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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