The physical study of Vegetables is one of those sciences which
require a multiplicity of observations and experiments beyond the
capacity of one man, and must consequently be a work of time; even the
observations themselves are seldom of much value till they have been
repeatedly made, and compared in different places and seasons, and by
different persons of similar ideas. It was for this purpose that Buffon
united with M. Du Hamel, to labour, in concert for the illustration
of a number of phenomena, which appeared difficult to explain, in
the vegetable kingdom, and from the knowledge of which may result an
infinity of useful matters in the practice of agriculture.
The frost is sometimes so intense during winter, that it destroys
almost all vegetables, and the scarcity in the year 1709 was a
melancholy proof of its cruel effects. Seeds, and some kinds of
trees, entirely perished, while others, as olives, and almost all
fruit-trees, shared a milder fate, shooting forth their leaves, their
roots not having been hurt; and many large trees, which were more
vigorous, shot forth every branch in spring, and did not appear to have
suffered any material injury. We shall, nevertheless, remark on the
real and irreparable damage this winter occasioned them.
Frost, which can deprive us of the most necessary articles of life,
destroys many kinds of useful trees, and which scarcely ever leaves
one insensible of its rigour, is certainly one of the most formidable
misfortunes of human nature; we have therefore every reason to dread
intense frosts, which might reduce us to the last extremities if their
severities were frequent; but fortunately we can quote only two or
three winters which have produced so great and general a calamity as
that in 1709.
The greatest spring frosts, although they damage the grain, and
principally barley, when it is but just eared, never occasion great
scarcities. They do not affect the trunks or branches of trees, but
they totally destroy their productions, deprive us of the harvest of
the vines and orchards, and by the suppression of new buds cause a
considerable damage to forests.
Although there are some examples of winter frosts having reduced us to
a scarcity of bread, and deprived us of vegetables, the damage which
spring frosts occasion becomes still more important, because they
afflict us more frequently, and their effects are felt almost every
year.
To consider frost even very superficially, we must perceive that the
effects produced by the sharp frosts of winter are very different from
what are occasioned by those in spring, since the one attacks the body
and most solid parts of trees, whereas the other simply destroys their
productions, and opposes their growth; at the same time they act under
quite different circumstances; and it is not always the ground in which
the winter frosts produce the greatest disorders, as that generally
suffers most from those in the spring frosts.
It was from a great number of observations that we have been able to
make this distinction on the effects of frost, and which we hope will
not be simply curious, but prove of utility, and be profitable to
agriculture; and should they not wholly enable us to escape from the
evils occasioned by frost, they will afford us a means to guard against
them. We shall, therefore, enter upon the detail, beginning with that
which regards the sharp frosts of winter: of these, however, we cannot
reason with so great a certainty as on those of spring, because, as
we have already observed, we are seldom subjected to their tragical
effects.
Most trees during winter being deprived of blossoms, fruits, and
leaves, have generally their buds hardened so as to be capable of
supporting very sharp frosts, unless the preceding summer was cool, in
which case the buds not being arrived to that degree of maturity, which
gardeners call _aoutes_[J], they are not in a state of resisting the
moderate frosts of winter; but this seldom happens, the buds commonly
ripening before winter, and the trees endure the rigour of that season
without being damaged, unless excessive cold weather ensue, joined to
the circumstances hereafter mentioned.
[J] Ripened or filled with sap.
We have, nevertheless, met with many trees in forests with considerable
defects, which have certainly been produced by the sharp frosts, and
which will never be effaced.
These defects are, 1st, chaps or chinks, which follow the direction
of the fibres. 2. A portion of dead wood included in the good; and
lastly, the double sap, which is an entire crown of imperfect wood. We
must dwell a little on these defects to trace the causes whence they
proceed.
The sappy part of trees is, as is well known, a crown or circle of
white or imperfect wood of a greater or less thickness, and which in
almost all trees is easily distinguished from the sound wood, called
the _heart_, by the difference of its colour and hardness; it is found
immediately under the bark, and surrounds the perfect wood, which in
sound trees is nearly of the same colour, from the circumference to the
centre. But in those we now speak of, the perfect wood was separated by
another circle of white wood, so that on cutting the trunks of them we
saw alternately circles of sap and perfect wood, and afterwards a clump
of the latter, which was more or less considerable, according to the
different soils and situations; in strong and forest earth it is more
scarce than in glades and light earth.
By the mere inspection of these cinctures of white wood, which we
in future shall term _false sap_, we could perceive it to be of bad
quality; nevertheless, to be certain of it, we had several planks sawed
two feet in length, by nine to ten inches square, and having the like
made from the true sap, we had both loaded in the middle, and those of
the false sap always broke under a less weight than those of the true,
though the strength of the true sap is very trivial in comparison with
that of formed wood.
