The following information on frost damage prevention comes from Five Acres and Independence by M. G. Kains. Five Acres and Independence is also available to purchase in print.
The killing of plants and plant parts by frost may often be prevented by simple, inexpensive, easily applied means. Before we discuss these, however, let us see what frost is and how it acts.
Frost is the term which indicates the conversion of a liquid into a solid by the reduction of temperature. Though this definition covers all cases such as the solidification of molten iron and other metals it is popularly understood to mean the formation of ice from water at the temperature of 32º F.
As air cools its power to hold water in vapor form decreases until it deposits more or less in tiny drops (dew) on objects cooler than itself, such as foliage. The temperatures at which this deposition occurs depend upon the proportion of water vapor in the air at the time. This varies as the cooling proceeds. During the summer the dew point, as the deposition or saturation temperature is called, is often above 60º; in winter often below zero.
When the dew point is below the freezing point the water vapor condenses on still cooler surfaces in the form of fine particles of ice, which because they reflect the sunlight and appear white we call hoar or white frost. Often a similar appearance or “false hoar frost” occurs when the temperature is several degrees above freezing point. Usually this is due to the way the light is reflected from the dew.
The condensation of water vapor tends to check the fall of temperature because what is called “latent heat” in this vapor is returned to the air which, scientific instruments prove, becomes measurably warmer. Thus within variable limits the deposition of dew protects plants from frost damage as the temperature approaches or in some cases even goes below the freezing point. This accounts for many escapes of plants that would have been killed by frost had the dew point been below, instead of above the freezing point. We can often take advantage of this phenomenon and save our plants, as explained further on.
Plants vary in their resistance to frost damage according to their origin and their condition. Those which originate in a cold climate (apple, cabbage) naturally are strongly resistant; those from a warm climate (orange, tomato) are weak and easily destroyed. Between these extremes are many intermediate grades. Those plants that, in a given locality, live through the northern or alpine winter in spite of deep freezing of the soil are called hardy in that locality; those that succumb to the slightest frost are tender, and those between are variously classed as half-hardy, semi-hardy, half-tender and semi-tender (fig, French artichoke).
Tender plants are usually injured more or less when the temperature continues at 32º, 31º, or 30º for several hours, especially when bright sunshine or quickly warming air strikes them in early morning. Few if any of them can stand lower degrees than these for even a short time; half-hardy plants often survive temperatures of 20º, but seldom lower degrees.
In all these cases plant condition plays an important part; for plants that have made rapid growth, have soft, immature tissues and are full of water, are far less resistant than those which have developed slowly, have denser, stockier growth and are less full (perhaps even in need of) water. This statement applies to hardy as well as tender plants; for even trees normally hardy in a given locality may be winter-killed because they made a late, sappy growth which did not ripen or which was full of water when cold weather arrived.
The mere deposition of frost on the surface of foliage does not necessarily indicate that the plants have been killed or even damaged. But when the air is too dry for dew to be deposited they may be frozen by a dry wind, or on a clear night without the deposition of either dew or hoar frost. In such cases damage is due to freezing of the water inside the plant and the consequent rupture of the tissues. When the sun shines on tissues thus injured the internal ice melts, the leaves have no chance to mend the broken cells, so the leaves droop, wilt and turn black; hence the term “black frost.”
Anything that will prevent the fall of temperature to or below the freezing point and anything that will shield the plants from direct sunshine while they are still frozen, covered with hoar frost or severely chilled will help ward off damage or even save plants that would otherwise die or be seriously checked in their development. A wind that springs up in the evening, clouds that appear during the night or early morning, or a rain that follows a frost will often either prevent the freezing or check the thawing process and thus save the plants. All these phenomena of Nature are, of course, beyond our control but we may imitate them as outlined further on.
We can largely control the development and therefore the hardiness of our plants in several ways. For instance, during spring we can avoid over-feeding our young plants with stimulant fertilizers such as manure and nitrate of soda and also avoid giving them excess of water. Both these tend to make sappy growth easily killed by frost. On the other hand, by keeping the plants cool, almost cold (“hardening them off”) as they approach the time for transplanting to the open ground we can increase the hardiness of hardy, semi-hardy and even tender plants. Plants so prepared will stand cold snaps whereas those of the same species not so inured would probably be killed or so chilled that they would “sulk” for several weeks before recovering or renewing a normal rate of growth.
Similarly we may prevent winter injury of hardy trees, shrubs and vines by supplying ample water during summer and early autumn, withholding it later, avoiding applications of stimulant fertilizers and manures from midsummer forward, counteracting any excess of these by liberal dressings of potash and phosphoric acid during early fall (Chapter 25) or by sowing buckwheat in July or rye in September or both these together in July. As these crops grow they remove excess water and nitrogenous plant food from the soil and develop plants which when plowed or dug into the ground in spring return the plant food and their own bodies to form humus.
Fortunately we can predict accurately enough for practical purposes when to expect frost. The daily forecasts by the United States Weather Bureau give suggestions as to the general weather to expect; but we can make our own observations and predictions. Local conditions influence temperature. For instance, a near-by body of water, such as a lake, the sea, a wide or a deep river, or even a large pond affects the rate at which air temperature changes. In spring, because the water is cold, it keeps the air also cold and thus more or less retards plant development. In autumn the reverse effect occurs; the water being warm not only warms the air but fills it with water vapor thus warding off frost.
