the branches of the other two species are green. The branches have conspicuous constrictions at the juncture between consecutive years growth (as does D. complanatum, which has broader, more conspicuously flattened branches and lacks the blue green color).

Diphasiastrum tristachyum grows from extreme eastern Canada west to Manitoba, south to Minnesota, Ohio and Maryland and in the mountains south to northern Georgia. In Wisconsin it is found mostly in the northern two thirds of the state, most often on dry, acid, unproductive, sandy soils in forests with light canopies and in forest openings, including disturbed sites.