This section is from the book "The Gardener's Monthly And Horticulturist V21", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
The following valuable paper, was read at a recent meeting of the Illinois State Horticultural Society:
We all know that many plants are killed by even a short exposure to a freezing temperature, and that others, ordinarily considered Winter-proof with us, are destroyed by an unusual degree of cold, especially in certain conditions of the plant tissues. It is further understood that, in some cases at least, quite as much depends upon the manner of thawing, as to the final effect upon the plant, as upon the freezing. Somewhat tender plants supposed to be frozen, and certainly coated with ice, may frequently be saved by placing them in cool, shaded positions, or in cold water while thawing. So apples in tight barrels, shaded from the sun, or buried in the ground, may be subjected to almost any degree of frost, and if afterward slowly thawed in these situations, are slightly or not at all injured, while if thawed in the sun they quickly decompose.
What is the explanation of these and similar phenomena with which every one is familiar? No doubt some of the changes and their causes are not yet well understood, but much certainly is known, the main portions of which can be given in a few words.
If fermented cider is exposed to a temperature below that at which water solidifies, some persons have found out that the part which becomes ice is not cider at all, but upon removal and thawing is very near pure water, while the unfrozen portion is cider with emphasis. On one occasion a very estimable young man, well known to the writer, at an evening party learned this latter fact in such a practical manner that he has never forgotten that apple cider kept in a barn in Winter and drawn by boring through ice is intoxicating. Those who hang wet clothes upon lines to dry in Winter speak of them as freezing dry. The fact is the water, closely held in the fabrics by capillary attraction, is extracted, brought to the surface, by the action of the frost, and is carried off by the air currents. The withdrawal of the water from the cloth is precisely similiar to its separation from solutions, as cider, sweetened water, salt water, etc. The process is somewhat as follows: Upon the withdrawal of heat, the molecules composing water, previously moving freely with little or no friction upon each other, thus forming a true liquid, approach and cohere, forming a solid.
But the attraction which thus binds together the molecules of water does not exist between those of water and the commingled molecules of other substances, hence these latter do not become attached; they remain floating in the still liquid portion. Ice having formed upon the surface, and consequently withdrawn particles of water from this immediate portion, equalization takes place in the remainder through the law of diffusion. Thus other molecules of water are brought in contact with those solidified and the ice becomes thicker from additions upon the liquid side. Diffusion and equalization again taking place, the process is continually repeated, the pure water being extracted and the solution gradually condensed. The more watery the solution the more readily the operations are effected, that condensed and thick yielding slowly or not-at all to the forces tending towards the result.
Now, all this is true in the case of the semifluid substances of plants. Green leaves and young stems may be observed coated with ice from the water extracted from the tissues, and yet close observation may show that the tissues themselves are still flexible, not frozen. This is beautifully shown in the little plant, often used for bedding purposes, known on account of the peculiar shape of the flower as the cigar-plant. If subjected to below 32° Fahrenheit in a damp still atmosphere, the roots being in moist soil not too cold, a layer of ice will be formed around the young stems of such thickness that it may be taken off as stout scales, separating readily sometimes from the still flexible plant. Upon close examination this ice is found to be composed of elongated crystals, radiating from the stem. Having received accessions from the plant end, the older portions of these crystals have been continually pushed outward. This loss of water leaves the half-fluid substances of the plant thicker or more condensed, unless, as may happen, the roots have kept up the supply. Often the ice forms within the tissues instead of outside, but it may then be found in localized places, the cells, perhaps, having been forced asunder by its expansion.
In this case the collection of the ice crystals is the same as indicated above. The simple rupturing of cells, or their forcible separation, though not a good thing for the plant, need not cause death. By the extraction of heat and water the bark and outer layers of wood may shrink with such force as to cause cracks more or less deep in trunks and large limbs, but the tree may survive without apparent injury and the wounds rapidly heal.
If after the water of the cell-sap is thus partially congealed, - either without or within the tissues, - thawing take place very gradually, it may be again absorbed and the normal condition restored. Anything which arrests evaporation, as sprinkling or plunging in water, has the same effect. The condensed, partially dried protoplasm and cell-sap receive the necessary amount of water to permit them to perform their usual functions, and life and health continue. But should the separated water be removed, death occurs from the dryness of the tissues. With rapid thawing the water runs away instead of becoming absorbed. With a small amount of ice-formation, and so little disturbance of the fluids, recovery upon thawing is easy, but becomes more and more hazardous the lower the temperature to which the plant has been exposed. In like manner the same degree of cold has very different effects, according to the proportional amount of water in the plant. When very much, as in most rapidly-growing parts, freezing occurs sooner and progresses to a greater extent than when the relative amount of water' is less, as in the Winter condition of trees.
Every school-boy knows that when ordinary black ink freezes, and subsequently thaws, it is no longer the homogeneous fluid it was before, but consists of water and separated particles of pigment; so starch-paste, frozen and thawed loses its adhesiveness, the water and solid portions not again uniting. Eggs behave in the same manner.
Just so it is with plants. Having passed a given point in the disturbance of the normal composition of the semi-fluids, varying with the species, the primary condition is no longer regained when thawed. Sap is no longer sap, but consists more or less of solid particles and water. As the roots cease to perform their functions when exposed to a given amount of cold, death may take place from the fact that while the drying effect of freezing proceeds above, the required corresponding supply is cut off below. Hence trees deeply planted and well established would not theoretically be so liable to injury by freezing, though the trunk and limbs be equally exposed, as those whose roots lie near the surface, or are in any way defective. It is said that the English ivy is capable of withstanding our winters if the roots are set below the frost line in the earth, and on the northern side of a building. It is certain that a vine has occasionally survived among the many that have perished, and this may be the explanation. As the roots of our ordinary cultivated plants are not adapted for healthful action in water itself, but only in a moist soil, another argument for deep drainage is presented.
Soil drained at least three or four feet deep, naturally or artificially, appears to be necessary to preserve the 'tops of trees from injury by frost, and six to ten feet would be still better as a safeguard. Early maturity and perfect ripening of wood are well known as the best prophylactics in this line at our command.