expansa simply means expanded.
Rhizome: erect, occasionally branching, covered in old stipe bases.
Frond: 80 cm high by 20 cm wide, tardily deciduous, monomorphic, blade/stipe ratio: 2:1.
Stipe: grooved, scaly at least at base; scales scattered, brown with dark brown stripe, vascular bundles: 7 in a c-shaped pattern.
Blade: 3-pinnate-pinnatifid, sometimes more at the base, deltate-ovate, slightly spreading to arching, herbaceous, yellowish-green, glabrous or with some glands.
Pinnae: 12 to 14 pair, well spaced, the lowest usually curving upwards, anadromic; pinnules first lower pinnules on the lowest pinnae equal to or longer than adjacent pinnules, and twice the legth of the first upper pinnule; costae grooved above, continuous from rachis to costae, narrow scales on lower surface; segments fairly flat; margins serrate, spiny; veins free, forked.
Sori: round, characteristically small, in 1 row between midrib and margin, but sparsely placed throughout the blade, indusium: reniform, at a sinus, sporangia: black, maturity: late summer.
Habitat: cool moist woods and rocky slopes .
Distribution: Pacific rim, northern Europe, northeastern North America.
Hardy to -35°C, USDA Zone 3.
basiscopic pinnule of the basal pinna is 2-3 times longer than the acroscopic one
Dryopteris assimilis S. Walker
Dryopteris dilatata (Hoffm.) A. Gray misapplied
Nephrodium expansum C. Presl
Dryopteris dilatata (Hoffmann) A. Gray subsp. americana (Fischer) Hultén