Zone maps are not absolute guides to plant survivorship, With minimal winter protection, palms can be grown up to two hardiness zones colder than their generally accepted zonal limits.
Plants are individuals, and thus even within the same species or variety there is some variation in cold tolerance. How you care for them during the growing season is very important.
Summer care (water & fertilization). Plants require lots of nitrogen and phosphorus to manufacture proteins, sugars, genetic material and energy-storage molecules. Palms also need a host of other mineral nutrients that must be dissolved in the soil moisture so that they can be taken up by the roots to feed the palm to help them get through the cold spells they may encounter during the winter months. Hooked On Palms has several palm fertilizers to keep your palms healthy.
Plant age (very old and very young are more fragile). The smaller a plant is the more sensitive it is to environmental extremes.
Time of establishment (the importance of acclimation).
The care your palms receive during the summer affects their viability and vigor during the winter.
It is important not to pamper your plants too soon. A little cold weather actually aids the onset of dormancy and makes the palms more cold hardy.
Most of a Palm’s body is water. If the temperatures drop below the freezing point (32 degrees F) the water can freeze. The formation of ice crystals within the plant tissues can rupture living cells, causing cell death and tissue damage. If damage is superficial or localized the plant can recover, but if most or all of the plant is frozen, the entire plant may die.
All Palms and plants have a genetically programmed minimum temperature below which their cells will always freeze and die, this minimum temp varies greatly between the different types of palms.
In Palms, the primary (or spear) leaf often is killed due to freeze damage in the embryonic stem tissue. Most cold hardy palms that lose their spears or are defoliated due to extreme cold damage still survive and recover the next growing seasonif you follow a few simple steps that you will find at the sub page on this web site called Treating Cold damaged palms at this link www.hookedonpalms.com/preppingpalmsforwinter/treatingcolddamage.html
Keep in mind, cold-hardy palms can and do adapt to freezing temperatures.
Heed the Tree-Year Establishment Rule. It takes at least three full growing seasons for a newly planted palm tree or bush to become fully established.