Winter hardiness is in the trees’ genes. Trees that are found in nature in mild climate zones are obviously not as frost resistant as trees from colder regions. Nevertheless it is also possible to ‘harden off’ individual trees. In the autumn, every tree builds up its winter hardiness once again. For the best possible hardening, the plant has to be exposed to temperatures under 5 °C for several nights. In addition, trees mustn’t be transplanted too quickly afterwards to regions that are considerably colder, such as elevated regions.
In the spring, as it gets warmer, the winter hardiness of all plants lessens. If the temperature stays above 15 °C for a few days, winter hardiness can decrease significantly.
Plants can be damaged by late frost, particularly in regions with föhn wind or when there is extreme cooling of the air after a warm spring. This means that when evaluating frost resistance, besides the absolute minimum temperature, it is also important to know when the plant is exposed to frost and to what degree the plant is hardened off.
Damage occurs when the plants are replanted too soon, in other words when they aren’t yet ripe. In view of this, time of delivery and proper handling on site (cool storage) are very important. This is certainly true for moving to areas with large temperature fluctuations and for valleys with föhn wind, but it also varies per tree. If trees bud early, which is determined by the genetic makeup of a plant, that is more likely to lead to damage in certain situations.
In order to be able to protect trees, it is important to know the potential causes of damage. That is particularly difficult with indirect frost damage. For example, after persistent severe frost, damage can occur through dryness that is very similar to pure freezing. Certainly the combination of frost, dry wind and strong winter sun forms a considerable danger. Evergreen trees cannot draw moisture from frozen soil and dry out under a glaring sun and strong winds. It is beneficial for these trees to be protected during their first few winters. The same applies for trunks of trees that have just been planted. But wrapping a reed mat around the trunk, they are well protected against the sun.
Information about the winter hardiness of special plant species and varieties can be found in the book ‘Van den Berk on Trees’. There is a USDA winter hardiness zone listed for every plant. This classification of plants in winter hardiness zones indicates the resistance of specific species and varieties in relation to the average minimum temperature. You can also ask the Van den Berk Nurseries employees for advice.