Dryopteris species are notorious for hybridizing, and many hybrid combinations have been documented from Michigan. Hybrids are best recognized by the combination of sterility (abortive spores) and morphological intermediacy and are not accommodated in the key. Only two, D. ×boottii (Tuck.) Underw. (D. cristata × D. intermedia) and D. ×triploidea Wherry (D. carthusiana × D. intermedia), are frequent. Species typical of forest understories sometimes persist in more open area after logging or tree death.
1. Blades densely scaly beneath, ca. 6–25 cm long; plants of granitic rock cliffs and ± exposed talus; western Upper Peninsula only.
1. Blades not or only sparsely scaly beneath, ca. 25–120 cm long except in very depauperate individuals; plants of various habitats but not normally vertical cliffs; widespread.
2. Blades 2-pinnate with the pinnules deeply cut (more than halfway to the base) or 3-pinnate.
3. Innermost pinnules (the one next to the rachis) on lower side of lowest pinnae clearly shorter than adjacent pinnule; midribs of pinnae, indusia, and rachis with minute glands; blades fully evergreen.
3. Innermost pinnules (the one next to next to the rachis) on lower side of lowest pinnae slightly to clearly longer than adjacent pinnule; midribs of pinnae, indusia, and rachis lacking glands (except in a rare form of D. expansa); blades dying back in winter.
4. Blades lanceolate-triangular; innermost pinnules (next to the rachis) on lower side of lowest pinnae 1.5–5 cm long, 1.2–2 (–2.3) times as long as the innermost pinnule on the upper side of the pinna; widespread.
4. Blades ovate-triangular; innermost pinnules (next to the rachis) on lower side of lowest pinnae (4–) 6–12 cm long, 2–3.5 times as long as the innermost pinnule on the upper side of the pinna; western Upper Peninsula only.
2. Blades once-pinnate (with the pinnae deeply lobed) to 2-pinnate with the pinnules shallowly cut (halfway or less to the midrib).
5. Sori situated very close to the margins of the blade; base of petiole with dense, pale brown scales.
5. Sori situated well away from the margin of the blade, ± in the middle of the pinnules or lobes of the pinnae or even closer to the midvein; petioles with scales either sparse or dark brown or both.
6. Petioles very short, 1/5–2/5 the length of the blade; rachis and midveins scaly; plants of dry, almost always rocky forests.
6. Petioles 2/5–3/5 the length of the blade; scales essentially restricted to the petioles; plants of rich, moist forests and swamps.
7. Lower pinnae triangular, widest near the rachis.
8. Larger blades 6–12.5 cm wide; pinnae of fertile blades tilted out of the plane of the blade (like the slats of a Venetian blind).
8. Larger blades 11–22 (–25) cm wide; pinnae of fertile blades not tilted, the frond essentially flat.
7. Lower pinnae narrowly ovate, the widest point well out from the rachis and narrowing both to the tip and to the base.
9. Blades abruptly narrowed to the apex; sori clearly nearer to the midvein; widespread (rare northwards).
9. Blades gradually tapered to the apex; sori midway between the midvein and the margins; rare in the southwestern Lower Peninsula.