We afterwards took several pieces of these two kinds of sap, and
weighed them both in the air and water, by which we discovered that
the specific weight of the natural sap was always greater than that of
the false. We then made a like experiment with the wood of the centre
of the same trees, to compare it with that of the cincture which is
found between these two saps, and we discovered that the difference
was nearly the same as is usual between the weight of the wood of the
centre of all trees and that of the circumference; thus all that is
become perfect wood in these defective trees is found nearly in the
common order. But it is not the same with respect to the false sap,
for, as these experiments prove, it is weaker, softer, and lighter
than the true sap, although formed 20, nay 25 years before, which we
discovered to be the fact, by counting the annual circles, as well of
the sap as of the wood which covered it; and this observation, which
we have repeated on a number of trees, incontestibly proves that these
defects had been caused by the hard frost of 1709, notwithstanding
that the number of some of their coats was less than the years which
had passed since that period; and at which we must not be surprised,
not only because we can never, by the number of ligneous coats, find
the age of trees within three or four years, but also because the first
ligneous coats, formed after that frost, were so thin and confined,
that we cannot very exactly distinguish them.
It is also certain, that it was the portion of the trees that were in
sap in the hard frost of 1709, which instead of coming to perfection,
and converting itself into wood, became more faulty. Besides, it is
more natural to suppose, that the faulty part must suffer more from
sharp frosts than sound wood: because it is not only at the external
part of the tree, and therefore more exposed to the weather, but also
because the fibres are more tender and delicate than the wood. All this
at first appears to wear but little difficulty, yet the objections
related in the history of the Academy of Sciences, 1710, might be here
adduced; by these objections it appears that in 1709, the young trees
endured the hard frost much better than old. But as these facts are
certain, there must be some difference between the organic parts, the
vessels, the fibres, &c. of the sappy part of the old trees and that
of the young; they perhaps will be more supple, so that a power which
will be capable of causing the one to break, will only dilate the other.
But as these are conjectures with which the mind remains but little
satisfied, we shall pass slightly over them, and content ourselves with
the particulars we have well observed. That this sappy part suffered
greatly from the frost is an incontestible fact, but has it been
entirely disorganized? This might happen without the death of the tree
ensuing, provided the bark remained sound; and even vegetation might
continue. Willows and limes frequently subsist only by their bark,
and the same thing has been seen at the nursery of Roule in an orange
tree. But we do not think that the false sap is dead, because it always
appeared to us in quite a different state from the sap found in trees,
which had a portion of dead wood included in the sound; besides, if it
had been disorganized, as it extends over the whole circumference, it
would have interrupted the lateral motion of the sap, and the wood of
the centre, not being able to vegetate, would have also perished and
altered, which was not the case, and which I could confirm by a number
of experiments; however, it is not easily conceivable how this sappy
part of wood has been changed so far as not to become wood, and that
far from being dead, it was even in a state of supplying the ligneous
coats with sap, which are formed from above in a state of perfection,
and which may be compared to the wood of trees that have suffered no
accident. This must nevertheless have been done by the hard winter,
which caused an incurable malady to this part of the tree; for if it
were dead, as well as the bark which cloathed it, there can be no
doubt that the tree would have entirely perished, which happened in
1709 to many trees whose bark was detached from them, and which by the
remaining sap in their trunk, shot forth their buds in spring, but
died through weakness before autumn, for want of receiving sufficient
nutriment to subsist on.
We have met with some of these false sappy part of trees which are
thicker on one side than the other, and which surprisingly agrees with
the most general state of the sap. We have also seen others very thin,
so that apparently there were only the outer coats injured. These were
not all of the same colour, had not undergone an equal alteration, nor
were equally affected, which agrees with what we have before advanced.
At length, we dug at the foot of some of these trees, to see if the
defect existed also in the roots, but we found them sound: therefore,
it is probable that the earth which covered them had repaired the
injury done by the frost.
Here then we see one of the most dreadful effects of winter frosts,
which though locked up within the tree, is not less to be feared, since
it renders the trees attacked by them almost useless; but besides this,
it is very difficult to meet with trees totally exempt from these
injuries; and indeed all those whose wood is not of a deeper colour at
the centre, growing somewhat lighter towards the sap, may be suspected
of having some defects, and ought not to be made use of in any matter
of consequence.
By horizontally sawing the bottom of trees, we sometimes perceive a
piece of dead sap or dried bark, entirely covered by the live wood:
this dead sap occupies nearly half of the circumference in the parts
of the trunk where it is found: it is sometimes browner than good
wood, and at others almost white. From the depth also where this sap
is found in the trunk, it appears to have been occasioned by the sharp
frost in winter, by which a portion of the sap and bark perished, and
was afterwards covered by the new wood; for this sap is almost always
found exposed to the south, where the sun melting the ice, a humidity
results, which again freezes soon after the sun disappears, and that
forms a true ice, which is well known to cause a considerable prejudice
to trees. This defect does not always appear throughout the whole
length of the trunk, for we have seen many square pieces which seemed
perfectly exempt from all defects, nor were the injuries of the frost
discovered until they were slit into planks. It is, nevertheless easily
to be conceived, how such a disorder, in their internal parts, must
diminish their strength, and assist their perishing.
