Leaves and stems:

Leaves are evergreen, alternate but spirally arranged with 6 leaves in a cycle, appearing as 6 columns when viewed from the side of the stem (6-ranked), and round in cross-section (like a bottle brush). Leaves are about ¼ inch (2.5 to 5.5 mm) long, to 1.2 mm wide, linear, medium green, toothless, and lacking a hair-like or spine-like tip. Horizontal stems run underground. At fairly regular intervals, erect shoots emerge, each with 2 to several fanning, horizontal branches and appearing tree-like. Leaves along the lateral branches are spreading to ascending, those on the vertical stem between the branches are prickly, more spreading and more widely spaced. Erect shoots are up to 12 inches tall but more typically about half that. Each year's new growth is not marked by a distinct constriction.

Spores:

Spores develop in spike-like or cone-like structures called strobili. Strobili are stalkless, single at branch tips, and ½ to 2+ inches long, usually 1 to 7 clustered at the top of the plant and occasionally a few in the lateral branches. Each tiny spore sac is attached to a scale (sporophyll) that is about 1/8 inch (3.5mm) long, triangular to tear-drop shaped and tapering to a slender, sharply pointed tip. 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:

Tree Clubmoss is a circumpolar species and common in forests in the northern half of Minnesota. It is among the species formerly all lumped into Lycopodium (L. dendroidium), which many references have now split into several genera and we have followed suit. Distinguishing characteristics of the new 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. The Dendrolycopodium species all have stalkless, cone-shaped strobili, underground stems, 6-ranked leaves that are not scale-like and lack a hair-like tip, and have a tree-like form like little spruce trees, so are often commonly referred to as ground-pines. Tree Clubmoss also has 1 to 7 stalkless strobili at the top of the plant, sometimes also a few in the lateral branches, leaves along the erect shoots between the branches are widely spreading, and branches are round in cross-section like a bottle brush.

Of the other Dendrolycopodium species in Minnesota, Hickey's Clubmoss (Dendrolycopodium hickeyi) has distinctly appressed leaves on the vertical stem between the branches. A third species, Flat-branched Tree Clubmoss (D. obscurum), has appressed to ascending stem leaves and the central leaf on the branch underside is distinctly shorter than the rest, the branch having a more flattened shape in cross-section. These 3 were once considered variations of the same species, Lycopodium obscurum, and all commonly called Princess Pine in various references. Of note is that much attention is given to the exact leaf spacing arrangement of these 3 species, but the differences between D. dendroidium and D. hickeyi are quite subtle and not always helpful in the field. Compare with other clubmosses with cone-like strobili: Spinulum species have stalkless strobili but lack tree-like branching, Lycopodium have stalked strobili and leaves with hair-like tips, and Diphasiastrum have stalked strobili and scale-like leaves. While several different clubmoss species may grow side by side, hybridization is not common.