Part 15

The sharp winter frosts produce, without doubt, many other injuries to trees, and we have remarked many defects, which we might attribute to them with great probability; but, as we have not been able to verify the fact, we shall pass on to the effects of the advantages and disadvantages of different expositions with respect to frost; for this question is too interesting to agriculture not to attempt its elucidation, especially as various authors have supported an opposition of sentiment more capable of breeding doubts than increasing our knowledge. Some have insisted that the frost is felt more strongly at the northern exposition, while others assert it is more sensible to the south or west, and all these opinions are founded on a single observation. We nevertheless perceive what has caused this diversity of opinion, and we are therefore enabled to reconcile them. But, before we relate the observations and experiments which have led us thereto, it is but just we should give a more exact idea of the question.

It is not doubted that the greatest cold proceeds from the north, for that is in the shade of the sun, which alone, in sharp frosts, tempers the rigour of the cold; besides, a situation to the north, is exposed to the north-east, and north-west winds, which are clearly the most intense, whether we judge from the effects which those winds produce, or from the liquor of the thermometers, whose decision is much more certain. It may also be observed along the espaliers, that the earth is often frozen and hardened all the day towards the north, while it may be worked upon towards the south. Moreover when a strong frost succeeds in the night, it is evident, that it must be much colder in the part where it is already formed, than in that where the earth is warmed by the sun; this is also the reason why, even in hot countries, we find snow in the northern exposition, on the back of lofty mountains: besides, the liquor of the thermometer is always lower at the northern exposition, than in that of the south; therefore, it is incontestible, that it is colder there, and freezes stronger.

It is therefore certain, that all the accidents which depend solely on the power of the frost, will be found more frequently at the northern exposition than elsewhere. But yet it is not always the great power of the frost which injures trees, for there are particular accidents, which cause a moderate frost to do them more prejudice than the much sharper, when they happen in favourable circumstances. Of this we have already given an example in speaking of that part of dead wood included in the good, which is produced by the hoar frost, and is found most frequently in the expositions to the south; and it is also to be observed, that great part of the disorders produced in the winter of 1709, are to be attributed to a false thaw, which was followed by a frost still sharper than what had preceded; but the observations which we have made on the effects of spring frosts supply us with many similar examples, which incontestibly prove it is not in the expositions where it freezes the strongest, that the frost commits the greatest injuries to vegetables. Not to dwell upon assertions, we shall proceed to a detail of facts, which will render these general positions clear and apparent.

In the winter 1734 we caused a coppice in my wood, near Montbard in Burgundy, to be cut, which measured one hundred and fifty-four feet, situated in a dry place, on a flat ground, surrounded on all sides with cultivated land. In this wood we left many small square pieces without felling them, and in a manner that each equally faced east, west, north and south. After having well cleared the part that was cut, we observed carefully in spring the growth of the young buds; the renewed tops on the 20th of April, had sensibly shot out in the parts exposed to the south, and which consequently were sheltered from the north by the tufted tops; these were the first buds that appeared, and were the most vigorous; those exposed to the east appeared next; then those of the west, and lastly those of the northern exposition. On the 28th of April the frost was very sharp in the morning accompanied by a north wind; the sky was clear, and the air very dry, and in which manner it continued for three days. At the end of which I went to see in what state the buds were about the clumps, and found them absolutely blackened in all the parts exposed to the south and sheltered from the north wind, whereas those which were exposed to the cold north wind, which still blowed, were only slightly injured; and with respect to the eastern and western expositions, they were that day nearly alike injured.

The 14th, 15th, and 22d of May, it froze pretty sharply, accompanied by the north and north-west winds, and I then likewise observed that all those sheltered from the wind were very much injured, but that all those which were exposed thereto had suffered but very little. This experiment appeared decisive, and showed that although it froze most strong in parts exposed to the north wind, yet the frost in that situation did the least injury to vegetables.

This circumstance is certainly opposed to common prejudice; but it is not less the fact, and is even easy to be explained; for this purpose, it is sufficient to pay attention to circumstances in which frost acts, and we shall discover that humidity is the principal cause of its effects, so that all which occasions humidify renders, at the same time, the frost dangerous to vegetables, and all that dissipates humidity, even if it should be done by increasing the cold (for every thing that dries diminishes the disasters of a frost) acts towards their preservation.

