On cold, sunny winter days, the trunks of young fruit trees are particularly vulnerable to frost. Their bark is still dark and very thin, as the protective bark only builds up over the years. Throughout the winter, it is therefore exposed to the strong fluctuations between day and night temperatures, especially if there is also snow, which additionally reflects the UV radiation. The temperature on the south side of the trunk is considerably higher than that on the north side. This can result in stress cracks, which serve as entry points for fungal or bacterial pathogens such as black bark blight (Diplodia), bacterial blight (Pseudomonas), etc.
Sunburn damage affects fruit tree trunks
As a result of climate change, sunburn damage in the form of sunscald occurs more frequently in summer. Even at air temperatures of around 30 to 35° C, the temperature in the trunk can heat up to 45° C. Excessive pruning in summer also promotes sunburn damage in the crown of the tree if it is no longer sufficiently shaded by the foliage.
To prevent frost cracks and sun plates, the trunk of young trees should be painted white. Ideally, this should be done at the time of planting. The white color reflects the sunlight and prevents the trunk from heating up quickly. This reduces the extreme temperatures of summer and the temperature differences of winter in the bark tissue. Another advantage of whitened trunks is that the still fine bark becomes unattractive to rodents.
The right time for whitewashing: what to consider
The whitewash should be applied in November or December if possible. A later date is also possible. It is important that it is only applied on frost-free and sunny days so that the paint can dry faster. As a general rule, the main branches should also be painted in addition to the trunk so that the crown of the tree is also protected from the effects of the weather.
To ensure that the paint holds well, the bark should be cleaned of loose pieces of bark, moss and lichen before application. Suitable tools for this are a tree scraper or a wire brush. Suitable products for painting are available from specialist dealers. An alternative to this is a self-made paint made from lime and wallpaper paste. However, this is not as weather-resistant and must therefore be renewed regularly.
