This section is from the book "The Gardener V3", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
It is said that in clear weather during April and May, when the Vines are growing, it is not unusual for the mercury to range between 80° and 90° in the shade at noon, and fall nearly to the freezing-point during the night. At such times Vine-growers tremble for their prospects; but it appears the Vine when shooting will stand, without being materially injured, a temperature that is not actually freezing. That there is nothing improbable in what I have stated regarding the above temperature in France, is proved by the statistics of the Scottish Meteorological Society. Owing to the insular position of Great Britain, extremes of temperature do not prevail as on the Continent; but a reference to the tables of this Society will show that the daily range of temperature in Scotland during the spring months is very considerable, and sometimes excessive. I am not speaking of mean temperatures, which are valueless to a great extent. For instance, at one station we read of the thermometer in April registering 72°.5 max., and three days later 27° min. At another station the thermometrical range between these dates is 47°.5. Again, at Dollar, we find the noon-day temperature on a certain occasion in April registered at 80°, and falling to 31° the same night.
Then, in East Lothian, between the 23d and 28th of July, the thermometer is fluctuating between 43° and 89°; and so on. It must be remembered, however, that the ranges here noted are between the lowest night temperatures and the maximum in the shade, and do not represent the actual extremes which vegetation endures. For instance, at one station we find the thermometer in the sun recording 140°. 8, where on the same day the highest in the shade was 60°. Thus, if we suppose the glass fell to 40° before morning, we have a range of 100° in 24 hours. This is what takes place in some parts of these islands, and the changes are more sudden, and therefore more destructive. But as under the clear skies of the Continent and some Eastern countries, the sun is more powerful by day, and radiation more excessive by night, we may conclude that the daily range is much greater, only, being more constant, vegetation will get inured to the conditions in the same way that Vines will bear a bright sun if it is steady, under which they would flag after a period of dull weather.
This is the kind of information "D. J." seems to be craving for, and it is to be hoped the above will satisfy him to some extent. I think, at least, the above facts will show that, in recommending a minimum temperature of 50° for Vines (in the early stages of forcing) after a high day temperature, rather than trust to hard firing to no purpose, I have kept within safe limits.
I can see no reasons, physiological or other, for thinking that Vines, up till the setting period we shall say, can be injured in any way whatever by allowing the temperature to fall to 50° or 45° before morning after a bright sunny day and a high day temperature. This is my argument, and I shall gladly listen to any facts "D. J." or any one else may advance to the contrary; but they must be conclusive.
I have seen the thermometer in a long Peach-case once under my charge run up to 90° and 100° during the sunny days of April and May, and fall to 45° or 40° before morning, and this would go on for days: there was only 2 feet of space for ventilation altogether. Yet the crops of Peaches and Plums were always excellent, and are to this day. It is my regular practice here to run up our early Peach-house to 85° or 90° in sunny weather before the fruit is stoned, rather than admit heavy draughts of cold air, and I am perfectly satisfied on such occasions if the thermometer stands at 40° in the morning. I have, of course, no objections to a high night temperature comparatively, if it can be secured without much fire-heat. Our early Vinery is allowed to fall to 50° or 55° before sunrise in clear frosty weather up till the setting-time; and our maximum night temperature for Muscats is 65°, often falling to G0° before morning. In mild dull weather this practice is, of course, modified considerably: the temperature is neither so high by day nor so low at night.
My only objection to a low night temperature is the time lost in getting up the heat in the forenoon in dull days, so as to have heat with light, when there are, perhaps, 700 or 800 feet of piping attached to one boiler. In a glass case here, devoted to Hamburgs and Peaches, we had at one time a Muscat Vine which invariably set well and equally, though it rarely had a night temperature above 60°, often not so high, while setting, unless the weather was mild, for there was not piping enough in the house to keep up a higher figure when the outdoor temperature was low, but we made the very most of the sunshine during the day.
It was this circumstance which first led me to doubt the propriety of a very high night temperature for the Vine in the first stages of growth. I was a believer in high and dry temperatures once.
In conclusion, let me say a word about the mean temperatures of Beyrout, Cadiz, and Catania, which "D. J." says are, according to the late Mr Thompson of Chiswick, "respectively 69°, 63°, and 71°, while Vines are in flower." I presume, "D. J." is quoting from 'The Gardener's Assistant,' by Mr Thompson. If he is, he might have quoted his author fairly, and told us that these were the mean temperatures of the month of May (Mr Thompson says nothing about the Vines being in flower), - the mean temperatures of the month be it noted; and as they are not very high, the daily and monthly range must have been considerable - i.e., much above and below these figures. To give an example, the mean temperature of the month of March in the south of Scotland is about 40°, but what is the range? Why, we read that the maximum for March 1871 was 68°.9, and the minimum 3°.6, or 29° of frost, and yet the mean temperature was above the average of the month. The mean temperature of the month of May is 50°, but the maximum for that month in the same year is recorded at 80°, and the minimum at 18°, or 14° of frost. These facts show abundantly how practically valueless are mean temperatures for conveying a just idea of climate.
It is worth noting also, that the month of May in 1871 was characterised by an unusual amount of sunshine, or clear weather; and the consequence was, the mean temperature of the nights was below the average, and that of the days above the average, thus showing that extremes meet, as I have pointed out in regard to some Continental countries.
Mr Thompson's tables of temperature are valuable, I admit; but his deductions with regard to treatment, under altered circumstances in this country, are not always applicable. For instance, he recommends as the lowest safe winter temperature for Pines a mean just 10° higher than the Editor of this paper does in his book on the Pine-Apple, and Mr Thomson's practice is that of all successful Pine-growers nowadays; but I shall conclude, and by way of encouraging " D. J." to do likewise, I subscribe myself fully, J. Simpson.