In forests, or woods, we meet with trees which strong winter frosts
have split according to the direction of their fibres; these are marked
with a ridge formed by the cicatrice that covers the cracks, but which
remain within the trees without uniting again, because a re-union is
never formed in the ligneous fibres when they have been divided or
broken; nor can it be doubted, that the sap, which increases in volume
when it freezes, as all liquors do, may produce many of these cracks.
But we also suppose that there are some which are independent of the
frost, and which have been occasioned by a too great abundance of sap.
Be this as it may, the fact is, we have found defects of this kind in
all soils, and in all expositions, but most frequently in wet ground
and in northern and western expositions; the latter may perhaps proceed
in cases when the cold is more intense, in such expositions; and in the
other, from the trees which are in marshy grounds, having the tissue of
their ligneous fibres weaker, and because their sap is more abundant
and aqueous than in dry land; which may be the cause that the effect
of the rarefaction of liquors by the pores is more perceptible, and
more in a state of diminishing the ligneous fibres, as they bring less
resistance thereto.
This reasoning seems to be confirmed by another observation; namely,
that resinous trees, as the fir, are seldom injured by the sharp frosts
of winter, evidently from their sap being more resinous: for we know
that oils do not perfectly freeze, and that instead of augmenting in
volume, like water, in frosty weather, they diminish when they congeal.
Dr. Hales says in his _Vegetable Statics_, _p._ 16, that the plants
which transpire the least, are those which best resist the winter;
because they have need of only a small quantity of nutriment to
preserve themselves. He says, likewise in the same part, that the
plants, which preserve their leaves during winter, are those which
transpire the least; nevertheless, we know that the orange tree, the
myrtle, and still more the jessamine of Arabia, &c. are very sensible
to frost, although these trees preserve their leaves during winter; we
must, therefore, have recourse to another cause to explain why certain
trees which do not shed their leaves in winter, so well support the
sharpest frosts.
We have sawed many trees which were attacked with this malady, and
have almost always found, under the prominent cicatrice, a deposit of
sap or rotten wood, and they are easily distinguished from what are
called in the forest terms, sinks or gutters, because the defects which
proceed from an alteration of the ligneous fibres, which is internally
produced, occasion no cicatrice to change the external form of the
trees, whereas the chinks produced by frosts, which proceed from a
cleft afterwards covered by a cicatrice, make a ridge or eminence in
the form of a cord, which announces the internal defect.
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.
But since the dew renders plants so susceptible of the spring frost,
might we not hope, that from the researches of Messrs. Musschenbroeck
and Fay, some inferences may be deduced which may turn to the advantage
of agriculture? for since there are some bodies which seem to attract
dew, while others evidently repel it, if we could paint, plaster, or
wash the walls with some matter which would have the latter effect,
it is certain we should have room to expect a more fortunate success
than from the precaution taken to place a plank in form of a roof
over the espaliers, which cannot prevent the abundance of dew from
resting on trees, since Fay has proved that it very often does not
fall perpendicularly like rain, but floats in the air, and attaches
itself to those bodies it encounters; so that frequently as much dew
is amassed under a roof as in places entirely open. It would be easy
for us to recapitulate all our observations, and continue to deduce
useful consequences, but what we have said must be sufficient to shew
the necessity of rooting up all trees which prevent the wind from
dissipating mists.
Since by cultivating the earth we cause more exhalations to issue,
great attention should be paid not to cultivate them in critical times.
We must expressly declare against sowing kitchen-plants on
vine-furrows, as by their transpiration they hurt the vine.
Props should be put to the vines as late as possible. The hedges, which
border them on the north side, should be kept lower than the rest. It
is preferable to improve vines with mould rather than dung. And in
choosing a soil we should avoid those which are in bottoms and grounds
which transpire much.
A part of these precautions may be also usefully employed for
fruit-trees; with respect, for example, to plants which gardeners are
forward to put at the feet of their bushes and along their espaliers.
If there are some parts high and others low in a garden, we should pay
attention to sow spring and delicate plants on elevated parts, at least
if we do not design to cover them with glasses, &c. but in cases where
humidity cannot hurt them it might be often advantageous to choose low
places, where they might be sheltered from the north and north-west
winds.
We may also profit from what has been said to the advantage of forests,
for if we mean to make a reserve of any of the trees, it should never
be in parts where the frost is severe; and in planting we should pay
attention to put in vallies those trees which can endure the frost
better than the oak.
When any considerable fall of timber is made we should make them in
roads, beginning always on the north side, in order that the wind,
which generally blows in frosty weather, may dissipate that humidity
which is so prejudicial to the underwood.
There might be also many other useful consequences drawn from our
observations; but we shall content ourselves with having briefly
adverted to some, because the ingenious man may supply what we have
omitted by paying a little attention to the observations we have
mentioned. We are well convinced there are a great number of further
experiments to be made on this matter; and perhaps even those which we
have related will engage some persons to work on the same subject, and
from our hints general and useful advantages may be derived.