Hi,
First time posting here. Much experience with cold tolerance in palms. Studied this since 1986.
Rhapidophyllum hystrix is most cold tolerant.
Our oldest flowered and produced a crop of fruit and seed in 2003; first palms we know of in Colorado to do that in 35 million years. Comfortable saying hystrix can go to -14F if soil is dry.
Sabal is good too. We have minor, and "birmingham," both of which are surviving, but not performing as well as Trachycarpus recently.
Trachycarpus are surprising us. Many failures at first, but a few years ago, learning from those failures, we developed a theory of equilibrium decline. These palms are never dormant, and while subzeroF always defoliated them, the bud often recovered. However if in the following growing season, they did not replace 100% of the lost mass, they entered "decline," where in a few years at best, the palm died, not from cold, but from exhaustion.
Equilibrium is a critical mass, when the crown of fronds achieves a unique balance, and the characteristic trunk develops. From that equilibrium point on, for every frond grown, one is shed.
If a Trachycarpus could be protected here in Zone 5b during those brief periods when subzero F cold occurs, usually only a few days each winter, limiting catastrophic cell and tissue collapse, a palm could enter a new growing season without the prerequisite of replacing lost mass, and instead increase mass, moving toward trunk formation equilibrium. It might be strengthened to more greatly resist lethal freeze, based on observation of Trachy experimenters in Zone 7. Our method was to construct a canvas tent in which the palm could be protected and kept dry, only when air temperature fell below 0F. The method was extremely successful. We currently have fortunei, takil, wagnerianus, nanus, and fortunei 'greensboro' in the ground, overwintering with minimal to no damage, for several years now.
This winter, an extraordinary development was observed. In early December 2005, we had a true Zone 5 period, with 36 sustained hours of subzero F cold, temperature bottom to -15F, coldest event in twenty years, and record cold for the date. I expected to see extensive damage on these Trachycarpus, based on the lab models of lethal freeze damage observed by Allen Hirsh and Dave Francko. Temperature inside the tent fell to at least -11F. When temperatures rebounded and the cover was removed, there was no visible damage to several of the palms. (Tissue collapse from lethal freeze is alwyas immediately visible) When I took leaf tissue samples for examination, I found cell walls in new growth exposed to subzero F cold last year, were up to 3x thicker than previously seen and measured growth. It appears Trachycarpus may have some adaptive ability after all.
Another observation in these palms is a peculiar creeping stem. While some less cold tolerant Trachycarpus exhibit a stem creep, some takil, wagnerianus and fortunei show a trait very similar to our alpine woody plants, in a sideways or even briefly upward growing root, to counter the effect of slope frost heaving, and quickly anchor the palm onto an alpine slope that creeps due to frost heaving and gravity. It was these Trachycarpus that also showed the massive thickening of cell walls, that resisted lethal freeze that we expected to severely damage them.
We are in an arid climate, and withold water most of the fall and all winter, limiting the palms to less than one inch total water per month. We found that the drier soil produces an osmolarity that increased resistance to lethal freeze temperature by several degrees F.
It appears Trachycarpus rank in order of cold tolerance, takil, wagnerianus and fortunei, but wagnerianus, with its smaller leaves, may eventually outperform all. Its osmolarity range is greater than the other species, suggesting it might be the most resistance to subzero F cold. There is also a great deal of hybridization, and fortunei, takil and wagnerianus might not be the separate species we think, but variations of a common ancestor, and adapted by changes because of specific and local Himalayans orogeny.
One final note. I am very skeptical about the so-called "tesan" claims, as most of those descriptions we observe in our specimens anyway. Importing plants is always risky, and severely limited by USDA regulations. Know them thoroughly before deciding to take a gamble. I personally doubt the tesan claims.
