Tree Ferns: An Introduction

Following an appeal in the last issue of 'Chamaerops', Peter Richardson joined the E.P.S. and at the same time submitted this comprehensive introduction.
Peter Richardson, Advanced Technologies Ltd, Science Park, Cambridge, U.K.
Chamaerops No. 5, published online 23-10-2002

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Left: Cyathea medullaris. Mature plants in logged bush regrowth
Right: Dicksonia fibrosa with skirt of dead leaves. C. medularis behind

The treeferns, with their giant green shuttlecock crowns on rough, fibrous trunks are attention grabbing plants both in the wild and in cultivation, evocative both of tropical lushness and a primeval, dinosaur ridden past. Indeed, like the cycads, treeferns are antiques among the world's present-day collection of megaphytes; a fossil Dicksonia has been found in Jurassic rocks in Yorkshire, from well before the appearance of any flowering plants.

The tree ferns are defined as the members of two fern families, Cyatheaceae and Dicksoniaceae. These are not the only ferns to have an upright stem, though few others have a stem to which the term trunk can be justifiably applied. Equally, both families have members with prostrate stems. There are only three genera commonly cultivated, although one of these, Cyathea, is huge, with 800 species. Dicksonia has twenty-five species and its relative, Cibotium, a very modest eight. Not surprisingly in view of their antiquity, both families are distributed worldwide (Europe excepted). Their favoured haunts are wherever seasonal variations in temperature are minimal and there is year-round high humidity, particularly rainy mountainous areas of the subtropics and tropics, and oceanic islands. Two species of Cyathea have the northernmost and southern- most natural distributions of any tree ferns, namely C. japonica on Kyushu, Japan, and C. smithii on the Auckland Islands southeast of New Zealand. A number of species including C. gleichenioides and C. muellerii grow with year round nightly frosts at 3700m in the highlands of Papua New Guinea.

Although it is their stems, which set them apart from other ferns, anatomically the trunks are not very different from stems of many other ferns. They do not have any wood and cannot even muster the kind of limited secondary thickening that Cordylines and Cocos nucifera show. Instead the soft centre of the trunk is propped up by the hard casing formed by the dead bases of old leaves, and very importantly, by adventitious roots. These are produced prolifically from the crown and grow all the way down the trunk amongst the leaf bases to the soil. Big old plants can have trunks 75cm through at breast height, though the actual stem will be only a slender core in a vast thatch of downward bound roots, living and dead. In dry surroundings the growth of the adventitious roots is inhibited and trunks are more slender and crowns smaller. Reliance on adventitious roots makes treeferns surprisingly resilient to damage so long as they are in a humid environment. When Dicksonia plants lean or fall against other trees in the rainforests on the west coasts of Tasmania and New Zealand, the roots abandon the trunk and grow straight down to the soil, making a black stringy curtain. Cibotiums in Hawaii respond to being undermined and felled by feral pigs, by rooting where they fall and the apex turns and starts growing vertically again.

Damage to the apex in Dicksonia and Cibotium often results in branched plants. The well-known species of Cyathea do not branch nearly so readily, but it would be hazardous to generalise to the hundreds of others.

Treefern trunks, with their fibrous, ever-damp thatch of roots are nature's own moss poles and are much favoured by epiphytes, including other ferns, and orchids. In Hawaii one of the major forest-forming trees, Metrosideros polymorpha, sometimes starts life as an epiphyte in the apex of a Cibotium, later extending prop roots down to the soil, an interesting reversal of the usual fern-on-tree relationship.

In Britain, only a few Australasian species of Cyathea and Dicksonia are available commercially. Among plants whose tolerance of European climates is marginal, these treeferns fall into the category of those, which will grow freely at ambient outdoor temperatures most of the year, but are susceptible to hard frost. All of these have firm evergreen fronds, which are tripinnate and vary in shape from narrowly ovate to triangular.

Dicksonias are typically forest-dwelling understorey ferns and their cultivation requirements reflect their adaptation to a consistently shaded and moist habitat with damp acid soil. The various species arc all but indistinguishable to the untrained eye when small - and even when mature are not easy to identify. The most commonly seen is Dicksonia antarctica, which is probably the hardiest, but slow. It grows wild in the Victorian mountains and throughout Tasmania. The leaves can be between one and three metres long and have a raspy, almost prickly texture. They are narrowly ovate and held nearly straight with only the tips drooping. Mature plants produce their leaves in vast annual flushes of up to forty leaves at once. The stem is stout and gains height slowly. Its true stem tissues only live a few years and after that it is completely dependent on its column of adventitious roots. Therefore it is particularly indifferent to what you do to the "trunk" so long as it remains damp. Nurseries in Australia sell it sawn off at ground level and purchasers simply sink the trunk in the ground deep enough to hold it upright, while the roots re-establish.

Two of the New Zealand Dicksonias are also imported and sold at a few nurseries. D. fibrosa from New Zealand is similar to D. antarctica, but the trunk puts on height faster and it is far less hardy. Its skirt of orangey-brown dead leaves help to distinguish it from other species. D. squarrosa has more slender trunks and in the wild it suckers to form groves. The leaves of some plants have a vaguely metallic sheen, which is very attractive, and the stipes are densely felted with long dark hairs. The fronds are smaller than those of D. antarctica, and arch out.

Cyathea contains species which prefer the damp shelter of a closed forest canopy (e.g. C. smithii) and others which arc most at home in disturbed sites such as logged out bush, and prefer their crowns to be out in the full sun while the base of the stem remains shaded by surrounding vegetation. The latter are the easier to cultivate in those gardens not closely resembling a mature rainforest, and for ferns, they are gross feeders.

