This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Vegetable organisms, more than animal, and especially than man, are subject to the influences of relatively slight deviations of both heat and cold in diverse degrees, according to the climatal conditions of their natural habitats. Taking those plants in general cultivation for our guide, an access of heat beyond what may be termed normal seems to be less injurious than an increase of cold. But even heat above the temperature natural to a plant will eventually kill it, though its action is slow in comparison with that of cold. Frost is very decided and rapid in its action, its effects being visible almost immediately after a fall in the temperature. This peculiar susceptibility is only understood by its results, though doubtless an explanation must be sought in the differences of' organisation of species restricted to different ranges of temperature. It is now almost universally conceded that by no process of acclimatization can we succeed in making a plant frostproof, even to the extent of a single degree. Under other-wise favourable conditions, it is true, a plant will withstand a somewhat lower air-temperature than it is subject to in a natural state. To illustrate the certain and unchangeable effects of temperature on plants, we need only mention such familiar examples as the Potato and the Dahlia. We mention these because they have been under cultivation with us for a long series of years, without producing any visible or appreciable alteration in their constitution, in so far as it concerns their power of enduring cold. From the same cause, acting inversely, it is difficult, and in some cases impossible, to grow mountain and arctic plants .successfully; that is to say, to keep them alive and in health for a number of years. In this case, perhaps, the actual increase of temperature is less injurious than the lengthened growing period to which plants from colder regions are subject under cultivation in this country; but, after all, this is a distinction with scarcely a difference. In practice, it is well known that the various plants employed for the summer decoration of the parterre have each and all of them clearly defined constitutions. One or two degrees. of frost for the shortest period will kill some plants outright, and the same species will succumb to a continued temperature many degrees above the freezing point; whilst other species will bear five, ten, fifteen, twenty or more degrees respectively without sustaining any permanent injury. The common Groundsel, and several other early-flowering somewhat succulent plants, will bear as much as forty degrees of frost for a short time without receiving any permanent injury. From the existence of these natural laws, it will be apparent that only those plants from countries enjoying a similar climate to our own will flourish in the open air all the year round without protection.1 Plants, it has been observed, will bear a lower air-temperature, under certain favourable conditions, than that to which they are normally exposed. These modifying conditions are, the nature of the soil and the situation, governed by the quantity of moisture in the soil and atmosphere. But we shall return to the consideration of this question when we come to treat of soils. It is evident from what we have said, that the average annual rainfall, winter and summer temperatures, and the extremes of heat and cold of different parts of the country, are, to a limited extent, a guide to the gardener as to what plants will succeed in his particular locality. Speaking generally, the farther southward and westward we get in Britain, the higher is the mean winter temperature; but there are quite local conditions, favourable or unfavourable, that render calculations based entirely upon the temperature and rainfall of a district almost valueless. These are chiefly dependent upon the nature of the soil and subsoil, and the altitude and inclination of the ground. One great modifying influence on the winter temperature of the south-western coast, especially of the British Isles, is the warm ocean stream that flows from the Mexican Gulf and washes our shores. To a smaller extent, this holds good for the whole country. As compared with the same latitudes on the continent, our winters are milder, and our summers some degrees colder. And, by way of compensation for our cloudy skies and frequent rains, we enjoy the delights of luxuriant verdure all through the heat of summer; whereas in many other parts of Europe vegetation is scorched up. The disadvantages are also important, especially a deficiency of solar heat for the maturation of fruits and seeds of many plants. There is a great divergence in the direction of the isothermal lines in summer and winter for the British Isles. The general direction of the summer lines is from west by south to east by north, with a slightly higher temperature inland in the centre and south of England; whilst the winter lines run nearly parallel with the east and west coasts. The mean summer temperature ranges from 63° or 64° in the south of England to 55° or 56° in the north of Scotland; and the mean winter temperature ranges from 37° on the eastern coast and inland, to 39° on the north-west and south-east coasts, and upwards to 43° or 44° in the south-west. But winter extremes, more than winter means, affect the gardener; and when the thermometer falls below zero the frost is very destructive. In round numbers, the mean annual temperature of the British Isles is about 50°, but it varies in different localities from 53° to 47°. The higher summer temperature of the east and centre raises the mean annual temperature considerably; but the difference of six or seven degrees in the mean winter temperature of different parts of Britain and Ireland, to say nothing of extreme degrees of cold, goes farther to determine the question whether certain plants can be grown in the open air without or with only slight protection in winter. Although the mild and comparatively equable climate of the south-west of England and the south of Ireland is favourable to the existence of tender subjects such as will not withstand the climate of the centre and east, the higher summer temperature of the last-named region, coupled with less rainfall and more sunlight, is of far more importance to the fruit and seed grower. Many plants that flower freely in the moist uniform climate of the south-west do not ripen their fruits; whereas the more continental summer of the centre and south-east is sufficient to bring them to maturity. There is a still greater disparity in the average annual amount of rain falling in different parts of the British Isles. The greatest fall is in Ireland and on the western coast of Britain in mountainous districts, gradually diminishing eastward, and reaching its minimum in the south-eastern counties. The average annual rainfall in Ireland and hilly regions in the west of Britain ranges from 80 to 150 inches, and in some localities even this large amount is exceeded, especially in the western Highlands of Scotland and in Cumberland. In less elevated parts of the west, it ranges from 30 to 40 inches, and in the east and south-east from 20 to 28 inches annually. During a period of forty years, the average rainfall at Chiswick, near London, has been about 23.5 inches. But these figures, by themselves, are of little use to the gardener. It is only when they are compared with those furnished by the countries whence we obtain our hardy exotic plants, and with purely local conditions, that they become really interesting and serviceable. As we have already stated, the insular position of Britain, and especially its exposure to the softening influence of the Atlantic Ocean, raises its mean annual temperature considerably above that of continental countries in the same latitude. The most important point in this increased mean annual temperature for latitude, is the fact that it is principally due to the high whiter temperature of those parts most favourably situated for receiving the full effect of the neighbouring ocean. The mean annual temperature of Britain in round numbers is 50°. Now, if we follow the lines denoting this mean temperature through the different countries of the world, both in the northern and southern hemispheres, we may form an approximate idea as to what countries will furnish us with hardy plants. In continental countries where this mean is raised considerably by a higher summer temperature, we may safely conclude that plants growing several degrees south or north of the line will prove hardy in the warmer parts of Britain. Again, insular countries on the same line, owing their mildness to the same causes as Britain, would naturally furnish us with plants that would flourish with us in some parts, whilst in others they would succumb to the rigours of winter. Another circumstance to be considered in conjunction with those already mentioned, in judging of the hardiness of a plant, is the elevation above the sea at which it grows in its native country. The rule for calculating for elevation is to deduct one degree of mean annual temperature for each hundred yards of height. As a rough guide, we may indicate the course of the north and south isothermal lines through those countries having a mean annual temperature of 50° Fahrenheit. But we must caution the reader against taking them as absolute and definite in the information they convey.
1 In the Introduction, a few remarks on the Geography of Plants hardy in Britain will be found, with indications of the pri