WINTER PROTECTION FOR TREE FERNS
Do I need to protect?
Hopefully your selecting of a treefern was done with consideration for the climate requirements of the fern and the amount of effort you hope to put towards for it. For example, if you live in Seattle and try to grow a truly tropical species such as Cibotium glaucum, you are going to have trouble if you only intend to protect it as much as you would a hardier species such as Dicksonia antarctica. Generally, treeferns are relatively easy to protect compared to other plants that are commonly grown near the limit of their hardiness, such as palms.
You should plan on protecting your treefern from cold if temperatures are expected to fall below the limit of its cold-hardiness. These temperatures, where known, are given under the description of each species listed on this page. That may sound simple enough, but there are some variables associated with cold hardiness, as discussed under Cold-hardiness and Climate Adaptablity. Also, it may be difficult to obtain climate data for your location to get an accurate sense of how cold your winters are. A sheltered microclimate may enable you to get away with growing more tender ferns with less protection, while an exposed situation may require more protection than you would expect.
There are a variety of different protection methods, described below. It is certainly not an exhaustive list of ideas for protection. In some cases, combinations of various methods can be used. Many of them will guarantee that you get funny looks from the neighbors, but that will be worth it when you have a treefern and they don't.
When should I protect?
There are two basic approaches to protecting treeferns from cold. There are advantages and disadvantages to each approach, depending on the particular method you use. More detail about this is found under the descriptions of each protection method.
The first approach is to protect the fern for the entire winter. That means that you would take whatever protection means you are planning on before the cold comes (say, in November for the Northern Hemisphere) and leave the fern under this protection until the damger of cold is over the following spring (say, March or April). This way you do not have to watch the weather forecasts being constantly concerned about impending cold weather on the way, because the fern is safe and sound all winter. This technique can generally be recommended for climates that consistently have long periods of cold weather every winter, or almost every winter.
The second approach is to protect the fern only at times when the temperature goes below the limit of the fern's hardiness, leaving it unprotected throughout the rest of the winter. This can be applied in climates where there is quite a bit of variability in winter severity (a cold winter often being followed by several very mild ones), and when the cold does come, it passes quickly. Examples of such a climate include the U.S. West Coast, milder parts of Britain, and maybe even the U.S. Gulf Coast. This approach has the advantage of keeping the fern in a natural open-air environment as much as possible, which lessens the risk of some problems associated with all winter-protection (see below). Also, it may be that the winter turns out to be mild enough that the fern does not need protecting all year, in which case it would have been a waste of time and expense to go to the trouble of an all-winter protection scheme. For example, the Seattle area has now (as of autumn 2002) had three consecutive very mild winters in which Cyathea cooperi could easily have overwintered outdoors in a shelered spot. But sooner or later a cold blast will come along and protection will be necessary for a brief period. This approach requires careful monitoring of the weather forecasts and you must always be prepared to act quickly to protect the treeferns if a hard freeze is iminent. Sometimes there may be very little notice as to the arrival and severity of the cold weather. On occasion the forecast may err the other way, and you will find you have been in a rush to take protection measures that turned out not to be necessary. But it can still be more practical than all-winter protection in some situations and climates.
What parts need protecting?
The trunk and crown are the parts of the treefern that are most important to protect. The fronds are much more expendible in comparison. Also, for most treeferns (a notable exception being Dicksonia squarrosa), the crown is just a little hardier than the fronds. The crown is the only point from which the fern is able to produce new top growth, so it is crucial that it is not exposed to temperatures that will kill those tissues. The trunk is usually just about as hardy as the crown, and, being little more than a mass of exposed roots and old stipe bases in the case of Dicksonia, it also needs protection or else the flow of water and nutrients to the crown will be cut off. In theory it is possible that, for some of the more woody species of Cyathea, the trunk may be a little hardier than the crown and have less need of protection, but I have never heard any reports to suggest this.
Fronds are not a permanent part of the treefern; they never live more than a few years anyhow. So, if necessary, they may be cut off leaving behind a stub that is not attractive for the winter but much easier to protect. Most or perhaps nearly all treeferns can survive if their fronds are cut off on an annual basis, provided the climate has enough heat in the summertime for a whole new set of fronds to regrow by midsummer (tropical ferns requiring more heat than temperate ferns). Growth of the fronds after they are removed can also be aided by generous feeding and watering in mid to late spring. Some ferns are more tolerant of this than others; for example Dicksonia antarctica and Cyathea australis can have all their fronds removed annaully with no problem and keep on going, while Cyathea medullaris is a bit more fussy and may be set back if it is not already well established. Cyathea dregei and C. smithii (and often C. australis) are by nature adapted to lose their old fronds over the winter anyhow.