We have often remarked, that in low places, where mists and fogs reign, frost is felt more sharply, and oftener than elsewhere. For instance, in autumn and spring we have seen delicate plants frozen in a kitchen-garden, in a low situation, while the like plants were preserved sound in another kitchen-garden situated on an eminence. So, likewise, in vallies and low forests the wood is never of a beautiful vein, nor of good quality, although the vallies are often by much the best soil. The coppice wood is never good in low places, although it shoots forth there later than upon high places, and which is occasioned by a freshness that is always concentered therein. When I walked at night in the wood I felt almost as much heat on eminences as in the open plains, but in the vallies I experienced a sharp and uncomfortable cold. Though the trees shoot out the latest in those parts, yet the shoots are still injured by the frost, which spoiling the principal buds obliges the trees to shoot forth lateral branches, and thus prevents their ever becoming straight and handsome trees fit for service. What we have just advanced must not be understood only of deep vallies, which are liable to those inconveniencies from northern expositions, or those inclosed on the southern side in the form of an alley, in which it often freezes the whole year, but also of the smallest vallies, so that by a little custom we can discover the bad figure of the shoots from the inclination of the earth; this I particularly observed on the 28th of April, 1734; on that day the buds of all the trees, from one year up to six or seven, were frozen in all the lower places; whereas in the high and uncovered places there were only the shoots near the earth which were so; the earth was then very dry, and the humidity of the air did not appear to have greatly contributed to this injury. Neither vines, nor the trees of the plain, are subject to frost, which might lead us to suppose they are less delicate than the oak; but we think this must be attributed to the humidity, which is always greater in the woods than in the rest of the plains, for we have observed that oaks are often very much injured from frosts in forests, while those which are in the plains are not hurt in the least.

Large timbers, even on eminences, may cause the young trees near them to be in the same state as if at the bottom of a valley. We have also remarked, that the young wood near large trees is often more injured by the frost than in parts remote from them, as in the midst of such woods, where a great number of branches are left, it is felt with more force than in those which are open. Now all these disorders are most considerable in such places, for as the wind and sun cannot dissipate the transpiration of the earth and plants, there remains a considerable humidity, which causes a very great prejudice to plants.

We have also remarked, that the frost is never more to be dreaded, with respect to the vine flowers, buds of trees, &c. than when it succeeds mists, or even rain, however slight, for they are all capable of enduring a very considerable degree of cold without being damaged, when it has not rained for some time, and the earth is dry.

Frosts likewise act more powerfully in places newly cultivated than in others, because the vapours, which continually rise from the earth, transpire more freely and abundantly from that which is newly cultivated. To this reason we must, however, subjoin the fact, that plants, newly set, shoot forth more vigorously than others, which renders them more sensible and liable to the effects of frost. So also in light and sandy soil the frost does more injury than in strong land, even though of equal dryness, because more exhalations escape from the first kind of earths than from the latter; and if a vine newly dunged is most subject to the frost, it arises from the humidity which escapes from it. A furrow of vine which lies along a field of sainfoin, peas, &c. is often all destroyed by the frost, while the rest of the vine is quite healthy, and this is undoubtedly, to be attributed to the transpiration of the sainfoin, or other plants, which bring a humidity on the shoots of the vine. In the vine also, the branches that are strong and cut are always less injured than the stock; especially when not attached to the props, as they are then agitated by the wind which dries them.

The same thing is remarked of timber, and I have seen in copses all the buds entirely destroyed by the frost, while the upper shoots had not received the least damage; indeed it always appeared that the frost did most injury nearest to the earth, commonly within one or two feet, insomuch that it must be very violent to destroy the buds higher than four.

All these observations, which may be regarded as very constant, agree to prove that in general it is not the sharpest frosts which do the greatest injury to plants, but that they are affected in proportion as they are loaded with humidity, which perfectly explains why the frost causes so many disorders in the southern exposition, although it should be less cold than that of the north, and likewise why the frost causes more injury to the northern exposition, when after a rain proceeding from a westerly wind the wind veers to the north towards sun-set, as often happens in spring, or when, by an easterly wind, a cold moist air arises before sun-rise, which, however, is not so common.

