Answer: The Tasmanian Tree Fern (Dicksonia antarctica) is the most cold hardy of the giant tree ferns. Still, despite the species name "antarctica" ? derived from the fact that the family as a whole lives around the lower half of the globe ? it really can't stand freezing temperatures.
It can be killed at 15 to 17 degrees Fahrenheit; 20 degrees is about it for safe low tolerance, & it's safe only then if it won't be that cold very often or very long. Despite the risk, I know that old specimens have been grown outdoors in Scotland, usually in sheltered outdoor gardens but sometimes unprotected. To get them through winters, don't break off the fronds, but tie them up (to protect the center or crown of the plant) & wrap the trunks in garden cloth or packaging bubble wrap. Don't leave the plastic on the trunk for extended periods of time, just long enough to get the exposed trunk through the worst of the winter weather.
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