Leaves and stems:

Leaves are evergreen, whorled or nearly so but spirally arranged with 8 to 10 leaves in a cycle, appearing as 8 to 10 columns when viewed from the side of the stem (8 to 10-ranked), and round in cross-section (like a bottle brush). Leaves are spreading to drooping, ¼ to 1/3 inch (5 to 8 mm) long, to 1.2 mm wide, lance-linear, dark green, often minutely toothed on the tip half and usually with a pale, sharp, spine-like tip.

Stems are horizontal, running above ground or just below the surface of the duff layer, but not underground. At fairly regular intervals, erect shoots emerge that are either unbranched or with 1 or 2 forked branches near the base. Each year's new growth is marked by a distinct constriction where the annual bud grew, typically with a whorl of smaller leaves at that point. Horizontal stems are 1 to 3 feet long and the erect shoots up to 12 inches tall.

Spores:

Spores develop in spike-like or cone-like structures called strobili. Strobili are single at branch tips, ½ to 1½ inches long, and stalkless. Each tiny spore sac is attached to a scale (sporophyll) that is about 1/8 inch (3.5mm) long, generally triangular and tapering to a slender, sharply pointed tip but lacks any hair-like extension. Scales are initially light green and tightly appressed, turning yellowish as they mature and light brown when dry, then become more spreading to release the spores in late summer into fall. The strobili persist through winter.

Notes:

Stiff Clubmoss is a circumpolar species, common in Minnesota's forests north of the Metro area. It is among the species formerly all lumped into Lycopodium (L. annotinum), which many references have now split into several genera and we have followed suit. Distinguishing characteristics of these groups are: whether spores develop in cone-like strobili or in leaf (or leaf-like) axils, whether strobili are stalked or stalkless, whether horizontal stems are above or below ground, whether branching on erect shoots is tree-like or not, the number of leaves in a spiral cycle, whether leaves are scale-like or not and whether they have a hair-like tip. Stiff Clubmoss is the only Spinulum species known to be in Minnesota; it has stalkless strobili that are single at branch tips, erect shoots are unbranched or few branched, horizontal stems are above ground, leaves are not scale-like (and often minutely toothed), and number 8 to 10 in a spiral cycle. The above ground stems, branching, and toothed leaves that lack a hair-like tip can help identify it even when strobili are not present.

Compare with other clubmosses with cone-like strobili: Lycopodium species have long-stalked strobili and hair-like extensions on leaf tips, Dendrolycopodium have stalkless strobili but tree-like branching, and Diphasiastrum have stalked strobili and scale-like leaves. While several different clubmoss species may grow side by side, hybridization is not common.