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| Ways to protect trees over winter To help trees survive over winter, there are some methods available for retaining heat in an orchard. Raising temperatures by as little as 1.5–2ºC can mean the difference between a bad burn and the death of a tree. Young trees are very sensitive to frost, so some protection is needed, such as wrapping the trunk with sponge, or covering the entire tree. In Argentina, young trees are wrapped with dry grass during winter. The time of year a frost occurs can determine how much crop damage is done. If the frost comes early, before temperatures have a chance to drop gradually, trees won’t have had time to acclimatise themselves, resulting in severe damage. If the frost hits later in the season, when temperatures have had a chance to decline gradually, trees will have developed a better cold tolerance. Soil absorbs heat from the sun during the day, which is then released back into the air at night (reradiation) and warms the orchard. Compacted soil conducts heat better than recently cultivated soil, and moist soil conducts heat better than a dry one, so if there is danger of a frost, don’t cultivate and keep soil moist to a depth of at least 30cm. Soil temperatures below 13ºC can affect water uptake by the roots. Weeds, grass and leaf litter between rows reduces soil absorption of the sun’s heat, which reduces reradiation. Grass should be kept short to maximise heat absorption and improve air circulation. Tree canopy size can also affect the amount of reradiated heat the tree captures. A large, fairly dense canopy (which can be achieved with proper pruning), can collect more heat reradiating from the soil than a small, open canopy. Another frost-protection method is the use of wind machines, which can be both an effective and economical way of providing frost protection (but is only really available to large-scale growers due to the costs). However, in some situations, such as cold polar winds, they can make things worse by drawing in dry cold air, which then intesifies the cold damage. They should be started before temperatures drop below 1.5ºC, after which it becomes difficult to rewarm the orchard. If you plan to use wind machines make sure you talk to an expert about your particular conditions and needs. Next – pests and diseases
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