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How hardy are they?

A quick overview of treefern hardiness will reveal several general facts.  First of all, one cannot generalize about the hardiness of all treeferns since each species is a little different.  Second, most come from either tropical areas or relatively mild temperate areas that lack extremes of temperature.  So, if you live in northern Minnesota and want to grow a treefern outdoors, unfortunately you may be out of luck.  Third, the ability of a particular fern to withstand a certain temperature will vary depending on other factors, such as the duration of frost, degree of exposure and size of the fern.  Therefore I must caution that the temperature figures given throughout this web page are meant only as a general approximation of a treefern's hardiness.

The hardiest treefern is probably Cyathea dregei, which might tolerate temperatures down to about 10°F or possibly a little lower (its exact limits are not known).  However, this species is rare and very difficult to find in cultivation, especially in the United States and Canada.  Cyathea australis is most likely the next hardy fern, bring probably just marginally hardier than Dicksonia antarctica.  (As far as I am aware, all ferns sold in the United States as C. australis are actually C. cooperi, a more tender species.)  If it is not possible to find these ferns, it can be said that the hardiest treefern that is easy to find is Dicksonia antartica (and, in Britain, D. fibrosa is not uncommon and roughly equivalent in hardiness).  D. antarctica is the treefern that has historically been most frequently experimented with in the Pacific Northwest and Britain, and throughout the Northern Hemisphere.  It is generally considered to be hardy to around 20°F, though it will come back from a bit lower temperatures than that with some frond damage.  Still, there are no success stories of this species surviving long-term in the Pacific Northwest without some help in the form of winter protection, since most of the region has all-time extreme low temperatures below 12°F.  Cyathea smithii and C. cunninghamii are about on this level of hardiness as well.

After this, there are many other treeferns that are hardy to temperatures somewhere in the 20's F (again, a little lower with frond damage).  A subtropical or very mild temperate climate with only light frosts can truly grow a wide variety of treeferns.  Some of the next hardy treeferns after those mentioned in the previous paragraph might be Cyathea robusta, C. spinulosa, C. leichhardiana, C. dealbata, Dicksonia squarrosa, D. youngiae and D. sellowiana.  More details about the hardiness of these ferns are available on the individual pages describing them.  Cyathea cooperi, a very common fern in the United States, is slightly less hardy than these.

Effects of frost on treeferns
 
 

Visible symptoms of frost damage
 
 

How treeferns respond to frost damage
 
 

Maritime and continental climates

A maritime climate is one that is dominated almost entirely by the influence of the ocean, and would include such places as the Pacific Northwest, Britain, and New Zealand.  These climates tend to have relatively high humidity and high rainfall, and moderate temperatures without a lot of extreme cold or heat.  Continental climates, on the other hand, experience much less influence from the ocean, and would include such places as central Australia, the eastern United States and Russia.  They generally have more extremes of temperature and may be very dry to moderately rainy and humid.

Most of the cold hardy treefern species come either from predominantly maritime climates, or from other climates that have similar conditions (for example, the highlands of Papua New Guinea or the Andean cloudforests are also lacking in extremes and very moist).  Therefore it can generally be said that treeferns adapt most readily to maritime climates similar to those where they are native.  However, it is still possible to grow many treeferns in continental climates provided they are not pushed beyond their tolerance for extremes of cold and heat.  The main setback to growing treeferns in continental climates of the Northern Hemisphere is that it is simply far too cold in the winter for them to survive in a permanent outdoor situation.  In the Southern Hemisphere however there are some fairly extensive collections of treeferns in continental climates.

