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Tree Ferns in the Pacific Northwest

This article first appeared in HPI Editions #44 and 45 in late 2000 and early 2001. I have given it a major revision in February 2007, as much new information about tree ferns has come forth since that time; and, admittedly, my writing style in 2000 needed some refinement!

As a young child who knew little about plants, I once had the good fortune of traveling to New Zealand. Although I can recall little from that trip today, one lasting memory I retained was one of gazing in awe at ferns that grew tall trunks and held their fronds high above my head. In later years, I discovered that these huge ferns are called 'tree ferns'-and, still fascinated with them, I began my search to find tree ferns that I might grow in my own garden. I have now grown tree ferns in Olympia, Washington for several years, and researched many tree fern species extensively. While I have not managed to collect a great number of species due to their scarcity, I feel that my knowledge of tree ferns has become sufficient to assess their potential and limitations as garden plants in our region. Because tree ferns remain scarce in the Pacific Northwest, much of the information in this article comes from correspondents in Australia and Britain, rather than local tree fern growers. Whether or not this information rings true in our environment can only be ascertained by experience over time.

The obvious, significant difference between a tree fern and an ordinary garden fern is the presence of a trunk that reaches high into the sky, holding the fronds (the name for fern leaves) high at the top. The tree fern trunk has a very different structure than that of our usual woody plants and trees. It consists of a thick woody substance called a caudex, which is formed out of a tangle of roots and other tissues, rather than annual rings of wood. Thus, the trunk does not increase in girth as the tree fern ages, except as more aerial roots coat its surface (this is especially apt to occur in a moist environment). The fronds develop continuously and expand from the 'crown' at the top of the trunk, while the trunk gradually increases in height. The crown, then, is the most vulnerable part of the plant: if it is destroyed, the fern will not be able to grow any more fronds (except in those species that produce offsets). Because tree ferns leave their root-covered trunks exposed to the air, the trunk is very vulnerable to drying out, though drought tolerance varies from species to species.

Dicksonia squarrosa at Fancy Fronds in Gold Bar, WA.
Tree ferns should be planted in a rich, loamy soil in the garden. They will appreciate occasional feeding with a balanced fertilizer, but in our climate this should probably only be done in the spring and early summer, so as not to encourage tender new growth late in the season that may be damaged my frost. A thick layer of mulch around the base of the plant also helps to provide a suitable environment for the roots. Well-rotted compost from 'green manure' and partially composted wood chips, are examples of suitable materials to use as mulch. This mulch helps the plant by providing nutrients, helping to retain moisture, and stimulating beneficial soil organisms, resulting in a much more robust specimen overall. Like most ferns, tree ferns grow well when they are given an ample supply of moisture, and they should not be sited in a dry part of the garden.

As a general rule, tree ferns perform well where they receive morning sun; but some protection from the hot, drying afternoon sun. Of the two main genera of tree ferns we can grow here, Dicksonia species are all shade tolerant, and will also thrive in some sun if the rest of its growing conditions are ideal; Cyatheas have variable needs depending on the species. Some Cyatheas such as C. cunninghamii and C. smithii require shade and shelter from wind and exposure to thrive. On the other hand, some Cyatheas require sun for good growth, which can limit the positions available to grow them without exposing them to frost. Positioning a tree fern under an overhang or high canopy of trees will aid in frost protection. It also shelters the plant from strong winds, which may damage or dry out the fronds. This replicates their natural forest habitat.

Trunk development of most species occurs at a rate that most gardeners would consider to be quite slow. In most cases, the trunk starts to appear about one to three years after the fern is planted from a three or five gallon pot. Dicksonia antarctica and D. fibrosa will typically grow about 1-2" of trunk per year, while some species that produce offsets such as D. squarrosa may grow up to 4" per year. Cyatheas are variable, but the hardier species generally grow at least 2-3" per year, and some less hardy species such as C. brownii over 12" per year (occasionally 24" per year) under good conditions. Young plants begin with small fronds, but new fronds increase in size quickly, resulting in an impressive plant even before the trunk forms. Once trunk development has begun, the fronds may be anywhere from 4 to 20 feet in length, depending on the species and the conditions in which it is growing.

