The mild course of this year’s winter in Europe means that trees are currently not at risk of frost losses. However, a sudden cooling may change this situation. How to protect trees against the effects of sudden and strong temperature drops? How and when should it be taken care of? Which trees are more resistant to harsh winter conditions?
Individual species of trees grown in European climatic conditions have different resistance to frost. Apple trees are the most resistant, followed by cherry trees, pear trees, plum trees and sweet cherry trees. Peach and apricot trees are the most affected by frost. Nowadays, the quality of wintering trees does not have to be determined only by the appropriate selection of species for planting. Fruit growers can take care of the proper preparation of trees for winter, even during the growing season.
In the second half of summer, European fruit growers reduce nitrogen fertilization, stop performing heavy pruning and abundant irrigation. All this affects the inhibition of tree vegetation, and in order to properly prepare for winter, trees must finish their growth, accumulate nutrients (which include proteins, carbohydrates and some minerals) and enter a dormant stage - and thus increase their resistance properly. The first stage of developing resistance begins in early autumn when the days become shorter, the second stage results from a gradual decrease in air temperature, and the third occurs under the influence of frost. And only such resistant trees should enter the wintering period.
There are two stages of winter dormancy of trees. The first stage – absolute dormancy – lasts quite a short time, from November to the end of December, and is very harsh. It is characterized by the fact that even significant warming does not stimulate trees to develop. After this time, absolute dormancy is interrupted and the stage of relative dormancy begins. At this stage, dormancy is conditioned only by the presence of low temperatures, and the plants are ready to start development at any time. This stage lasts until the end of winter and it conditions the occurrence of any frost damage. At this time, any increase in temperature limits the plants’ resistance and stimulates their development. If the warming period is short, the plants regain their resistance (however, this re-acquired frost resistance is already weaker than before the period of higher temperatures). In winter, sudden and strong temperature drops after a longer warming period and in the phase of relative dormancy are very dangerous. Serious losses may occur because the plants are not able to regain their resistance.
This is where good agricultural technology comes in handy. It was found that trees with larger and undamaged leaves during the growing season better tolerate unfavourable weather conditions in winter, in contrast to those with a limited assimilation surface of leaves, and less nourished. It’s not just about fertilization during the growing season, but about providing the trees with nutrients before the dormancy period. It is observed that trees with abundant, and especially excessive yield, even despite proper nutrition during the growing season, are subject to greater damage in winter than those with optimal or poor yield.
Striving to increase the productivity of orchards, European fruit growers must ensure proper post-harvest nutrition of trees tired of heavy yields. Important ingredients introduced into the soil at this time are calcium, magnesium, potassium and phosphorus. Foliar fertilizers include boron and zinc, as they improve the quality of flower buds and increase the resistance of plants to frost (often with microelements and biostimulators based on natural amino acids of plant origin). They also apply phosphorus and potassium, as well as ingredients the deficiencies of which were observed in trees during the growing season. It is important that these treatments are performed while the leaves are still green and fully active.
Currently, trees on dwarf M.9 rootstocks are mainly used to establish orchards in Europe. They are susceptible to damage by low temperatures and therefore trees growing on them, especially during snowless and cold winters, should be properly protected by spreading compost, manure or peat substrate around their trunks.