Cyatheas differ from Dicksonias in having chaffy scales on their frond stalks as well as hairs. They generally have larger and broader leaves as well. The scales of C. cooperi are very long and dense and persist unlike those of several others. Its shaggy appearance and fast robust growth make it popular as a garden plant in its native Australia. Its sharply pointed pinnae are another way of distinguishing it from the more rounded leaf outlines of two favourites from New Zealand.

C. dealbata, used as the emblem on the All-Black's rugby shirts, is famous for the silvery-white undersides of the leaves, a feature it shares with several other Cyatheas and also with Cibotium glaucum of Hawaii. It is a trait associated with ecological tolerance of rather drier conditions than most treeferns. The stipes are slender, and silvery when young. Like D. antarctica the trunk is slow to attain height, and the great majority of specimens in the wild are 4m or less high, though they are capable of reaching lOin eventually. C. medullaris is New Zealand's monster fern, massive in all its parts, but one of the most graceful. Rich soil and a mild damp climate in the North Island enable adolescent plants to throw out leaves four to five metres long, though as the trunk gets higher (up to 20m) the leaves get smaller. The stipes are thick, and purple-black with a glaucous bloom. When the leaves die and fall they leave a neat hexagonal pattern of leaf scars on the trunk.

The fat, chunky new croziers have a covering of dark chaffy scales but these all fall off by the end of the summer when the leaves have matured. The pinnae droop gracefully and the whole thing looks very tropical. Unfortunately it is the least hardy of those from New Zealand and mature plants will sustain severe damage at three or four degrees of frost. C. dealbata seems to cope with that level of cold but plants were killed outright by the cold spell in Britain in February 1991. These two species are happy to have, their leaves in full sun in Britain, but further south in Europe shade becomes necessary. The netting roof of the Estufa Fria in Lisbon enables treeferns to be grown there without sun scorch. Woodland shade provides even better conditions in Portugal. Away from the Atlantic seaboard, Europe's range of climates is not good for treeferns; along the Mediterranean coast the summer is too dry and going northwards the winters quickly get too cold. Even so, Cyathea is almost certainly underexploited in Europe as a garden subject. Populations of C. australis, C. smithii and C. dealbata exist in highland areas of Victoria in Australia and the South Island of New Zealand respectively which experience winters comparable to those along Britain's southwest coast. Such provenances should be safe planted outside there except in the occasional cold spells associated with air masses from continental northern Europe, for which there is no equivalent in Australasia. Some of Australia's species such as C. woollsiana can tolerate summer dryness in a Mediterranean climate.

Nearly all Cibotiums are tropical (variously native to Central America, Hawaii and Southeast Asia) and young plants quickly develop very large, broadly triangular leaves, so they are not suited to cultivation in temperate areas, though they are extremely handsome. A Mexican species, C. shiedei, is now an established resident of suburban California. One specimen is kept in the temperate house at Oxford botanic garden.

The great majority of treeferns for sale (in Britain at least) have been imported from wholesale growers in New Zealand. They are often in less than pristine condition by the time they are put on sale and many people have had difficulty in keeping them alive for long after purchase. Therefore if one is considering buying imported plants it is worth looking for plants that have been in the nursery for some time and have settled down and put out some leaves since arriving. They are never cheap to buy. Like most palms, treeferns are trunkless for their first few years while the apex increases to its adult size.

Possibilities for vegetative propagation of treeferns are limited because of their basically monopodial habit. In New Zealand, where there is an abundance of mature Dicksonias, sections of trunk are cut up and planted, and a proportion of them produces a side sprout and establish. Cyatheas cannot be treated in this way.

Treeferns are no different to raise from spores than many herbaceous ferns widely cultivated, so people experienced with other horticultural ferns should have no trouble. The problem is usually getting good spores in the first place. The packets of spores sold by some British seeds men contain mostly empty spore cases and few spores. This seems inexcusable in view of the incredible number you can harvest yourself from just a portion of one fertile frond. For those who will grow treeferns no matter what, better sources are the seeds men of Australia. There is the added fuss and expense of airmailing and dollar bank drafts though. Visits Down Under may provide an opportunity for some to gather their own. Dicksonia antarctica reaches reproductive maturity in Cornish gardens and inside at Kew, so that is a tack worthwhile pursuing. For those new to raising ferns from spores a full and excellent how-to-do-it description can be found in Jones (1987) -see bibliography.

Bibliography:

Jones, David L. Encyclopaedia of Ferns, British Museum (Natural History) 1987
Wrigley, J.W. & Fagg, M. Australian Native Ferns. Collins, Sydney 1979
Sporne, K.R The morphology of Pteridophytes. Hutchinson, London (4th edition) 1975
Rock, J.F. Indigenous trees of the Hawaiian Islands. Tuttle 1913 reprinted 1974 (Out of print again; the Botany School library in Cambridge has one).
Brownsey, P.J. & Smith Dodsworth, J.C. New Zealand Ferns and allied plants. David Bateman 1989

Suppliers:

Burncoose and South Down Nurseries, Gwennap, near Falmouth, probably have the best selection of treeferns in the UK. Be wary of their labelling.

M.L. Farrah pty Ltd. P.O. Box 1046 Bomaderry NSW 2541 Australia produce a substantial export seed catalogue listing several native treeferns as well as palm seed and hundreds of Myrtaceace and Proteaceae and other Australian natives.

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