Tree ferns are magnificent specimens native to the moist forests of the Caribbean, Mexico, Central America, and across the entire Southern Hemisphere where they stand tall and dignified. They are indeed treelike and range in height from 3 to 60 ft. (90 cm to 18 m) with members of the Cyatheaceae being the tallest. The arching fronds, among the largest "leaves" of any plant, flush from the top of the trunk and can reach 20 ft. (6 m). Like most ferns they are ancient plants, predating the dinosaur days and surviving the calamity that destroyed them. Look for tree ferns (or their replicas) as evocative features in those primeval movie scenes where the plot calls for a pre-his-toric atmosphere.
While widely admired, tree ferns are not extensively cultivated in temperate gardens, being best suited to humid pockets in Zones 9 to 11. Dedicated optimists coax Dicksonia antarctica, the cold-hardiest of the lot, into focal point displays in sheltered locations in Zone 8, where they do indeed make a dramatic fashion statement. However,
because their roots run down the outsides of the trunk, they are cold-vulnerable, high-maintenance plants requiring special winter protection. Larger, well-rooted specimens are better able to withstand some cold and should be sited in warm pockets away from winds. Otherwise, outside of Zones 9 to 11, they must be winter-protected. Attention ranges from a hastily applied skirt of bubble wrap or a quick toss of a gauze throw to secure burlap bundling or elaborate coffinlike structures that encase almost the entire tree.
So aided dicksonias have survived in some remarkably challenging areas including the garden of British Pteridological Society past president and tree fern specialist, Alastair Wardlaw, in Glasgow, Scotland. After years of experimenting he has developed an elaborate system for promoting winter survival. Basically (but greatly oversimplified) it is this: tree ferns without trunks and still in pots are overwintered in an insulated cold frame which does not go below 28°F (-2°C). Trunks
Lines of tree fern trunks at a New Zealand nursery.
on ferns planted in the ground are wrapped in a single layer of aluminum foil, which is then enclosed in a double layer of large bubble wrap. The top of the trunk is covered with 6 in. (15 cm) of horticultural fleece which is tucked into the crown and capped by aluminum foil. The whole is then protected by a transparent umbrella-like structure positioned above the fronds and designed to protect as much of the frond area as possible. The winter minimum air temperature in Glasgow is generally around 24°F (-5°C). Thanks to this care, Cyathea australis, C. dealbata, C. smithii, Dicksonia antarctica, D. fibrosa, and D. squarrosa not only survive but also have trunks of between 5 and 7 ft. (1.5 to 2.1 m). The purpose of the various systems is to keep the crown frost-free and dry, the trunks warm, and also, if possible, to avoid browning of the fronds.
Cultural requirements are similar to those of most ferns. Water is the exception. As water must be transported some distance from soil to frond, tree ferns need extra moisture, including an occasional hosing of the trunk, as well as wind protection. When will they form trunks? Essentially trunk formation is an ongoing process that begins as soon as there are fronds (although it may not always be obvious). Like humans, when well nourished they grow most measurably as youngsters and slow down with age. Dicksonias are rather slow with trunks, gaining about an inch (2.5 cm) a year. Cyatheas may increase by a foot (30 cm) or more. All rates are, of course, affected by the cultural conditions.
Propagation techniques include the traditional spore method, at its best with very fresh spores. Large-scale commercial production for some species, which enables shipment worldwide, is done from logs cut from the fern's "trunk." With the roots coming down from the fern's apex, the fern is "topped" and the resultant log reset in soil to form a new plant.
One of the most interesting operations I have ever seen was an air layer on a Cyathea that was literally threatening to raise the roof of the conservatory at the Hamburg Botanical Garden in Germany. As with woody plants a cut was made into the trunk and the wound wrapped in an envelope of sphagnum-stuffed plastic wrap. The newly rooted tree was then cut back and reduced, at least temporarily, to a more manageable size.
For gardeners whose tree ferns have not survived and who have an unadorned lonely trunk standing as a testament to the loss, George Schneider (1890,1:166) suggests playfully that they be decorated with young sporelings of hardy ferns to create a personalized "hardy tree fern."
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