COLD PROTECTION FOR TROPICAL FRUIT TREES
With the increased interest in growing tropical fruits in Far North Queensland more and more fruits will be grown in cooler areas than they are accustomed to.
If you happen to live in an area that gets light frosts or quite cold mornings, then a number of steps can be taken to improve success. Often it is just in the first few years of a plant's life that it needs protection. As a rule a tree becomes more able to withstand changes in environmental conditions the larger it is.
Knowledge of cold and frosts becomes helpful when planning the planting of your trees. Cold air flows like water, and will settle in the lowest possible areas. Good cold-drainage can eliminate cold pockets, however we must be aware that obstacles, such as houses, roads, and vegetation can stop cold flow. Planting on a slope as high up as possible or planting amongst other trees often gives encouraging results. Bodies of water trap and store heat and thus it is rare to find frosts nearby.
A healthy tree is less susceptible to cold damage, as also is a tree that has a full canopy of leaves. The careful timing of pruning so the canopy is full during winter is often the difference between success and failure.
Most cold weather in Far North Queensland occurs in the early morning when there is very little air movement. For the first 10 metres or so above ground there is a temperature inversion. It is the heat radiated from the soil, buildings and plants that is lost to the air. The colder the air temperature, the greater the heat radiation.
Besides planning measures, there exists a number of other practices to keep your tree warm. Probably the most effective is the covering of the tree to trap the radiation. This may be as simple as a bag placed over, but not directly on the tree, to a miniature greenhouse being built. Plastic coverings are good, but stay away from metals as they radiate heat rapidly, and as dew forms on the cold metal, evaporation takes place and further heat is lost. It is important to allow for air circulation during the day, otherwise lethal temperatures can be reached. In orchard development it may prove cost-effective to build quite a few mini-greenhouses of 2 or 3 different sizes to move from planting to planting as each gets older.
The baring of the soil around the tree increases the radiation into the canopy. Mulches tend to prevent heat entering and leaving. Moist soils conduct heat better than dry or cultivated soils. Compacted moist soils are best.
Sprinklers can be used in frost protection too. If turned on in the early hours of the morning and left on till all danger of frost has passed, they can save your trees. But if just wet intermittently, it can cause severe damage as evaporation can be the critical limiting factor in the cooling process. A dry tree is less susceptible to cold than a wet tree.
Wrapping of trees is an almost ancient practice, which has even been developed into an art at some Japanese temples. Materials such as straw, newspapers, blankets and even soil are used to cover as much of the tree as possible. The Japanese managed to cover, each year without fail, whole tender trees and palms to over 15 metres in height, with straw and bamboo mats. The principle is as an insulation, often just buffering a few degrees. This method of cold protection requires dry trees and often invites insect, rodent and rotting damage if left on for a long period of time.
Wind generators, air machines, even giant fans are employed in different parts of the world to mix cold and warm air together. This can raise the temperature by one or two degrees.
Smudge pots, or the burning of oil-soaked sawdust has also been used extensively, to create a cloud of dense, but warm air in the canopy of the trees.
Whatever the method or practice, tropical trees can often be grown in a sub-tropical area with cool winters. Growth is slower, but the rewards are worthwhile.
DATE: July 1985
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