Hybrids between species of Diphasiastrum are apparently fertile. They are intermediate in morphology, but can occasionally occur without one or even both parents. The commonest hybrid is D. × habereri (House) Holub, between the widespread D. digitatum and D. tristachyum. This is best distinguished by having a relatively shallowly buried horizontal stem but with lower leaves of the branches much less reduced than in D. digitatum. The hybrids D. complanatum × D. tristachyum (D. × zeilleri (Rouy) Holub) and D. complanatum × D. digitatum (D. × verecundum A. V. Gilman) are much rarer.

1. Strobili sessile; collected long ago in Keewenaw Co.

D. alpinum

1. Strobili (or groups of strobili) short to long peduncled; widespread.

2. Strobili 1 (-2) on short to elongated, rarely forked peduncles; base of strobilus with a few sporophylls scattered along peduncle; stomata on both leaf surfaces; known only from Chippewa Co.

D. × sabinifolium

2. Strobili 2-4 on forked peduncles; base of strobilus compact and abruptly distinct from peduncle; stomata on lower leaf surfaces only.

3. Horizontal stems almost always buried ca. 4-10 cm in the soil; ultimate (outermost) branches squarish in cross section, 1-2 mm wide; plants usually blue-green (except in deep shade); lower leaves the same size as the upper.

D. tristachyum

3. Horizontal stems superficial, or slightly buried (< 4 cm) in the litter; ultimate (outermost) branches flat in cross section; 2-4 mm wide; plants not blue-green; lower leaves reduced to a leaf base and a short free appendage.

4. Ultimate branches symmetrically in one plane forming a layered appearance; annual constrictions mostly absent; peduncles of strobili 0.6-1 mm in diameter and staying green until after spore discharge in autumn (Sept. -early Oct.); widespread.

D. digitatum

4. Ultimate branches not evenly layered, giving an irregular appearance; annual constrictions uniformly present; peduncles of strobili 0.4-0.7 mm in diameter and losing green by spore discharge in late summer (late July-early Aug.); Upper Peninsula and northernmost Lower Peninsula south (very rarely) to ca. 44° 30’ N.

D. complanatum