Sphagnum inexpectatum is frequently collected with
S. tenellum,
S. pacificum,
S. andersonianum, and
S. rubellum in weakly minerotrophic blanket mires. It is similar in size to
S. subsecundum, with which its range completely overlaps. The latter species has many of the branch leaves subsecund while those of
S. inexspectatum are straight. The stem leaves of
S. inexspectatum are also conspicuously larger than those of
S. subsecundum.
Microscopically Sphagnum inexspectatum has a stem cortex that has enlarged thin-walled cells that form 1-2 layers, whereas S. subsecundum has only one. The stem leaves of S. inexspectatum also have numerous commissural pores in the hyaline cells in the distal half of the concave surface, whereas S. subsecundum has only a few if any in this region and these are more free than commissural.