I am trying both Rhapidophyllum hystrix and Sabal minor in my Toronto garden. I bought the plants in 1995 from Plant Delights in Raleigh (bare root, unfortunately and unnecessarily) and followed my usual procedure with test plants: I keep them in an unheated or barely heated (depending on temperature), small acrylic cold frame, about the size of a porch, for at least one winter. Subjecting plants to this regime of hardening off seems, consistently, to improve garden performance.
This is the first winter the palms are outside, and of course they must be monitered for much longer. As with you, damage does not usually make itself conspicuous until March, when the longer days, warmer sun and occasional cold temperatures combine to dessicate the foliage of broadleaf evergreens. It may be then that my note to you is premature, but the success or failure of the experiment over time may interest you.
Our climate here, on the shore of Lake Ontario, is much like the rest of Northeastern North America. The city itself is zone 6, USDA. I base this remark on my researches into 140 years of data, not on the pictures in books: those maps, even when well reproduced, are based on airport data, and it is safe to say that the airport is just on the border of 6A/5B. The city, which is where I garden, with the moderating effects of the lake, is considerably milder; it allows me to grow such borderline plants as Mahonia japonica 'Charity', and Aucuba japonica, as well as a variety of evergreen azaleas. Naturally they do not grow with the same lushness as those of the American south, and a very bad winter may well damage or kill some of them. In any case, where I find a difference between Philadelphia, for instance, and Toronto, is that our temperatures (aside from being slightly lower in the extremes) are more often consistent: a winter day is more likely here to be just below freezing (around 30F), which in my opinion keeps the plants at a kind of steady state without being too stressful. Approximately 4 years out of 10 the temperature falls in the range 0F to -5F.(As to summers, they are warm and humid, like much of the Great Lakes region).
This year, until mid January, we had pleasantly mild, wet and very overcast weather. It then became cold, once falling to 0F, with a brief spell at -10F (it doesn't sound so dramatic in Celsius: -18C to -23C, which is the system we use). This was not remarkable, but not altogether common, and we had snow cover. I measured the temperature under the snow, over the palms, and found that while it was -5F outside, it was +23 under the snow. It does indeed make a difference. What will happen during a dry winter, I cannot say yet, nor can I guess how they will fare when the snow melts on their crowns, or if it rains and freezes in late February or March. I noticed a leaf of Sabal minor sticking out from the snow two days ago, which meant that that leaf had been subjected to all the worst weather. It looked splendid. Unless the weather remains unusually cold, we can expect the snow to melt soon and I will have a proper look.
I forgot to mention that I planted both palms about 8 feet from the house, in a bed with Rhododendron and Osmanthus (heterophyllus Ogon, another experiment). I find it a favourable spot, allowing me to grow things like Crinum, Phygelius, the so-called tender Rhododendron 'Crest' and so on, for several years. There is some shelter from a surrounding hedge.The amended soil is slightly acidic and friable, with the natural soil being quite sandy/silty, and the drainage good.
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