“Good timing increases winter hardiness”

Questions about the winter hardiness of the trees provided by Van den Berk Nurseries are frequently asked in certain parts of Germany, Austria, Switzerland, Scandinavia and Eastern Europe. A multitude of deliveries throughout Europe have proven that the trees from Van den Berk Nurseries have no problems in colder regions due to the minimum temperatures of -5 to -10 °C (source: Royal Netherlands Meteorological Institute) that trees in the nursery in the Netherlands experience regularly. We asked Mr Claus Heuvemann of Van den Berk Nurseries to explain the winter hardiness of trees.

What influences winter hardiness?
“Winter hardiness in trees is genetically determined. Trees that thrive in temperate zones are, of course, less resistant to cold than trees from colder regions. However, it is also possible to ‘harden’ individual trees. Each tree regains its winter hardiness every autumn. For optimum hardening, the plant must be exposed to night time temperatures under 5 °C for several nights. Moreover, there must be a delay before the trees are planted in regions where it is much colder, e.g. areas at a high altitude.”

How can you protect trees against frost damage?

“It is important to know the reason of the damage. This is especially difficult with indirect frost damage. For example, damage can be caused by persistent severe frost and dryness that is very similar to pure freezing. The combination of frost, drying wind and intensive winter sun is certainly a significant threat. Coniferous trees are unable to draw moisture from the soil and dry out due to the effects of the strong sun and dry wind. These trees benefit from being well covered during the first few winters. The same applies to the trunks of newly planted trees. Wrapping a reed mat around the trunk protects it from the sun.

More information on winter hardiness
Information on the winter hardiness of individual plant types and varieties in the extensive range stocked by Van den Berk Nurseries is contained in the book ‘Van den Berk on Trees’. The USDA winter hardiness zone is indicated for each plant.

This classification of plants into winter hardiness zones shows the resistance of certain types and varieties in terms of the average minimum temperature.

Naturally, you can always ask Van den Berk Nurseries for advice.

Is winter hardiness connected with the temperatures of the previous winter?
“This is certainly not the case. Trees do not have a ‘temperature memory’. In the spring, when temperatures rise, the winter hardiness of all plants decreases. If the temperature remains above 15 °C for a few days this may significantly reduce the winter hardiness. This is especially the case with hot, dry winds or if extreme cooling of the air takes place after a warm spring. Late frost can severely damage plants. This means that the evaluation of frost resistance involves important factors such as the first contact with frost and the extent of hardening in addition to the absolute minimum temperature.”

When can frost damage occur after movement?
“Damage occurs when plants are transplanted too early. In other words: when they are not yet ripe. In this regard, the correct delivery time and handling on site (cool storage) are extremely important. This certainly applies to movement to regions with major temperature fluctuations and valleys with hot and dry winds. But each tree is different. Some types of winter hardy plants are also sensitive to frost. Early shooting frequently results in damage. This does not depend on the supplier and is determined by the genes of the plant. Nevertheless, these plants can also be used in other regions without problems.”