There are advantages to leaving the fronds on, however. For one thing, it will look nicer at once when the protection is removed, rather than being an unsightly stub until the new fronds grow. Also, it will grow more quickly in the spring since it is able to photosynthesize, which feeds the new developing crown. Cutting the old fronds off in any case will not encourage more new growth but only slow it down. Species with relatively short, tough fronds such as Dicksonia fibrosa are really not that much more difficult to protect with the fronds left on.
Protecting from light frosts
There are a lot of treeferns that are hardy to the upper 20's F, or just below freezing, and can be made to survive temperatures down to the mid 20's with just a few tricks. Any form of overhead cover can be used to protect treeferns from light frosts. This may be done by planting the treefern under a tree canopy to begin with, draping plastic or some such thing over it just for a night, or if it is potted, move it under a lath structure, etc. Any method that is sufficient to keep white frost from forming on the leaves is at least somewhat helpful.
A form of "protection" from light frosts is also to keep the fern on the dry side. Of course, the fern must never be allowed to dry out all the way, or it will be severely damaged. But to keep the treefern only just moist enough to live will add a couple degrees of frost tolerance to many species. But, in climates such as Britain or the Pacific Northwest, there is no real way (at least, not that I have thought of, for in-ground ferns) to regulate the amount of soil moisture this way, since it rains all winter and the soil remains saturated. So it is really more useful in places that have dry winters and receive more of their precipitation (or watering) in the summer, such as interior Australia.
The heavy mulch method
A young, pre-trunking treefern may be protected by burying it in heavy mulch a foot or two deep. In climates that are wet in the winter, it is best to use a mulch of something relatively dry and light such as pine needles or oak leaves, which will provide protection from the cold while also allowing the plant to breathe, rather than be stifled in decomposing muck and start rotting away. However I have also used fairly wet leaves on Cyathea cooperi and it has not had any problems with rot. Even ferns with a couple feet of trunk may be protected by putting up a round wire cage and filling it up with leaves (a popular way to protect the pseudostem of hardy bananas). If parts of the fronds stick out, no big deal; they might freeze off but there will still be some left inside the cage to help fuel next year's growth.
Snow, especially dry snow, also has insulative properties. If a cold blast is preceeded by a foot or two of snow, then no worries for a pre-trunking treefern (of one of the hardier species), as long as the snow stays put.
The fiberglass insulation method
A fairly successful method of protection treeferns is to wrap them in fiberglass insulation. This can add roughly 10-15°F of hardiness depending on how much insulation is used. It has proven a successful means of growing Dicksonia antarctica, D. fibrosa and D. squarrosa in Seattle and Portland. The fronds can either be tied up straight, or removed. The challenge though is to keep the insulation dry without completely cutting off air circulation to the fern. What I have done is to first wrap the insulation in plastic and then wrap the treefern - but, it might be just as successful to just wrap the insulation on first and then put plastic over it. Better yet, find a material that is just a little breathable but will not allow enough moisture in to damage the insulation, or cover it some kind of tarp held at some distance above the plant that would keep rain off while allowing sufficient air circulation. Or, if the cover will be removed immediately before any wet snow or rain starts falling, then maybe waterproofing the insulation would not be necessary at all.
Here is an example of Dicksonia fibrosa, its fronds tied up with rope and a stick to keep it upright, and the crown securely protected in insulation with plastic over it. I also tied a couple blankets over the top of the fronds, and the fern survived 15°F undamaged with this protection.
This Dicksonia squarrosa was wrapped with plastic-covered insulation and then another larger piece of insulation that covered the top half. I unwrapped it and the fronds looked fine, but it never made any new growth because the crown had frozen. The outdoor temperature was 15°F - so, I was a bit surprised, but would conclude that it just needed an additonal piece of insulation around the base to keep the crown a little warmer. D. squarrosa is a species that forms multiple crowns and is capable of coming back from the roots, but this plant did not have any strong offests at the time, and died - bummer.
The disadvantage to this method (and the following) is that, if the insulation is left on all winter, it may be difficult to maintain an environment in which the fern can be kept warm without being completely suffocated. In certain situations the fern may also get too dry, or have problems with fungal infection from too much moisture. It would be difficult to monitor this if the fern is buried deep in layers of insulation. Some ferns are more succeptible to these problems than others - Dicksonia antarctica is usually relatively trouble free.
In theory, it might be feasible to incorporate the use of a heating cable with this method to overwinter treeferns in very cold areas. The heating cable should be set at a very low temperature (perhaps 35°F, certainly no warmer than 40°F) so as not to encourage active growth while the plant is stifled in insulation and plastic. The cable could be wrapped around the length of the trunk and then the insulation put on over it. I would probably not use Christmas lights under the insulation (as is often done with palms) as they are liable to burn the roots growing on the trunk. I have never actually heard of anyone trying this but it just might work in USDA zones 6 - 7 or perhaps even colder.
