TREE FERN COLD-HARDINESS AND CULTIVATION
Though my experience with tree ferns has taught me quite a bit, much of the following has been passed along from other people are more knowledgeable than I.
Tree ferns can probably be grown outdoors anywhere from USDA zone 6 to the tropics. The question here is not so much about exactly how hardy they are, but how much effort the grower is willing to put into protecting them from the cold. The standard procedure is to tie up the fronds and wrap the trunk in fiberglass insulation when cold weather threatens to come along. With insulation, perhaps velcro, some sort of breathable plastic and a little intuition, it is not hard to make little jackets that can be strapped right over a treefern with hardly any hassle at all. This method adds at least a zone of hardiness to the plant, perhaps more if more insulation is used. Some may think of the idea of wrapping a plant in insulation to protect it from cold as ludicrous. But, considering that plants as interesting as treeferns are bound to have unique needs, I question whether it is so much more outrageous than weeding the vegetable garden or deadheading the marigolds. Trunk wrapping is the standard gardening practice that comes with planting a tree fern in a cold climate.
Insulation is not the only means of keeping a treefern alive through the cold times of winter. Some have used straw, burlap, or blankets. Be creative. Keep in mind the main objective is to keep the treefern as warm as possible, particularly the crown where the tender new fronds will emerge in the spring. Young plants with no upright trunk can merely be covered in blankets or insulation as protection.
Some of the tropical species, or for that matter any species pushed to the limit of its hardiness (e.g. a Dicksonia in zone 6), may require more elaborate protection measures, such as stacking straw bales up around the trunk and fronds, and tying a polystyrene sheet over the whole conglomeration. If it is wrapped up tightly for a long period, such as an entire winter, care must be taken that it is not allowed to dry out. Water should occasionally be poured down the trunk, but not into the crown as this combined with poor air circulation can lead to rot and disease. My climate (western Washington) is mild enough that I need only to keep a close eye on weather forecasts and wrap treeferns for a few days at a time (some winters not at all), in which case drying out and poor air circulation are not concerns. If circulation is good, a light mulch of leaves can be set into the crown as temporary protection.
Siting is also important. Planting a treefern close to the house, under a canopy of trees, and away from frost pockets has obvious benefits of shelter and warmer microclimate. Although some have argued that small plants are hardier because they are less exposed to the elements, the general consensus is that larger plants are hardier than young ones, as with most of the rest of the world's plants. As the trunk becomes thicker it insulates the pre-emergent croziers.
If a tree fern is to be grown without these protection measures (or if it is to be pushed to the limit of its hardiness even with protection), then the importance of other factors, such as spore provenance or plant origin come into play. The cold tolerance of some treeferns, particularly the temperate zone ones, may rely not so much on which species it may happen to be as how cold it is where the individual plant is growing. For exapmle, Dicksonia antarctica spore collected from a mild coastal area will not produce offspring as hardy as D. antarctica spore collected up in the mountains where it is far colder. Most species which have a large natural range are bound to vary somewhat in cold-hardiness. Dispute occurs between reference works based on cultivated plants, because different people will have grown one species from spore collected throughout different parts of its natural range, where temperature can vary considerably. For every species I know of, low elevation, cold-tender stock was the first to be introduced to cultivation and is dominant today. This may give the entire species a bad reputation for hardiness, though more montane provenances of that species would be hardier.
In contrast to the temparate species, some of the tropical species have a remarkable tolerance for coolness and frost far beyond what they are subjected to in their native environment. Using more elaborate protection measures, it is possible to overwinter some strictly tropical species, such as those from Hawaii or Oceana, in places like Britain. It is even remarkable that such ferns will tolerate how cool our temperate summers are. Here lies the yet unexplored realm of cold-hardy treefern cultivation--out of a thousand species, who knows how much extra "built-in" hardiness all the yet untested tropical species may have?
I have now written up a more complete summary of winter protection schemes here.
Other cultural requirements
When transplanting a treefern, follow the basic rules of popping it gently out of its pot (taking care not to damage it), untangling the roots, and setting it in a prepared bed of well-cultivated soil. Sometimes it can be a little tricky to get a larger plant out of its pot, loosen the root ball, and set it into the ground without damaging the crown. A rich humus is appreciated, but most seem remarkably tolerant of a wide variety of soil types, so long as the pH is neutral or slightly acidic. Anything lacking in the soil can be made up for my amending it at the surface.