Cyathea smithii near MacLean Falls, New Zealand.
(Photo courtesy of Peter Richardson)
The main factor that limits our ability to easily cultivate tree ferns in the Pacific Northwest is our cold winter temperatures. Tree ferns from temperate latitudes are what some of us have dubbed 'three-seasons-out-of-four' plants-they are perfectly happy here in the spring, summer and fall; but winter means trouble. Most tree ferns originate in the Southern Hemisphere or the tropics. (The truly tropical species are not cold-hardy and will not be considered here.) None of the regions of the Southern Hemisphere where tree ferns grow is subjected to 'Arctic blasts' comparable to those that occur every so often in the Pacific Northwest. This is because the Southern Hemisphere landmasses where tree ferns are found (Australia, New Zealand, South America, and South Africa) are disconnected from the Antarctic landmass by an expanse of ocean which moderates any Antarctic air mass that crosses it. By contrast, much of the temperate Northern Hemisphere is directly connected to the Arctic regions by landmasses that may be very cold in winter. Not surprisingly, this has significant implications for the adaptability of Southern Hemisphere native plants to Northern Hemisphere gardens, including the tree ferns. So, although most Southern Hemisphere temperate tree ferns can tolerate some frost, they are generally unable to endure temperatures below around 18 - 26°F, depending on the species.

This means that in the Pacific Northwest, tree ferns will need occasional to frequent protection from cold weather, depending on the locale and the species chosen. For the present, it seems that no tree fern can be considered fully hardy for Pacific Northwest gardens unprotected from the cold.

However, protecting tree ferns from winter cold needn't be overly difficult if you stick with the hardiest species. A common protection method is to put sawdust or pine mulch (or something similar) into the crown, and wrap the trunks with fiberglass insulation. Young plants with little trunk can simply be covered with some kind of insulative mulch and/or blankets.

Cyathea australis in habitat under Eucalyptus viminalis
forest near Toolangi, Victoria, Australia.
(Photo courtesy of Scott Ridges)
For tree fern species that are not in the 'hardiest' group (i.e. around 20°F), winter protection requires a bit of innovation and careful monitoring of the weather. In the milder coastal regions such as Tofino and Oregon's south coast, which usually experience numerous consecutive mild winters in between the occasional Arctic blast, success may be had with some of the less hardy species by digging the tree fern up out of the ground (tree ferns can recover from major root loss), cutting off most of the fronds (or, some can be left on if they don't get in the way), and storing it somewhere warm when an Arctic blast threatens; and then placing it back where it was after the weather moderates. This is probably only feasible where the fern can be allowed a couple years to recover in the ground in between the Artcic blasts. Also, depending on how much available manpower one has, this procedure may become unmanageable once the fern gets very large. Other methods I have heard of involve such techniques as stacking bales of straw around the tree fern and covering straw, tree fern, and all with polythene plastic. This is probably more work than the average gardener wants to go through to grow tree ferns.

The two tree fern species you are most likely to encounter in Pacific Northwest gardens and nurseries are Cyathea cooperi (sold as the 'Australian tree fern') and Dicksonia antarctica (sold as the 'Tasmanian tree fern' ('Tasmanian' is a bit misleading since this species grows throughout the mainland of temperate southeast Australia also). Cyathea cooperi is relatively easy to find, very fast growing, and fun to grow; but it is not really hardy here, and cannot be expected to survive a severe winter without heavy protection. Dicksonia antarctica is hardy enough to grow here with basic protection methods. The fronds of this species will survive temperatures down to about 20°F before they are severely damaged. It usually grows about 1 inch of trunk per year, but it can be compelled to grow 2 inches per year under good conditions, ultimately achieving a height of 20' or higher. The fronds have a coarse, leathery texture; and the upper trunk and fronds have reddish brown hairs. If the fronds are lost in the winter, it will produce a whole new 'flush' of fronds at once in the spring, as long as the crown has not been killed by freezing. (Heavy feeding can encourage this.) I would recommend this species as an excellent tree fern for gardeners wishing to introduce themselves to tree ferns, since it is relatively easy to grow and not too difficult to find in nurseries.

Now for a wider overview of tree fern species we may eventually have to choose from for our gardens. I will begin in New Zealand, which contains three endemic species of Dicksonia. D. fibrosa is very similar to D. antarctica, but smaller in overall stature, making it useful where a smaller substitute for D. antarctica is desired. The fronds are slightly coarser in texture than those of D. antarctica. It is at least as cold-hardy as D. antarctica, and also quite easy to grow and overwinter.

Cyathea dealbata in a Seattle garden, showing the silvery undersides of the fronds.
D. squarrosa is less hardy than both, but it is a very beautiful fern if the dead fronds are removed. It has grey-brown hairs, gently arching fronds, and a rather slender, dark trunk resulting in a more refined look than the other Dicksonias. It sends up offsets from the ground at a distance from the main plant, a trait which enables established plants to recover if the main trunk dies (i.e. from frost). This fern may not be easy to grow without elaborate protection, except in very mild gardens: the crown seems hardy to about 25°F, and may be destroyed before the fronds show any damage. Although it tolerates exposure to sun and wind well, I recommend a sheltered site in the garden because of the need for frost protection. Both of these species have been available in the past in specialty nurseries, including at least two in the Pacific Northwest, though they remain very rare.

