This section is from the book "The Gardener V2", 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.
Your leader in the March number of the ' Gardener ' on low night temperatures raises a question the importance of which it would be difficult to overrate. I had almost said raises the whole theory of gardening under glass. To definitely solve the various relations of heat, moisture, air, light, and darkness as far as their relations are concerned in the maturation of a perfect plant - that is, plants thoroughly developed in leaves, wood, flower, seeds, or fruit - at the least possible outlay, would be of incalculable value. A plant uses only a given amount of heat, moisture, and air in passing through its various stages to maturity. What is given over the required amount is waste, and what is given under this required amount I would characterise as the least desirable of the two extremes. What we have to aim at is to try to reach the closest approximation to what is necessary - this is the seeming difficulty. To thoroughly clear up these points would be worth years of labour. Although we may have no record of such a thing, I cannot but think it has been, and is being, arrived at possibly by some quiet practical men, who, as a rule, do not approve of much noise about their doings.
What is a low night temperature? and what is a high night temperature? Your correspondent in the tropics mentions that the thermometer goes down to 40° or below it. Now to adopt 40° as a night temperature, and keep our houses for tropical plants and fruits by day, as they generally are kept at the present time, we would have a very poor result; and so long as day temperatures are kept as they generally are, it would not be very good practice. Of course I am speaking of plant-stoves and forcing-houses - the greatest consumers of coal, and which require the highest temperature. We never have, as a rule, aimed at a strictly tropical day-heat. Indeed it would in winter be all but impracticable to do so. We have rather been contented with 50° to 60° night-heat, and from 60° to 70° day-heat, and lower in very cold weather, and higher with sun-heat, which is such a scarce commodity with us from October till March that it can enter but little into our calculations. A fall to 40° in the morning sometimes occurs, but on such occasions the heat is got up 5° or more as quickly as possible, as compensation. The only perceptible harm I ever could detect in going down to 40° happens to the flowers.
I have long acted upon the theory that a low night-temperature must have a corresponding high day-temperature, but the middle course is the safest. In coming to summer treatment, say from March to September, this is the time in a good many cases that the coal bill may be lessened, not only with no evil result, but with much positive advantage. It is the proper balancing of heat and moisture at the roots and in the air that gives us the most luxuriant growth. Nothing can operate so much against a growing plant as hot, dry, hard, thin air. Do we as a rule utilise sun-heat as we ought? I rather think not. The usual practice is to do all, or nearly all, the watering at night and morning, which is all right as far as it goes; but something more remains to be done, and that is to feed the air properly with moisture during the day when the houses are hottest. It is far more needed then than at any other time, because it mixes with the hot air, and is then made into proper food for the plants.
Moisture and heat must - if I may use a homely phrase - go hand in hand if you wish a steady luxuriant growth. It is far too often the practice to allow the best and fattest of the air to escape by the top ventilators, before it has given any material assistance whatever to the plants. There is no greater inconsistency in practice than having heat in our power, and allowing it to pass away without first having secured all that is valuable out of it. I would say that far too much air is given during sunshine. The roofs of our houses, as a rule, are pitched at the best possible angle to catch as much of the sun as we can, which soon runs up the thermometer to tropical heat. This is the golden time for plenty of moisture in the air. But what do we do? We open top and bottom ventilators, until we have it down to 80° or 85°. Why not give plenty of moisture to the air, and allow it to run up other 10° or 15°, and then the plants would enjoy the benefits of a cool temperature at night? The air should always be circulating; never at rest, but always moving - of course more by day than night. A high thermometer is no indication of itself that growth is progressing according to the heat. It is only when duly tempered by moisture that it can be most acceptable to the plants.
What I would propose would be something like the following: To collect in a tabular form all the information from your numerous correspondents on the points at issue, which, to be of any available use, must to the minutest degree be trustworthy. Say, to note the outdoor thermometer and barometer three times a-day; the same with indoor thermometer; also the quantity of water made use of, and how; with the weight of coal consumed per day or per week, and what other conditions you might think proper to impose; with notes on the various stages of growth up to maturity. If you could only set this machinery in motion, it would, in my opinion, lead to a great reform, although it may be years before we arrive at thoroughly reliable data. Could not some of our experimental gardens have solved this for us before they were abolished? They certainly would have been entitled to our lasting gratitude. I have to apologise for taking up your valuable space, and, with your permission, I purpose in a future paper giving notes and observations bearing upon the above, which I have made from time to time.
T. Sy. M. [We will be glad to have the notes referred to. - Ed].