Although it is still winter, and in early March the air temperature in Central and Eastern Europe oscillates around 0°C, local orchards are slowly waking up from their winter dormancy. What is overwintering of plants and why is its assessment so important?
Fruit species cultivated in European climatic conditions require a period of winter dormancy, i.e. a period of cooler temperatures, to flower and bear fruit. Trees have two stages of winter dormancy. The first stage – winter rest – is quite brief lasting from November to the end of December, and is very strong. It is characterized by the fact that even significant warming does not stimulate tree development. After this time, winter rest is interrupted and the stage of after-rest 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 period lasts until the end of winter and it is critical for the occurrence of any frost damage. At this time, any increase in temperatures causes dehardening of plants and stimulates their development. If the warm period is short, plants can reharden.
The situation becomes more dangerous when, in winter, after a longer warm period, there are sudden and strong temperature drops. Then, serious losses can occur, because plants are not able to reharden.
Winter 2022/2023 in Europe
So far this year’s winter in Europe has been quite mild, but it has not yet said its last word. There were years during which, even in March, significant and strong temperature drops damaged trees waking up from winter rest. Also spring frosts are dangerous for trees, and especially for their fruiting potential, which in the period from the second half of April to mid-May can destroy flower buds, flowers, and even the youngest fruit buds, significantly reducing the future yield. However, European fruit growers will face this problem when the need arises. Meanwhile, it is worth focusing on the evaluation of orchard overwintering.
Although the winter of 2022/2023 was not frosty in Europe, fruit growers assess the wintering condition of trees in their orchards during the pre-harvest season. In March, effective measures can still be taken to help regenerate possible frost damage. Trees growing on dwarf rootstocks (type M.9) and young plantings (one- and two-year-old) are the most vulnerable to winter damage. Overgrown trees, over-fertilized with nitrogen and those planted in frost hollows, freeze more often.
To save the trees
In order to assess the overwintering status of trees, fruit growers cut trunks and thick branches of trees (bark and wood) with a knife, as well as shoots and buds on long and short shoots. Browning and black streaks indicate their frost damage. Shoots and branches with minor damage often regenerate on their own, more severe damage requires action. The most dangerous symptoms of frost damage to trees are longitudinal cracks in the bark observed at the base of the trunk, reaching up to 20 cm in length. Such extensive wounds are very dangerous for trees. They can contribute to their death and also be a source of infection with bark and wood diseases. Such wounds are treated by nailing the bark back to the wood (with small nails) and smearing the wound with a healing emulsion. Trees can also form gangrenous wounds. In such a case, the bark collapses, turns brown, dries and flakes. Such damage is treated by cutting the wood to healthy tissue, and the wounds are also protected with a suitable emulsion. These activities, although time-consuming, are extremely effective in the case of apple trees and allow the trees to recover before the growing season begins.
European fruit growers also assess the degree of bud frost damage. To do this, they take twigs with buds from the trees and place them in a container with water at room temperature. Such twigs start vegetation and after a dozen or so days their condition can be assessed, i.e. the health and type of buds (leaf or flower), which will determine the intensity of flowering during the season. If the buds are in poor condition, it is recommended to fertilize at the beginning of the growing season. First of all, it is important to properly nourish the rosette leaves with nitrogen - they are responsible for the production of assimilates and the nutrition of flower buds. This element in several small doses (preferably three) is applied to the soil before flowering. It is also worth supplying trees with potassium because it has properties that accelerate the regeneration of this type of damage.