Winter hardiness: different from tree to tree

Winter hardiness is in the genes of trees. Understandably, trees which naturally occur in temperate zones are less tolerant of frost than trees from cold regions. The maps show Europe’s different climate zones with their corresponding minimum temperatures and the trees are classified according to their resistance to frost. Using the tree filter on this website, you will be able to find which trees suit which zones in terms of winter hardiness. The right winter hardiness zone can be selected under 'site requirements' filter.

Hardening of trees: every year again

What’s more, each tree rebuilds its winter hardiness in autumn. For optimum hardening the tree has to be exposed to temperatures below 5ºC for a few nights, after which they should not be transported too quickly to considerably colder locations, such as upland areas. Make sure that a tree hardens off and builds up proper winter hardiness before transplanting it. Damage can occur if trees are transplanted too soon and this certainly applies in areas with large temperature variations and valleys with a Föhn wind.

In spring, as the weather gets warmer, the winter hardiness of all trees decreases. If the temperature remains above 15 ºC for a few days, the winter hardiness can be considerably reduced. In areas where there is a Föhn wind or where there is extreme cooling off of the air after a warm spring, late frost may cause damage. Sensitivity varies from species to species. Trees which sprout early on - something which is determined by the genetic predisposition of a plant - are at a greater risk of damage. If we put all of these factors together, we can see that when evaluating frost resistance, not only the absolute minimum temperature, but also the hardening off and the time of transplanting are relevant.

Protect trees from frost

In order to protect the trees it is important to be aware of possible causes of damages. Indirect frost damage in particular is not always straightforward. For instance, after a persistent and harsh frost, drought damage can occur which bears many similarities to pure frostbite. The combination of frost, drying wind and intense winter sunshine certainly poses a great threat. Evergreen trees are unable to extract any moisture from the ground and dry out when faced with bright sunshine and strong wind. Such trees benefit if they are properly sheltered during the first winters. The same thing applies to the trunks of recently planted trees. Wrapping a coconut mat around them will protect them against the sun.

You can find information about the winter hardiness of individual plant species and varieties under the species descriptions in the tree filter. A USDA plant hardiness zone is stated for each tree. This classification of trees into hardiness zones indicates the resistance of certain species and varieties relative to average minimum temperatures. The following maps show the various zones.

Click here for the climate zone maps.

Trees for this application