Ferns generally tend to be associated with moist, humid environments such as streambanks or rainforests. Tree ferns are certainly no exception to this stereotype--if anything, most species are even more demanding of moisture than other ferns because they leave their trunks exposed to the drying effects of heat and wind. Some species will grow reasonably well without constant high humidity, but the healthiest treeferns with larger, beautiful fronds are generally achieved by maintaining a humid environment. Such practices as watering down the trunk and fronds on dry summer days will help to ensure happy tree ferns. The trunk is little more than a mass of moisture-seeking roots, and the fronds lose moisture quickly on hot days. Some people even install mist systems that will keep the air constantly moist. Heat and drought tolerance is noted for each species whenever it is known. Cyathea dregei may be one of the best treeferns to attempt in hot, dry areas.
Where heat and dryness are likely to be problems, there are other things you can do to make up for it. Some species that would normally grow in the sun in moist environments seem to be happy in the shade in climates with more heat. Nearly all species can tolerate some shade (but Cyatheas will not tolerate a really dark shade), since many grow as understory and deep in shady gullies in their native environments. They will often grow darker green fronds in more shady conditions. Also, placing a treefern near a pool or a cool stream can be advantageous, since the water can have a cooling and humidifying effect on hot, dry air. Many species will grow well in full sun in cool, maritime climates such as the extreme west coast of the United States, and the western coasts of the British Isles. In warmer climates, afternoon sun is really not necessary and tree ferns are much easier to manage without it. Morning sun, however, can be helpful for its warming effects on the soil on cold winter days.
Many tree ferns detest wind, and respond with burnt frond tips. The best specimens are usually found in wind-sheltered positions. The Dicksonias are probably slightly more wind-tolerant overall, and perhaps Cyathea medullaris. The combination of subfreezing temperatures and cold winds can be extremely damaging where adequate protection measures have not been taken.
The soil should be rather moist year-round as well. Dicksonia antarctica, D. herbertii and Cyathea australis are more tolerant of very wet soils than other species, but I do not know just how wet they can get before they suffer. All treeferns will suffer if allowed to dry out, and many tree fern species, particularly (but by no means exclusively) those from New Zealand, will not recover if they are allowed to go completely dry. Others such as Dicksonia antarctica, Cyathea australis and C. spinulosa seem better able to recover eventually as long as the drying is brief. Particular attention should be given to soil moisture when they are growing in pots.
Tree ferns generally benefit from a slow release fertilizer of some sort. I usually cover the base of the trunk with a thin layer of organic fertilizer or compost every spring to keep them happily growing. This also keeps the base of the trunk from becoming too exposed, which might lead to drying out. Tree ferns respond to well-rotted organic matter, and often show their appreciation by cranking out broad, lustrous, deep green fronds; but they resent overfertilization, and heavy fertilizing is not necessary or recommended. They can make good pot subjects, but once they become rootbound they need a steady supply of slow release fertilizer to keep them going. Without it, they will grow small, sickly fronds. But too much fertilizer, particularly liquid fertilizer, can be damaging or even lethal to treeferns.
Treeferns in the house and conservatory
Special care must be taken when growing tree ferns as greenhouse subjects. They are succeptible to diseases and insects that can cause poor crown development and stunted fronds. Sudden changes in temperature and humidity can also be damaging to some species. Some species with tougher, more plasticy fronds can be maintained indoors, though it is a challenge to keep them looking good. Dicksonia antarctica seems to do reasonably well indoors, provided it is given enough light. Most species do best with surprisingly cool temperatures when they are in the dark, dry environment of the house. It can help to place the tree fern over (but not in) a tray of water to promote humidity as the water evaporates.
The forest environment/Garden situations for treeferns/Siting for cold/heat/sun protection (coming soon)
Transplanting treeferns (coming soon)
If a treefern outgrows its space in a house or conservatory, some species can be air-layered, though it is a bit tricky. One successful method is to cut a pot in half vertically, cut a trunk-sized hole in the bottom, tape the two halves of the pot pack together onto the treefern and somehow keep it secured in that position, and fill the pot with peat. Then the adventitious roots that cover the trunk will wander into the pot. When the pot has filled up with enough roots, the trunk can be severed just underneath the pot. Dicksonia antarctica, and possibly other species of Dicksonia, are much more tolerant of rough treatment when air-layering, and usually even recover if they are simply cut off at the desired point, repotted, and kept in a very humid environment. Cyatheas cannot survive this treatment.
Importing treeferns and caring for imported treeferns (coming soon, but only a very brief commentary)