The third New Zealand species is D. lanata which is very cold hardy but usually has a prostrate, creeping trunk, and is considered by some to be difficult to grow.

New Zealand is home to five species of Cyathea. The most spectacular is C. dealbata, the silver fern, whose fronds have beautiful white undersides. This beautiful species is the floral emblem if New Zealand. In the wild it is a forest-dwelling species preferring a sheltered spot, but it is able to adapt to some exposure. It is hardy to about 24°F, and specimens at Logan Botanical Gardens in Scotland have survived 16°F without protection (at which temperature they were heavily damaged, I am certain).

Impressive young plants of Cyathea medularris at a New Zealand hotel.
(Photo courtesy of Peter Richardson)
Probably the hardiest of the New Zealand Cyatheas to cold is C. smithii, which is common in the mountains on the southewestern South Island. This large, attractive tree fern may lose most of its fronds below 22°F, but this is no matter, since this often happens in its native habitat anyhow. A cool, moist forest dweller, this species has shown a complete intolerance of any heat or exposure in cultivation, and should be planted in a very sheltered garden only.

Cyathea medullaris is also from New Zealand, where it is found in lowland and coastal areas, frequently on very exposed sites. It can achieve large dimensions quickly in cool climates and, if not damaged by frost, rapidly becomes an impressive plant. It has attractive black stipes (the word for fern petioles) and a dark crown, and the fronds may exceed 12' in length. Alas, its cold tolerance is only about 26°F, or perhaps slightly lower. Although it has attracted attention from hardy tree fern enthusiasts, I would not really recommend it as one of the best ones to try in the Pacific Northwest. It does not like losing its fronds and requires a couple years to recover if it is severely damaged.

C. cunninghamii is a montane relative of C. medullaris that occurs in both Australia and New Zealand, where it is found occasionally in sheltered gullies. It is a very large fern that may reach 60' tall or more in the wild! Although hardier than C. medullaris, it is very rare in cultivation. It demands a cool, shady situation, but it is very fast growing, and holds promise for Pacific Northwest tree fern enthusiasts.

The fifth species, C. colensoi, is very cold hardy but does not usually form much of a trunk. New Zealand also contains many other large and interesting ferns that do not form trunks, and a few trunk forming species of Blechnum.

Mass planting of Dicksonia antarctica at Golden Gate Park, San Francisco.
Australia is home to many tree fern species, including the aforementioned Dicksonia antarctica and Cyathea cooperi. Contrary to what some have advertised, there are probably not any super-cold-hardy forms of Dicksonia antarctica growing in high in the Australian Alps waiting to be introduced to cultivation. The highest populations are found at about 4,100', at which altitude they may live for a few years, but are unable to grow very tall before they freeze from the absence of snow protection. Clearly they are on or past the fringe of their survivability in this habitat, and have only sprouted up there because the spore was carried on the wind from larger ferns lower down the mountain. Although there is variation in Dicksonia antarctica's morphology and probably its hardiness, the various forms appear to consistent throughout all altitudes in a given region: there appears to be no empirical evidence that specimens found at higher altitude are necessarily more resistant to cold than those at lower altitudes. There is also no evidence to support another popular claim that Tasmanian provenance of this species are hardier than those from the mainland, as some have purported.

Australia also contains two other rare Dicksonias. D. youngiae grows in cool moist forests in southeast Queensland. It roughly resembles D. squarrosa in habit and size, but has coarse, reddish-brown hairs rather than the soft brown-grey hairs of D. squarrosa. Despite such a northerly place of origin, it is at least as cold-hardy as D. squarrosa, and should be added to the collection of any tree fern enthusiast who fancies D. squarrosa.

The other Australian species is D. herbertii, which grows in moist mountain cloudforests in the north of Queensland. Little information is available regarding its appearance or potential cold-hardiness.