Open and flat country and small villages are more likely to suffer from late spring and early autumn frosts than are large cities and their near-by suburbs because, in the former, heat loss by radiation into space is more rapid in clear, clean air and under cloudless skies than in the latter where the air is filled with smoke and dust and where the fires in countless houses, factories and other buildings directly raise the temperature.
Dark colored, sandy and well drained soils absorb and hold more sun heat than do light colored, clayey and poorly drained ones so are less likely to be frosty. Other conditions being equal, southern and eastern slopes are also warmer than western and northern ones because they more quickly absorb the sun’s rays. Though this favors earliness of plant development it often makes the growing of certain fruits (apricot, peach, Japanese plum, more especially) precarious or impossible because the flowers are encouraged to open so early that spring frost kills them and thus prevents fruit production, though not usually killing the trees.
We can discover for ourselves that cold air, being heavier than warm air, flows like water from high to low ground and “settles” in hollows or “pockets” unless it can drain to still lower levels or be driven out by wind; that frosts are much more likely to occur when the air is still, the sky clear and the stars brilliant than when there is wind or clouds, especially when the former is strong and the latter cover the whole sky. The direction and force of the wind also help in making a local forecast. One that blows strongly from the north is far more likely to bring cool or cold weather than one from the south, just as one from the east is likely to bring clouds and rain and one from the west clear skies and colder weather. The rate at which the barometer rises also helps because it indicates the approach of clear weather and, if rapid, also of cold weather.
An unusually warm spell is almost sure to be followed by a cooler or cold one because our general weather moves in prodigious waves from southwest to northeast across the country. Hence a light frost following a warm spell is likely to do more damage than an even more severe one following cool weather. For this reason we should be on our guard when one of these warm spells occurs in spring—be ready to protect our seedlings, newly transplanted plants and the flowers on our fruit trees and bushes.
When the sky is cloudy, when there is fog or even when a haze occurs during or toward evening, frost is less likely to occur than when the night is clear because these conditions of moisture in the air prevent loss of heat from the earth.
A reliable sign of approaching frost is the rate at which the temperature falls during the late afternoon and early evening. Starting with 50º or less, clear skies and no wind, a fall of 2º or more an hour between four and eight o’clock usually indicates that freezing temperatures will be reached before morning unless clouds or winds develop or unless we do something to prevent frost.
In a small way individual plants may be protected by inverted flower pots, peach baskets, and other receptacles placed over them, by newspapers spread and held in place by stones or clods of earth. A more convenient adaptation of this way is to use a light screen of burlap mounted on a frame placed over the plants or beds. These all tend to hold the heat around the plants.
Smouldering fires which produce abundant smoke and steam form artificial clouds which check radiation in the same way as do true clouds. When the air is still the smoke spreads out evenly and proves effective as a protection nearly as far as the clouds extend. This method is infeasible where the smoke would prove objectionable to neighbors. Numerous small, bright fires of wood, coal or (preferably) oil are used extensively by commercial growers of fruit and vegetables to heat the air. They are less useful in small areas than the methods already presented and those that follow.
The most generally feasible method is to fill the air with water vapor by sprinkling the plants, the ground and the adjacent area with a hose nozzle that breaks up the water into small drops, or by using an over-head irrigation system for this purpose. The water evaporates and as the vapor condenses it liberates latent heat and thus checks the cooling process.
Freezing of the ground may injure even established trees, shrubs and vines of some kinds, so anything that will reduce the depth of frost penetration or prevent alternate freezing and thawing will tend to prevent such injury.
Experiment has proved that under a sod freezing reached a depth of 8″ whereas in an adjacent sodless area it reached 18″. Peach trees on the sod ground made healthy, uniform growth whereas in the sodless soil they were slow to start, had many dead branches and made poor development.
In another experiment, just before winter a few forkfuls of manure or shovelfuls of soil or peat were banked around the trunks of exceptionally vigorous peach trees with the result that every tree so treated came through the winter without injury whereas a few not banked all died.
In order to have extra early beans, corn, melons and cucumbers I have often sown seed much earlier than was locally popular, thus risking frost damage. When no frost occurred I was ahead of competitors and when it did come I usually saved the plants by one of these methods. I have never thus risked transplanting eggplants, tomato or pepper plants because, even though not frozen, they “sulk” if chilled and start fruiting late.
In case you have not protected your plants and a frost has occurred during the night you may be able to save them, even those covered with frost or whose tissues have been ruptured by freezing of their sap, provided you will spray them with cold water as soon as possible after dawn or before sunrise and also shield them from direct sunlight after the sun appears until they have thawed out and apparently resumed normal activity. Better keep them so shielded until between ten o’clock and noon. The most conveniently applied shield is the screen mounted on a frame with short legs.
When water freezes it swells and lifts the crust of frozen earth above the unfrozen ground below. As it does so in autumn and early winter it also lifts shallow rooted plants, roots and all. When it thaws the soil settles back but the plants do not. They are left with more or less root exposed. Each succeeding freeze lifts them some more and each thaw leaves them farther out of the ground with the result that they dry and die. Hence the importance of applying a mulch in the fall.
In the spring equally fatal results may follow unmulched plants because when the surface thaws above a lower layer of still frozen earth the thawed layer settles and when it later freezes and lifts it breaks the roots of small plants by pulling them. Hence, again, the importance of a mulch; for beneath a sufficient layer of such loose material heaving and settling are reduced to a minimum and thawing of the ground proceeds from below upward until the mulched soil has thawed out and thus eliminated danger of root breakage.