There are likewise circumstances where the frost does most injury to the eastern exposition; but as we have many observations on that subject, we shall first relate those which we made in the spring frost in 1736, which occasioned so much damage. It having been very dry previously, it froze for a long time before it injured the vines; but it was not so in the forests, apparently because they contained more humidity. In Burgundy it was the same as in the forest of Orleans, the underwood was injured very early. At last the frost increased so greatly that all the vines were destroyed, notwithstanding the dryness still continued; but instead of this frost doing much damage under the shelter of the wind, those parts which were sheltered were the only ones preserved, insomuch, that in many closes surrounded by walls the stocks along the southern exposition were very green, while all the rest remained dry; and in two quarters the vines were saved, the one by being sheltered from the north by a nursery of ash-trees, and the other because the vineyard was stocked with a number of fruit-trees.

But this effect is very rare, and this happened only because the season had been dry, and because the vines had resisted the weather till the plants had became so strong, from the time of the year, that the frost could not injure them, independently of the external humidity and other particular circumstances.

But there are other causes to be assigned why frost produces injury more frequently to the east than to the west, and which are drawn from the following observations:

A sharp frost causes no prejudice to plants when it goes off before the sun comes upon them: let it freeze at night, if the morning be cloudy, or a slight rain fall, or, in a word, if by any cause whatever the ice melt gently, and independently of the action of the sun, it seldom does any injury; and we have very often saved very delicate plants, which had by chance remained exposed to the frosts, by returning them into the green-house before sun-rise, or by simply covering them before the sun had shone upon them.

One time in particular a very sharp frost happened in autumn while our orange-trees were out of the green-house, and as it rained part of the night they were all covered with icicles: but this accident was prevented from doing any injury by covering them with cloths before the sun rose, so that there was only the young fruit and the most tender shoots injured, and we are persuaded they would all have been saved if the covering had been thicker.

Another time our _geraniums_, and many other plants which cannot bear the frost, were out, when suddenly the wind, which was south-west, veered to the north, and became so cold that the rain, which fell abundantly, was frozen, and in almost a moment all that were exposed to the air were covered with ice; we thought, therefore, that all our plants were irrecoverably destroyed; nevertheless we had them carried to the furthermost part of the green-house, shut up the windows, and by that means they sustained but little damage.

This kind of precaution is always observed with regard to animals; when they are stricken with cold, or have a limb frozen, great care is taken not to expose them hastily to heat, but they are rubbed with snow, dipped in water, or burned in dung; in one word, the greatest attention is paid that they shall gradually be brought to warmth. It is almost certain, with respect to fruit which may be frozen, that if thawed with precipitation it invariably perishes, whereas it suffers but little if thawed gradually.

In order to explain how the sun produces so many disorders in frozen plants, some have imagined that the ice, by melting, is reduced into small spherical drops of water, which form so many small burning mirrors when the sun shines upon them. But however small the form of a mirror may be, it can only produce heat at a distance, and can have no effect on a body it touches; besides, the side of the drop of water which is on the leaf of a plant is flat, which removes its focus to a greater distance. In short, if these drops of water could produce this effect why should not the dew-drops, which are also spherical, produce the same? Perhaps, it may be thought that the most spirituous and volatile parts of the sap melting the first, they evaporate before the rest are in a state of moving in the vessels of the plant, which might decompose the sap.

But in general it may be said, that the frost increasing the volume of fluids, dilates the vessels of plants, and that the thaw cannot be performed without the parts which compose the frozen fluid enter into motion. This change may be made with sufficient gentleness not to break the most delicate vessels of plants, which will by degrees return to their natural tone, and then the plants will not suffer any injury; but, if it be done with precipitation, these vessels will not be able to resume their natural tone so soon after having suffered a violent extension, the liquors will evaporate and the plant remain dry.

Although we might conclude with these conjectures, with which I am not myself perfectly satisfied, yet the following data are irrevocably constant.

1. That it seldom happens with regard to fruit, either in spring or winter, that the plants are injured simply by the force of the frost and independently of any particular circumstances, and when it does, it is at the northern exposition that plants meet with the greatest injury.

2. In frosty weather, which lasts several days, the heat of the sun melts the ice in some places for a few hours; for it often freezes again before sun-set, which forms an ice very prejudicial to plants, and it is observable that the southern exposition is more subject to this inconvenience than all the rest.

3. It has been observed, that spring frosts principally disorder those plants where there is humidity, the soils which transpire much, the bottoms of vallies, and in general all places which cannot be dried by the wind and sun are the most injured.

In short, if, in spring, the sun which shines on frozen plants occasion a more considerable damage to them, it is clear that it will be the eastern exposition, and those next the south which will suffer most.

