HELPING PLANTS SURVIVE THE COLD
This is a transcription of a talk given by Seymour Goldweber of the Agricultural Extension Service, Homestead, Florida, to a meeting of the Rare Fruit Council International.
Mr. Goldweber spoke about cold protection, or programming one's plants for winter. He said it really meant programming oneself in order to get ready for what might be a very serious problem. He said we who live in south Florida have the luxury of living at the end of a peninsula that is surrounded by great masses of water. One would expect constant temperatures, but the weather in this area is not very consistent, as evidenced this month of November which is ordinarily the most beautiful month weather-wise. This year we have had drizzling rain early in November, chilly cool weather and then it became very hot, and after that more rain. Mr. Goldweber commented that this does not necessarily forecast a cold winter as is commonly supposed.
He reported that in 1977 we had one of the most accurate weather forecasts in recent years. In preparing plants for winter weather, one must pay attention to the temperature and wind forecasts, and in order to keep alert, one should purchase a radio that tunes in solely to VHF weather. Many tropical trees do not tolerate freezing temperatures, and some such as breadfruit, mangosteen, etc. will not even tolerate down to the freezing temperature.
In preparing plants for a cold winter, they should be "in condition". Temperatures seriously effect the enzyme systems in plants. The enzyme system acts like the governor on an engine; it controls most of the activities of a plant. There are other things involved too, such as the condition of the stored material in the plant. Plants that are in poor condition, that have nutritional deficiencies or disease problems, or insect problems, or plants that have suffered from drought just prior to the cold, can be more quickly injured than plants that have been on good programs, have no deficiencies, and have all the stored foods that they should have for that particular period of time.
Plants that are subjected to gradually decreasing temperatures can also more easily tolerate cold. This slows down the respiration (the process of combining oxygen with the stored foods to release energy for plant growth) and the plant builds up more reserves. As the reserves are increased, the plants appear to be much more cold-hardy than plants that are not in good shape. Plants in poor condition may be damaged at higher temperatures.
Mr. Goldweber said that in the process of getting the plants ready for winter, one should try to control as much of the internal condition of the plant as possible. One cannot control the weather but can control the condition of the plant. Every time a plant comes out in a flush of growth, the reserves of food in that plant are markedly reduced. This seems to have a serious effect on the damage that can be done. Research on how that affects the enzyme system and how the enzyme system affects that is still in progress. Pure water freezes at 32°F, and every molecule of salt added to the water lowers the freezing point of that water. This gradually increases the concentrations of salts and sugars in those systems and continues to lower the freezing point, or the point at which damage can be done internally.
Mr. Goldweber remarked that then we have to go back to the question, what about the enzymes? There is the problem - it is not known what can be done about the enzyme systems except that it is known that the two work together, and plants that are in good condition appear to be able to tolerate more of the lower temperatures.
The other matter to consider is how to get plants in the condition of having high stored foods so that they will be able to resist cold and come on with a good strong bloom that will provide strong embryos in their fruits.
If your trees are deficient, now is the time to put on an application of a suitable fertilizer. Mr. Goldweber emphasized the importance of having moisture in the ground before applying fertilizer; it is even more important than applying moisture afterwards. He also stated that it is never too late to apply nutritional sprays. One of the reasons we have good fruit crops here in south Florida is because nutrients can be absorbed through the leaves, whereas if many of the nutrients were applied to these alkaline soils, they would be tied up in an insoluble form and do no good for the plants.
Pruning trees at this time of the year is not recommended because it disturbs the canopy of the trees, or in other words, the leaves which are the food manufacturing organs. Pruning can initiate a lot of new lateral growth that will reduce the food reserves and the resultant immature tender tissue can be severely damaged if there is a cold spell. The tree itself will have its reserves reduced by putting out new growth. In removing the leaves and young branches, all the stored material is removed; what is more, the tree becomes exposed to excessive radiation that takes place. The canopy tends to hold the heat in.
Mulching trees is another item to be given consideration. When cold weather is forecast, all mulches should be removed from under the trees. Mulches on top of the ground keep the heat in the soil and prevent that heat from rising and warming the tree. One should remember to replace the mulch after the freeze is over in order to protect against loss of moisture. Another advantage to removing the mulch is that it enables one to apply moisture which in turn compacts the soil and makes it a better conductor of heat; the trunk of the tree will be protected.
Young seedlings or small plants that cannot be protected by a sprinkler system may be covered. Do not use materials that are conductors of heat such as metals or glass as they are good conductors of cold. If planning to use covers, do not permit any of the material to touch the plants. In this way, the cover will reflect the radiant heat given off by the plant. Newspapers make good covers, as do blankets or sheets over frames, and plastics may be used if nothing else is available. One might place an electric light bulb under the cover, or even place a can of water under a tree as the water itself gives off some heat.
In south Florida, most of our freezes are called radiation freezes, which means there is a minimal movement of air. Cold air, which is much heavier than warm air, stays close to the ground. The density of the cold air that moves in determines how high that cold air mass extends above the ground. Therefore, we must be concerned with the differences in temperatures at ground level, at one foot, four foot, or whatever, above ground level. (This information is available from the VHF weather forecasts).
There are means of mixing the warm air that is overlayering the cold air (with fans) which, even if it raises the temperature only one or two degrees, might be the critical point. Heaters are no longer used in groves because they are too expensive, too difficult to store and also require a good deal of maintenance.
Banking, or piling soil up around trees in order to protect the bud union in young trees is another method of cold protection but is not practised in Dade County because we don't have sand.
Mr. Goldweber emphasized that the most effective means of protecting against cold is a properly designed sprinkler system. They are not the final word, but they are effective. It may be that one can only attach sprinklers to hoses in which event one can expect only part protection with some damage on the fringe areas, but they can be effective if there are enough of them. Water from sprinklers that goes over the tops of the trees releases heat as it goes through the air and raises the temperature. This method is effective if the trees you are protecting can stand 32°F temperatures without being killed or damaged. The trees will be covered with ice, even might be loaded with ice, but they will be protected. In 1977, temperatures in Dade County went down to the low 20s and trees were protected by sprinklers that were left on for 14 to 16 hours.
When using sprinklers on hoses, you have an under-the-tree sprinkler system which means that the heat lost from the water droplets rises and drifts through the tops of the trees. There was some freeze damage in the top centers of the trees watered from underneath, but this can be pruned out later. Using the over-the-tree system, some branches are broken because of the weight of the ice.
Once the water system has been turned on for cold protection, it is vitally important that it be left on. Washing the frost off the plants in the morning can be fatal, because unless water is applied continuously, evaporative cooling takes place which can be very damaging. If you have a home sprinkler system that operates off one pump and only functions in sections, Mr. Goldweber recommended that you select the section that you want to protect and concentrate on that. Don't try to rotate from one section to another.
Mr. Krome, a member in the audience, commented that in the event of a power failure, one should be aware that often the failure is of short duration and one should stay alert in order to reset the button to the transformer. In this way, the water can be supplied with only a few minutes gap in some instances.
Wrapping trees to protect the trunk against freezing is another means of cold protection. Mr. Goldweber commented that if the wrapping material can be kept totally dry, it will be somewhat effective, especially if the temperatures do not stay down too long. The wrapping material is just delaying the temperature drop inside the tissue long enough for the temperature to come back up from the critical levels. The tree does not have much ability to give off heat. There are some new plastic materials that have been developed to wrap and protect the trunks of trees but it has nothing to do with protecting the top of the trees. By keeping the trunk of the tree warmer, you do not circulate warmer sap into the top of the tree.
DATE: March 1983
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