Rusty woodsia is a small fern (although our largest Woodsia species), with red-brown scales and coarse hairs along the stem. It occurs in rocky slopes and outcrops from the lowlands to the alpine zone. The fronds are 3-20 cm tall, lanceolate or elliptic in outline. The fronds are 2-3 pinnate-pinnatifid (more divided than other woodsia species). The base of the stem is dark brown or purplish. Pinnae are longer than they are wide, ovate or triangular, tapering at the ends. There are light brown sori on the backs of the leaves near the margins, surrounded by hairs. The very scaly stems and pinnae twice as long as they are wide distinguish it from other woodsia species in the park. The similarly narrow, alpine Dryopteris fragrans has horseshoe-shaped indusia covering the sori.
Rusty woodsia is perennial and deciduous. The timing of spore production and establishment in Woodsia ilvensis is unknown.
Like all ferns, rusty woodsia is spore producing. Spores germinate in suitably moist habitats into haploid gametophytes, tiny heart-shaped plants. If fertilized, gametophytes sprout new fern plants.
Rusty woodsia is a circumpolar species with a boreal-montane distribution. In North America, this species occurs from Alaska to southeastern B.C. in the West. This species has an eastern disjunction from Nunavut the Great Lakes, and occurs southward to Virginia. The species also occurs in eastward to Greenland and Europe, where it is highly endangered in Britain. Rusty woodsia is widespread in Alaska, although infrequent, occurring from the northern edge of the Brooks Range across the mainland to the Alaska Peninsula and in southeast Alaska. Woodsia ilvensis occurs in occasional sites on both sides of the Alaska Range in Denali.