But it may be said, if this be the case, we must no longer plant to the southern exposition _en a-dos_ (which are slopes, or borders of earth, thrown up in kitchen gardens or along espaliers) gilliflowers, cabbages, winter lettuces, green peas, and such other delicate plants as we would have stand the winter, and preserve for an early crop in spring; and that it is to the northern exposition alone that we must in future plant peach and other delicate trees. It is proper to destroy these objections, and shew that they are false consequences of what we have advanced.

Different objects are proposed when we set plants to pass through the winter in shelters exposed to the south, and sometimes it is to expedite vegetation: it is, for example, with this intention, that along espaliers we plant ranges of lettuces, which for that reason are termed _winter-lettuces_; these will tolerably well resist the frost in whatever part we plant them, but are always most forward in this exposition; at other times, it is to preserve them from the rigour of this season, with an intention of replanting them early in the spring. This practice is also followed in winter cabbages, which are sown in this season along an espalier border. These kind of cabbages, like brocoli, are tender and cannot endure the frost, and would often perish in these shelters, if care were not taken to cover them during the sharp frosts with straw or dung supported on frames.

To forward the vegetation of some plants which will not bear the frost, as green peas, &c. it is usual for that purpose to plant them on borders exposed to the south, besides which, they are defended from sharp frosts when the weather requires it.

It is well known, without being compelled to dwell any longer on this point, that the southern exposition is more proper than all the rest to accelerate vegetation, and we have shewn that this is also what is principally proposed when some plants are set in that exposition to pass through the winter, since, in addition, we are also obliged to make use of coverings to guard those plants which are very delicate from the frost. But we must add, that if there be some circumstances wherein the frost causes more disorders to the southern than to other expositions, there are also many cases which are favourable to this exposition: for example, in winter, when there is any thing to fear from the ice, it frequently happens that the heat of the sun, increased by the reflection of the wall, has sufficient force to dissipate all the humidity, and then the plants are almost perfectly secure against the cold. Besides, dry frosts often happen, which unceasingly act towards the north, and which are scarcely ever felt towards the south. In spring, likewise, we perceive that after a rain which proceeds from the south-west, or south-east, if the wind change to the north, the southern espalier being under the shelter of the wind, will suffer more than the rest; but these cases are very rare, and most often it is after rains, which come from the north-east or north-west that the wind changes to the north, and then the southern espalier having been under shelter from the rain by the wall, the plants there will have less to suffer than the rest, not only because it will have received less rain, but also because there is always less cold here, than in other expositions. It is likewise to be observed that as the sun dries much earth along the espaliers which are to the south, the earth transpires there less than elsewhere.

It is well known that what we have just advanced must be considered as applying also to peach and apricot trees, which it is customary to put in this exposition and in that of the east. We shall only add, that it is not unusual to see peach trees frozen in the east and southern expositions, while those are not so which stand in the west or north; but notwithstanding this we can never rely on having many, nor good peaches in this last exposition, for great quantities of blossoms fall off entirely without setting; others, after having set fall from the trees, and those which remain with difficulty arrive to maturity. I have an espalier of peach-trees in a western exposition, a little declining to the north, which scarcely ever produce any fruit, although the trees are handsomer than those to the southern and northern. We cannot, therefore, avoid the inconveniences of the frost with respect to the southern exposition without feeling others that are worse.

All delicate trees, as fig, laurel, &c. must be set to the south, and great care taken to cover them; it is only requisite to remark that dry dung is preferable for this purpose to straw, because the latter not only does not so exactly cover them, but also from its always retaining some grain which attracts moles and rats, who sometimes eat the bark of trees to quench their thirst in frosty weather, when they can meet with no water to drink, nor herb to feed upon; and however singular this may appear, it is a circumstance which has happened to us several times; but when dung is made use of it must be dry, without which it will heat and make the young branches grow mouldy.

All these precautions are, nevertheless, very inferior to the espaliers in niches, as in that manner plants are sheltered from all winds, except the south, which cannot hurt them; the sun, which warms these places during the day, prevents the cold from being so violent during the night; and over these defended places we may put a slight covering with great facility, which will hold the plants there in a state of dryness, infinitely proper to prevent all the accidents which the spring frosts and ice might produce; and most plants will not suffer from being deprived of their external humidity, because they scarcely transpire in the winter, or in the beginning of spring, so that the humidity of the air is sufficient for their supply.