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Cyathea australis, also known as the Rough Tree Fern, is a species of tree fern native to southeastern Queensland, New South Wales and southern Victoria in Australia, as well as Tasmania and Norfolk Island. It grows in moist shady forest, both coastal and montane, at an altitude of up to 1280 m, often in the company of Dicksonia antarctica. The massive erect trunk is usually up to 12 m tall, although specimens reaching 20 m have been reported from Queensland, Australia. Fronds are bi- or tripinnate and may reach 4 m in length, occasionally even 6 m. These form a distinctive crown that is dark green above and lighter green below. The Tree Fern has quite adventitious roots, tubercles and hair-like follicles on its ‘trunk’.
Plants growing in southern Australia, often lose their fronds by the end of winter, as is the case with Cyathea dregei in South Africa. Characteristically of this species, stipe bases are often retained around the trunk long after withering. They are covered with scales and conical, blunt spines towards the base. The scales range in colour from shiny brown to bicoloured (pale and brown) and are often distinctly twisted. Sori are circular and occur on either side of the fertile pinnule midvein. True indusia are absent, although reduced scales may encircle the sori. C. australis is a highly variable taxon, with several subspecies and varieties.
C. australis is a highly variable species. Plants from Norfolk Island, belonging to the subspecies norfolkensis, are larger, more robust and differ primarily in scale characteristics. They are rare in cultivation. Further study is needed to determine whether this taxon represents a separate species or not. C. australis appears to be related to Cyathea woollsiana.
C. australis was described by Robert Brown in 1810 from a specimen collected on King Island in Bass Strait, off the coast of Tasmania. It is the type for the genus Alsophila, which has now been reduced to a section. The specific epithet australis means "southern" and refers to this southerly location.
C. australis is a relatively hardy species and a popular landscape and container plant. Provided moisture levels remain high, it will tolerate frost and full sun, or shade in warmer regions. Although well known in its native country, this species is not common in cultivation outside of Australia.
In the horticultural trade, most plants labeled as C. australis are in fact Cyathea cooperi. Much confusion has existed between the two, especially in the United States, although they are quite distinct from one another. C. australis is relatively stout trunked and has a large number of closely spaced fronds emerging at one time,with a slower increase in trunk height. C. cooperi in contrast, grows more quickly with fewer fronds emerging each year and a much narrower trunk with the frond bases aligned vertically for some distance ("hugging" the trunk as it were), before arching outwards.