"Arctic blasts" vrs. overnight radiation frosts

There are, generally speaking, two different types of frosts or freezes.  Overnight radiational frosts occur during fair weather patterns when the air is still and the temperature drops below freezing at night.  During such freezes the temperature usually rises during the day and often the weather will be sunny.  "Arctic blasts" occur when a cold air mass moves over an area.  This is known as an advective freeze.  Advective freezes will often be associated with snow, and sometimes the temperature will not rise above freezing during the day.  In the Northern Hemisphere, sometimes (but not always) a freeze may begin as an advective type pattern, but develop into a radiational freeze as the cold air mass settles over the area and moderates gradually.  In the Southern Hemisphere, there is pretty much no such thing as a true advective freeze (or "Arctic blast") because all of the land masses are cut off by the ocean from Antarctica, the only potential source of severe cold in the Southern Hemisphere.  The worst freezes in the Southern Hemisphere are always the result of radiational frosts that occur from cold air buildup on the continents themselves.

This difference is important to consider, since most of the hardiest species of treefern originate in the Southern Hemisphere.  Even the hardier Northern Hemisphere treeferns come from regions that are sheltered from advective freezes (for example, C. spinulosa in northeast India).  Generally, treeferns are more tolerant of brief overnight frosts in which the temperature rises above freezing during the day.  A treefern will suffer more damage in an advective freeze than in a radiational freeze of the same temperature.  This is because the temperature during an advective freeze is lower for a longer period, and in some instances the cold winds associated with these "Arctic blasts" may have a dehydrating effect on the fronds.  It might be worthwhile to experiment with an anti-dessicant such as wilt-pruf to see whether it enables the fronds to hold up better to cold temperatures in an advective freeze.

There are a whole lot of treeferns about as hardy as Cyathea cooperi that will tolerate radiational frosts in places such as lowland Australia or Southern California, where the temperature generally rises well above freezing in the day after a frost.  Under such conditions they may tolerate 25°F without harm.  But their tolerance of such temperatures will not be quite as good in climates such as the Pacific Northwest or Britain - the fronds may be lost at temperatures around 28°F.

Heat tolerance

Different tree fern species vary in their degree of heat tolerance.  Cyathea smithii and C. cunninghamii are perhaps the two least heat tolerant treeferns, and can be very difficult to grow where average high temperatures in summer exceed 80°F.  The temperate Dicksonias such as D. antarctica, D. fibrosa, D. squarrosa and D. youngiae are somewhat more heat tolerant than this, as are Cyathea australis, C. dealbata, C. dregei and C. medullaris.  Many of the tropical treeferns are of course very heat tolerant provided they have enough moisture, including some from very high elevations where temperatures are cool.  Treeferns respond to heat damage by frond scorch, or outright death.  If you are in a hot climate, you might increase your chance of success by planting it just at the beginning of the cool season and ensuring it is well established before the heat comes back.  Siting it in more shade than would otherwise be necessary would also help, as well as ensuring it is very well watered when the mercury climbs.  If possible, mimic the conditions of a cool sheltered gully with high humidity - it is in these conditions that many of the more heat sensitive species are found in habitat.  A more in-depth review of this subject is coming soon.

Humidity, shelter and rainfall needs

As alluded to above, treeferns generally have a preference for an abundance of humidity and rainfall.  There is no set way to determine just how much of each is necessary for a particular fern, since other variables such as heat also come into play.  However, when in doubt you can usually not go wrong by supplying your treefern with plenty of moisture and humidity.  It must be considered that there are many examples of treeferns that find their niche within only the most sheltered, moist places in a given climate.  For example, Dicksonia antarctica is found primarily on streambanks and moist forests of Southeast Australia.  It is not found in exposed places except where there is an overabundance of precipitation and very cool humid conditions.  D. youngiae hides in a small area of cool rainforest in southeast Queensland.  Cyathea cunninghamii and C. smithii also prefer very sheltered situations in gullies with high humidity, even in high rainfall climates.  There are also many ferns that will grow out in the open such as C. medullaris, and some even colonize disturbed areas.  However these treeferns that do not require shelter are always found in very rainy climates.  It is beneficial to consider the climate and setting a particular fern is from in order to try to replicate it, even if many treeferns can be grown far beyond their range of natural adaptation.