Spinulum annotinum has erect, leafy stems arising at intervals from horizontal (also leafy) stems spreading along the surface, or beneath fallen leaves, but not beneath the soil. The leaves spread from several sides of the vertical stem, giving a round cross-section to the branches (i.e. the branches not conspicuously flattened in appearance--see the Diphasiastrum species for examples of flattened branches). Spores are produced in sporangia located only in specialized "strobili" or cone-like structures at the ends of the leafy vertical stems. S. annotinum is separated from other species in this group by having the strobilus sessile on the tip of branches (no narrow elongated stalk between the leafy stem and the strobilus--see Lycopodium clavatum for the alternative), and by the conspicuous narrowings along the vertical stems, called annular (or annual) constrictions. The annular constrictions are the result of shorter leaves being formed at the end of one year's growth and the beginning of the next year's growth.

This species was long known as Lycopodium annotinum.