Cyathea cooperi at San Diego State University, California.
Australia has a number of Cyatheas, the hardiest of which is probably C. australis. (C. cooperi is sometimes sold under this name in the nursery trade, but it can be distinguished by the conspicuous white scales covering the stipes and crosiers of nursery-grown plants.) Cyathea australis is very promising for the Pacific Northwest, since it grows along with Dicksonia antarctica but preferring more exposed situations, tolerating more sun and invariably encountering more frost. Most of my Australian friends feel that it is slightly hardier to cold than D. antarctica, and it too has no trouble recovering if it loses its fronds. It is not common in cultivation because it is relatively slow growing. However, it is a particularly attractive tree fern and deserves more attention from growers in cooler areas. It adapts well to a range of garden environments. Cyathea cunninghamii also grows in Australia, and another tree fern, C. X marcescens, is presumed to be a hybrid between C. cunninghamii and C. australis. C. X marcescens is magnificent and hardy, but very rare, and can possibly be obtained by sowing a large amount of C. cunninghamii spore in proximity with a small amount of C. australis spore.

Other Australian Cyatheas worth trying include Queensland's C. celebica, which is a little bit hardier to cold than C. cooperi; and C. leichhardtiana, a forest dweller from the Southeast which is commonly ignored in cultivation though it may be quite hardy. The really adventurous tree fern enthusiast may wish to try C. brownii, from Norfolk Island, which can grow to 100' tall; or C. robusta or C. howeana from Lord Howe Island. These three ferns have similar hardiness to C. cooperi (C. robusta perhaps slightly hardier!) but are far more spectacular in appearance.

Interestingly enough, Papua New Guniea contains a few possibilities. On the very highest mountains, tree ferns can be found growing in grassland as high as 13,500', where they are subjected to intense sunlight, perpetually cool temperatures and occasional frosts down to 20°F. Species from this region include Cyathea muelleri, gleichenoides, macgregori, and tomentosissima. Some of these tree ferns have the remarkable adaptation of being so covered with hairy scales that they are nearly self-insulating and are able to capture the sun's warmth very effectively. These special adaptations for protecting the central bud from the cold are common to many taller tropical highland plants, such as the giant Lobelias and Senecios of the Central African highlands. It should be noted that the frosts these ferns endure are overnight only-thus, they may have trouble adapting to a freeze that lasts longer than a few hours in cultivation. Much more work needs to be done with these ferns to discover their adaptability to cultivation. C. tomentosissima, at least, is adaptable enough to be grown easily in a pot.

A young plant of Cyathea tomentosissima.
Central and South America contain a great number of tree ferns which are not very well known, and not very hardy. However, a few are worth mentioning. Cyathea caracasana and C. fulva grow at altitudes upwards of 14,000' in the Andes, and would be worth trying if they could be obtained.

Dicksonia sellowiana ranges all the way from Central America down to Argentina, and surely must be quite hardy at the southern end of its range. It somewhat resembles D. antarctica, but with yellow rather than red hairs. It is a variable species: D. sellowiana var. gigantea, from Mexico, is on the large side, and may form offsets on the trunk.

Chile's Juan Fernandez Islands contain the interesting but very rare Dicksonia berteriana and D. externa, and possibly others.

I am not aware of any Central American tree ferns that may be hardy, although Mexico may have a few-Cyathea mexicana is known to survive unprotected in southwest Ireland. Mexico's Cibotium schiedei may be worth trying for the silky white hairs on the light green fronds.

Asia may be home to a few possibly hardy tree ferns, including the Sino-Himalayan Cyathea spinulosa.

Our last stop will be South Africa, which contains two tree ferns, Cyathea dregei and C. capensis. Little is known about the hardiness of C. capensis, which grows in moist, sheltered gullies.

Cyathea dregei at Pietermaritzburg Botanical Garden.
(Photo courtesy of Ed Brown.)
C. dregei, on the other hand, is quite an exciting plant: in fact, it may be the hardiest of all tree ferns! Its large native range includes the Drakensberg Mountains, where it grows at a high enough elevation to be subjected to frosts of 12°F some winters. Interestingly, this tree fern has adapted to fire and frost in South Africa by behaving essentially as a deciduous fern: the crown is able to survive much lower temperatures than the fronds. The fronds freeze off at about 30°F, which is no cause for alarm for anyone fortunate enough to acquire this species. It is probably one of the toughest all-around tree ferns, requiring only a consistent supply of moisture to do well. Alas, it remains very difficult to find, and it is excruciatingly slow-growing.

That is not quite a complete overview of all possible tree ferns worth trying in the Pacific Northwest. But I hope I have given some idea of what may lie ahead, as an increasing number of these fabulous tree fern species continue to make their way into cultivation. Tree ferns are beautiful plants and I hope I have encouraged more of you to try growing them. I wish you the best of success!

Anyone desiring to learn more about tree ferns, and see many more pictures, may visit my Cold Hardy Tree Ferns web page. There is also an excellent Tree Fern Internet Discussion Group on the web.

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