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.