Cyathea australis - Australian Tree Fern

Cyathea australis (or Alsophila australis) is less common than C. cooperi but is worth discovering for its ornamental qualities. It forms an almost black stipe (false trunk), which is highly decorative, supporting a crown of large, light green, finely cut fronds with a striking appearance. This tree fern instantly brings a tropical feel wherever it is planted. One of its advantages is its relative hardiness, down to approximately -10°C (14°F), although its foliage suffers from temperatures below -6°C (21.2°F). Preferring moist and well-drained acidic soils, devoid of limestone, as well as a fairly high air humidity, it will thrive in mild and humid oceanic climates. With relatively slow growth, it is a plant reserved for enthusiasts.

Cyathea australis belongs to the Cyatheaceae family, which includes the largest tree ferns in the world (up to 30m (98ft) tall!) and whose members populate the humid tropical and subtropical regions of the southern hemisphere. This genus comprises around 470 species, and this one is native to the humid mountains of eastern Australia (Queensland, Victoria, New South Wales), where it grows at an altitude of approximately 1200m (3937ft), which explains its relative hardiness. It has a rough "trunk" (in reality, it is a stipe, as in Palms), hence its nickname "rough tree fern". This stipe is wider than that of the better known Cyathea cooperi (or Sphaeropteris cooperi), and it is black in colour. Moreover, it is distinguished from other species by the presence of small spines at the base of the leaves, which are called fronds in ferns. In its natural habitat, it can reach a height of 12m (40ft) with fronds measuring 3 to 4m (10 to 13ft) in length.

Away from its natural habitat, Cyathea australis generally does not exceed 6m (20ft) in height with a diameter of 4m (13ft), which is already sufficient to give a fiercely exotic appearance to a bed! With slow to moderate growth, the stipe can grow by 10cm (4in) per year under favourable conditions. This false trunk is composed of intermingled rhizomes covered with dry adventitious roots and the brown, scale-like, papery remnants of the petioles of old leaves. The base of the leaves bears small spines, which give it its rough appearance. The young, coiled leaves, called croziers, unfold into large, finely cut fronds measuring 2m (7ft) in length, of a light green colour. They are produced at the upper part of the stipe, one after the other, uninterrupted if the plant is kept above 10°C (50°F), thus giving it a deceiving palm-like appearance. The fronds are evergreen and each has an average lifespan of 2 years when not exposed to severe frost. Indeed, from -6°C (21.2°F) onwards, they will be damaged by the cold and may die, but new ones will form in the following spring, as the plant itself can withstand temperatures around -10°C (14°F), or even -12°C (10.4°F).

Like other tree ferns, Cyathea australis requires humidity to thrive. Prehistoric plants, emerging from the water before others, they remain dependent on it for reproduction. 

Tree ferns are not known for their great hardiness, except for the quite common Dicksonia antarctica, and this Cyathea expands the range. This tree fern only thrives in the shaded and humid atmosphere of an understory. It seems to tolerate wind slightly better than its sisters, but in cooler climates, it will need to be planted in a container so that it can be brought indoors during winter. For gardens that can accommodate it in the ground, it will create a strong visual element for a tropical-inspired scene. It can be paired with giant foliage plants, such as Gunnera manicata, the Giant Brazilian Rhubarb, along the water's edge, or the equally spectacular Tetrapanax papyrifera 'Rex' (be careful, it produces many suckers, so it needs to be contained with a root barrier like bamboo). On a smaller scale, Farfugium, available in several species, will also be a good companion with its extremely decorative leaves.