The jacket method
This is just a refined version of the insulation method. Rather than wrapping the fern, just make a "jacket" out of insulation and plastic that can easily be easily tied or clipped on (or use velcro?). This would potentially be a good way to quickly protect a large number of ferns, but once you make a jacket of a certain size it does not accomodate future upward growth of the treefern.
The straw method
This is basically similar to the insulation method. Straw is perhaps a little bit superior material, since it is very breathable, and no worries if it gets wet. However it is a bit more difficult to tie it onto the fern and it may require two people spending a considerable amount of time to protect all the trunks and crowns of a large treefern collection.
The leaf-litter-in-the-crown method
Leaf litter often offers protection to treeferns in their own habitat. Placing some leaf litter in the crown will help to ensure that the crown is not destroyed by the cold, but does nothing for the trunk or fronds. So if the fern is small, or if the trunk is hardier than the crown, it might help just to throw some leaf litter in the crown to protect it. I would consider this to be more of a quick-fix if you don't have time to do more, or if you just need to add a degree or two of protection, rather than being a very reliable and broadly applicable protection method.
The polyethylene or bubble-wrap temporary fabrication method
This technique is most often used on ferns that will be protected for an entire winter, and would include any kind of fabrication that does not come into direct contact with the fern. A structure built from wood or plastic tubing and clear polyethylene sheeting or bubble wrap is erected around the fern (with or without fronds cut off) large enough to hold it with room to spare. Any parts of the fern that come into contact with the covering are likely to get scorched if it gets too cold outside. Multiple layers may be used to enhance the insulative qualities, and a little heat can perhaps be added. It is a good way to overwinter even very tender ferns in temperate climates, or somewhat hardy ferns such as Dicksonia antarctica in USDA zones 6 - 7. This might be the only method for which Christmas lights would be a good source of heat, though they should still not come into contact with roots on the trunk (perhaps another protective layer could be put on the trunk, with Christmas lights over it!).
Of course if you're going to go to all the trouble of this method, you might just as well build a greenhouse over the fern to begin with. But it might still be worthwhile to have the look of a fern in the ground over the summer without a permanent structure. With a little ingenuity it would not be difficult to come up with some kind of collapsible structure that could be put up very quickly and easily and is just the right size for your situation.
The dig-it-up-and-store-it-in-the-garage method
It might sound funny, but this is quite seriously a good technique to use in climates where most winters are very mild, with a severe winter only once every 5 to 20 years. Such climates would include the immediate coasts of Oregon and Vancouver Island, and the mild west coast of Britain. Most winters the treefern can be left in place and it will not be damaged by frost. But you must take care to watch the weather forecasts in winter - if Arctic air seems to be headed your way, dig the treefern up and store it in the garage, or in any other cool environment where the temperature will remain above freezing, but not so warm that the fern starts making new growth. Get as large of a rootball as you can lift - of course you will be cutting off many roots, so it is a good idea to cut off some fronds to compensate for the root loss. (You may want to cut them all off if that makes the fern easier to store.) Then after the freeze passes, replant the fern back where it was. (Or if you have a lot of different treeferns, rearrange them all just to confuse your guests!)
This might not work so well in a climate where this has to be done most winters, because the fern will have to grow a whole new root system every year and will often be enfeebled unless it is a very tough species. If it would have to be dug up more winters than not, better to just keep the fern in a pot and sink the pot into the ground in the garden so that it is easy to lift out with less stress on the fern. A potted fern sunk into the garden will of course need more summer water than an established fern.
Recovery from freeze damage
In the spring you finally find out whether all that trouble was really worth it. First of all, remove the protection as soon as possible. When the fern is no longer in danger of freezing, it will be good for it to get some air which is necessary for the plant to metabolize. It may be a little while before it is certain that the fern will recover. New growth will not begin until the fern has enough heat to meet its growth requirements, so don't be too impatient. If the crown does not produce new growth in the spring and it is still firm to the touch, maybe it will come back eventually. But if it is mushy and slimy then it must have frozen and will probably not recover.
Recovery can be sped along (perhaps even induced, for challenging species) by generous feeding and watering of the fern in mid to late spring. Heat is also necessary for ferns that originate in warm areas. Subtropical ferns in a temperate climate may not start growth until early summer - if it is mid summer and you still can't get your fern to grow, it may not be a viable long term endeavor even if the plant is cold-hardy enough to grow with relatively easy protection measures. However, ferns from New Zealand and other temperate, forested areas, as well as high-elevation cool cloudforest, will generally regrow from frond loss in most climates that have mild enough winters to support them to begin with.
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