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 a branch (6-ranked). The leaves in the central column on the underside of a branch are much smaller than the others so the branch appears more or less flat in cross-section. Leaves are about ¼ inch (2.5 to 5.5 mm) long, to 1.2 mm wide, linear, medium to dark green, toothless, and lack 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 as well as those on the vertical stem between the branches are appressed to ascending. Erect shoots are up to 12 inches tall but more typically about half that. Each year's new growth is not marked by any distinct constriction.

Fruit:

Spores develop in spike-like or cone-like structures called strobili. Strobili are stalkless, single at branch tips, and ½ to 2+ inches (1.2 to 6 cm) long, 1 to 10 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 fall. The strobili persist through winter.

Notes:

Flat-branched Tree Clubmoss was unknown in Minnesota prior to 2020, but only because no one was looking for it. In 2019 the DNR began looking at Wisconsin records to get a better understanding of its habitat and distribution in counties neighboring Minnesota and the following year began field surveys in areas with promising habitat. Those surveys located 5 populations, and at least 2 more have been found since. It is likely more common than current records indicate and the hunt continues.

It is among the species formerly all lumped into Lycopodium (L. obscurum), 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. Flat-branched Tree Clubmoss also has 1 to 10+ 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 appressed to ascending, and the central leaf on the underside of a branch is distinctly smaller than the rest, giving the branch a flattish appearance in cross-section.

Of the other Dendrolycopodium species in Minnesota, Hickey's Clubmoss (Dendrolycopodium hickeyi) also has appressed leaves on the vertical stem between the branches, but all its branch leaves are equal in size and the branch is round in cross-section. Tree Clubmoss (D. dendroideum), has branch leaves all equal in size and its stem leaves are widely spreading. 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.