Much confusion of these large ferns seems to exist.
The Fern Grower's Manual (Hoshizaki & Moran, Timber Press 2001) states that Plants circulating in US and England as B. chilense (Kaulfuss) Mettenius; B. tabulare (Thunberg) Kuhn, and B. magellanicum (Desvaux) Mettenius, all seem to be B. cordatum. This is hardly surprising since the name B. cordatum is the currently accepted name for B. chilense.
Description of Blechnum cordatum (Desvaux) Hieronymus
Blechnum cordatum has dimorphic fronds. The sterile fronds have oblong, dark green blades that are somewhat hard and leathery, with the terminal pinna resembling the lateral pinnae; the pinnae are oblanceolate and finely toothed. On the lower surface of the rachis opposite each pinna stalk is a small elongated bump, a type of aerophore. Aerophores may be incospicuous or absent on dried ferns but are easy to see on fresh fronds. They are usually whitish yellow and contrast strongly with the surrounding tissue. The fertile pinnae are linnear, with the margins strongly enrolled when old.
Description of Blechnum capense (Linnaeus) Schlechtendal
Blechnum capense has one-pinnate, dimorphic fronds. The sterile fronds are elliptic with 10-15 pairs of crowded, oblong to linnear pinnae. The basal pinnae are highly reduced and rounded. The pinnae on the fertile fronds are linnear but rounded and sterile at the base of the blade.
Sue Olsen, Encyclopedia of Garden Ferns disagrees. Blechnum cordatum is closely allied to Blechnum chilense, "although the two species are separated by hundreds of miles. The easiest observable difference is that B. cordatum pinnae do not have the lower lip of the pinnae overlapping the rachis. In addition Mickel & Smith (2004) observe«B. cordatum ... wide-spread and common in South America and with many synonyms, is closely related to B. schieanum [of Mexico] and perhaps not distinguishable. Monographic study is needed on this group of blechnums before names can be applied with confidence.» To which my colleagues and I say, "Amen."
Rickard (2000) observes that "few popular garden plants can have been so frequently misnamed as this one"
Description (Olsen) of Blechnum chilense
The rhizome is long creeping, tossing up sturdy erect fronds at random. Grooved stipes, with abundant basal scales, are the color of weak tea and are one-half of the frond length. The once-pinnate, narrowly oblong blades have 10 to 20 pairs of rubbery pinnae on a bit of a stalk. The extended apex resembles an elongated pinna. The lower edges of the pinnae overlap the rachis, and appear to be "holding hans" or at least thumbs. This characteristic is one of the few distinctions between this and the extremely closely related and visually similar Blechnum cordatum (both ready, along with their growers for taxonomic classification). The fertile fronds are tall wands of narrow, linear pinnae that are solely composed of sori running out on the horizontal from the rachis.
Range and habitat: Blechnum chilense is abundant in Chile and also grows in Argentina, Brazil, and Uraguay. In the low-lands the new growth is green, but at higher altitudes, the new fronds are a very attractive red. The new fronds unfurling are called in Chile costillas de la vaca.