Treefern: Dicksonia

• Dicksonia
  This genus is most easily distinguished from Cyathea by usually having fronds of a more stiff, leathery substance, and the absence of scales on the croziers as mentioned previously. The individual fronds are more convex when viewed from the top, as opposed to the often flat fronds of many Cyathea species. There are also important distinguishing features on the reproductive structures. Some species, especially D. antarctica and its close relatives, can take on sort of a "shuttlecock" or "vortex" appearance, as if all the fronds together are like a funnel, with the crown at the center. By far the most common Dicksonia encountered in cultivation is D. antarctica, the Tasmanian Tree Fern. D. fibrosa, a species from New Zealand, is sometimes sold as and mistaken for this species in California. There are also several cold-tender tropical species of Dicksonia, making about 25 species all together.

Dicksonia antarctica -"Soft treefern" "Tasmanian treefern" Dicksonia antarctica is probably the best known of all the treeferns. It has been grown in Britain for years for its ease of cultivation, great beauty, and cold-hardiness. Since it is also in cultivation in many other parts of the temperate world, it is not nearly so uncommon as most of the other species discussed on this page, and relatively easy to get hold of. It is probably the commonest treefern in Australia as well. In the wild it can be found growing throughout the forests of eastern Australia, Tasmania, and some sub-Antarctic islands where the temperature seldom if ever rises above 65°F. The name "Tasmanian treefern" is misleading since the vast majority of them are not in Tasmania. In some parts of Australia it grows almost into the alpine zone, higher than Cyathea australis, where it is often set back by severe frosts, and usually fails to put on a trunk so that it can benifit from the protection of winter snow cover. In the western end of its range where the climate is drier, a more stout-trunked form exists which could one day show promise for cultivation in hot, dry climates.
This species is probably the largest of the Dicksonias, occasionally reaching the incredible height of around 50' (though 20' is more usual), with a trunk that can be a couple feet in diameter or more if given enough moisture, and a life span of up to 400 years. The fronds most often grow to about 8-10' in length, but under excptionally good conditions, fronds as long as 14' are not out of the question. The crown can contain a great many fronds; up to 60 or 70 have been recorded on one plant! The uncurling croziers and stipes are covered with soft, reddish-brown hairs. Growth is moderate, but trunk development is relatively slow - about 1" per year is all that can be expected in cool areas, or even a bit less. In the Southwest of Ireland it grows 2" per year, and some in coastal California have grown 3" per year. In pot cultivation the fronds will be smaller and trunk development will be slower.
Dicksonia antarctica, like most temparate treefern species, seems to vary in its hardiness according to the provenance from which it originated. Most of the plants so far introduced to cultivation have proven relatively cold-tender, and are often destroyed from temperatures of 15-18°F. Despite this setback, specimens were established outdoors many years ago at Logan Botanical Gardens in Scotland, and other sheltered gardens along the west coast of Britain, and most are still alive today. In North America, very large unprotected specimens can be found along the south coast of Oregon, as well as a few growing in British Columbia. Only recently have efforts been made to introduce hardier, more montane provenances of this species, and once these provenances find their way into cultivation it may spread the region of outdoor treefern growing even farther north. With a great deal of careful protection, the hardy provenances may be able to grow in USDA zone 6; and parts of zone 8b which have not grown treeferns unprotected before, such as London and Seattle, may eventually see established Dicksonias growing without protection in vrey sheltered microclimates. The hardiest provenances, which are most likely to come from Victoria and New South Wales, might be able to withstand temperatures down to near 16°F before they are severely damaged.
Despite its origin in rather cool mountain forests, Dicksonia antarctica is also one of the more heat-tolerant treeferns, although in very hot climates it will not look good. Provided enough water it will grow from central Texas eastward in the United States, as well as California's hot Central Valley. It is also very adaptable to growing in shady conditions (shade is more of a necessity in hot climates), and in fact it grows as an understory on the Australian forest floor where few other plants will grow. Its general resilience and tolerance for adverse conditions also enables it to serve as a good indoor subject.

