Other species of Dicksonia
In addition to the species listed individually, the following species may also one day be successful in temperate climates. Some of these are extremely rare, and any guess at their cold-hardiness would be pure speculation.
Dicksonia arborea (St. Helena)--This species grows up to 9,000' above sea level on the tropical island of St. Helena in the Atlantic, in completely open sites with no shade or windbreak on the mountains of Diana's Peak National Park. It is tolerant of cool temperatures and wind, and if it is anything like most other tropical highland tree ferns, it probably even tolerates a few degrees of frost. It is thought to be the last remnant of a treefern population that once existed on the African mainland. This population has been reduced even further by competition from introduced plants such as flax, and because of the absence of any tree canopy, which has all been destroyed by European settlers. It is not believed to be in cultivation, but it should certainly be introduced since wild populations are on their way to extinction. It forms a trunk, and, while the fronds are rather short and "scruffy-looking" on wild plants, this is likely an adaptation to its exposed habitat situation, and it might take on a much more lush appearance in cultviation. The trunk reaches about 12' in height, and the fronds, compared to other trunking species of Dicksonia, are relatively short, on wild plants at least.
Dicksonia baudouini (New Caledonia)--A rare species whose frost hardiness is not known.
Dicksonia berteriana (Juan Fernandez Islands, off the coast of Chile)--A very rare species, hardly in cultivation at all, about which little is known. It is said to thrive in a sheltered, humid site (as most treeferns do). Despite its proximity to South America, it is more closely allied to the Western Pacific species than to the geographically closer D. sellowiana. This species is native to Alejandro Selkirk Island and is a smaller fern than D. externa. According to one reference, Dicksonia berteriana is less inclined to grow very much upright trunk.
Dicksonia externa (Juan Ferandez Islands)--This stout-trunked, large species grows in extensive thickets on slopes with full exposure to the sun and wind. In this situation the fronds are rather short, but it could make a magnificent fern in a sheltered site in cultivation. Plants in habitat may grow as tall as 25' with a 1' diameter trunk. This species is from Robinson Crusoe Island and is even less well known than D. berteriana.
Dicksonia herbertii (northern Queensland)--A species from cool monsoonal highland rainforests which is only beginning to make its way into cultivation. It is generally similar to D. youngiae, having greyish-green fronds with reddish brown stipe-hairs, but it is always single-trunked. It is likely to thrive on a great deal of rain and constant humidity.
Dicksonia karsteniana (Central America)--Another species from Central American cloudforests. Possibly fairly hardy.
Dicksonia thyrsopteroides (New Caledonia)--This species has a much more slender trunk than most species of Dicksonia, with only a few fronds in the crown at once. It despises sun and heat. Its frost hardiness is not known.
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Lophosoria quadripinnata (next)
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