Dicksonia fibrosa -"Wheki-ponga" "Golden Treefern" This is a smaller species of Dicksonia from New Zealand that looks almost like a miniature of D. antarctica. It grows in formidably cold areas of New Zealand, at elevations up to 2,700', and some of its provenances are nearly as hardy as D. antarctica, able to withstand 18-20°F. The trunk develops rather slowly and attains a mature height of only about 16'. Its fronds closely resemble those of D. antarctica, but shorter, growing only 5-7', and even more "plasticy." The hairs on the stipes do not have quite such a reddish look, but more of a golden-brown. A few have worked their way into cultiavtion in the United States and Britain, but sadly they often seem to be mistaken for D. antarctica in California.
Dicksonia fibrosa is very beautiful and easy to grow, and useful in situations where D. antarctica is too large to fit. Like D. antarctica, it will do well in a site with morning sun and afternoon shade, but tolerates quite a bit of sun in cool climates. In Britain it is often planted under trees, since it will fit nicely there, and the trees offer some shelter from the cold. Root development of imported plants is such that the young trunks often take on a conical shape.

Dicksonia lanata -"Prostrate Treefern" This small species from New Zealand is not much like the other Dicksonias in its general appearance, but it is nevertheless an attractive fern. Although very cold-hardy (at least 20°F), this species usually forms a prostrate, creeping trunk; and is not really a tree fern. Sometimes, however, it will form a small trunk up to 6' tall. It often grows in clumps like Lophosoria quadripinnata. It has not really found its way into cultivation in the Northern Hemisphere and remains a collector's item.

Dicksonia sellowiana-This species has such an enormous natural range, all the way from Mexico to Argentina, that it varies considerably in its appearance, as well as its hardiness which to my knowledge remains untested. It does sound very promising, as anything that grows in Argentina must tolerate some frost; and also that it would be adaptable enough to span over such a huge area. Most plants are about the size as D. antarctica, or a little smaller; and many have rather yellowish stipe-hairs as opposed to the red-brown of D. antarctica. However the variety D. sellowiana var. gigantea (which is often just referred to as D. gigantea), which is found in Mexico, can form quite a massive plant with suckers arising from the base and lower trunk.
An English nursery has rated this species to 20°F, but I do not think this figure can be relied upon for all plants of this species; the tropical ones being less hardy and the most southerly ones perhaps more so.

Dicksonia squarrosa -"Wheki" This species is quite distinct from D. antarctica and D. fibrosa, and it is probably my favorite of the Dicksonias. It can be found over quite a large part of the South Island of New Zealand, except in the coldest areas, often not far from Cyathea smithii. It grows in full sun above the Franz Josef Glacier and Fiordland, one of the wettest places on Earth. In sunnier climates they prefer a bit of shade, and do not tolerate heat well. Nor is it quite as cold hardy as D. antarctica or D. fibrosa--in my observation, temperatures near 24°F will do it some damage. But it is still hardy enough to do well outdoors in most of Britain and the Pacific Northwest with the standard protection procedures. It is in cultivation, though rare, in Britain and the western United States.
What distinguishes it from the aforementioned species is its decidedly slender trunk, the stipes covered with hairs that are a soft brownish-grey, and a beautiful crown of gracefully arching fronds up to 7' long that have a whitish undersurface. The trunk does not tend to form aerial roots, and the old stipes can always be seen on the trunk in an attractive pattern. Interestingly, it also sends up offshoots near the base of the trunk, or sometimes as far away as 4 feet from the parent. An added bonus is its relatively fast growth as Dicksonias go, 3" per year being typical of a healthy specimen with an ultimate height of about 20'. A unique feature of this species is its ability to resprout from dormant adventitious buds lower down in the trunk or ground if the top is destroyed by cold. This species is happiest if it is never allowed to dry out--in the wild, the dead fronds cling to the trunk to prevent it from losing moisture.

Dicksonia youngiae-This species comes from cool highland rainforests in Queensland and Australia, and is somewhat resemblant of D. squarrosa, perhaps even a bit cold-hardier than D. squarrosa according to one source. It, like D. squarrosa, sends up suckers to form multiple trunks as high as 12'. Unfortunately the trunks can sometimes be weak and have an inclination to deteriorate. The fronds are a glossy, deep, bright green; and coarse, brittle reddish hairs densely cover the stipes and croziers.
Dicksonia youngiae is not particularly heat tolerant and needs shelter from wind. Another potential problem is that it may tend to produce fronds in the winter which are left exposed to damage before they mature; however, its growth cycle may change entirely if brought into a cool enough climate. Being a heat-intolerant species in an overall rather hot part of the world, its presence and isolation so far north in Queensland is somewhat of a mystery, and clearly it would not have survived if it had not been able to retreat into the cool highlands and find its niche there.

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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