This section is from the book "The Gardener V1", 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 would be difficult to name two more important or sternly-practical operations connected with Horticulture, than those represented by the couplet with which we have headed these observations. Heat and air are two of the most subtle elements - if we may term the first named an element - with which man can intermeddle. Yet the success of the Horticulturist in the cultivating of many plants and fruits depends to a very great extent on the way in which they are applied and regulated. Hence we venture to say that there is probably more anxiety and irritation arising from these two sources, in the case of those who have to manage ranges of hothouses, than from all others put together. Those who act in the capacities of stoker and ventilator are generally not long in finding out, from a fastidious superintendence, that their hottest tests arise, in more senses than one, from the stokehole, and their coolest judgment must be exercised, and sometimes overruled, in the matter of ventilating. We feel certain that we have just written down what many an under-gardener and their superintendents can fully endorse.
The dangers arising from potting and watering, stopping and training etc, are nothing in subtleness as compared to those which often follow in the train of injudicious firing and air-giving. This may perhaps sound strangely in the ears of the novice and unexperienced, but a little consideration will serve to show that we have not drawn upon fancy or imagination for such a statement.
Exotic flowers and fruits in forcing-houses and pits are in circumstances so thoroughly artificial in respect to two such important conditions as those of heat and air, that the appliances which are necessary to regulate these conditions are the very sources from which danger and harm to delicate plants arise. It is not possible by any known means to produce and regulate that amount of warmth required by exotic plants of the most tender description, without enclosing a volume of air in a transparent glass house, and heating it by artificial means, in order to compete with the rigorous variableness of our climate. True, at certain short intervals we could, as it were, entrap within the limits of our glass cases as much or even more heat for a short time than is required; but even in the height of our best summers this would be most uncertain and fitful, and therefore the aid of fire-heat, unnatural as it is, becomes indispensable. Yet it is from the proper use or balancing of these two sources of heat that the most thoroughly satisfactory results, in what is generally termed forcing, to a very great extent depend.
The more sun-heat that can be made available, and the less fire-heat that is made use of to keep up temperature to a proper degree, the better is it for the wellbeing of plants, and vice versa. Of course the horticulturist has no control over clouds and sunshine; nevertheless he can very effectively store up the sunbeams, and save his coals, while he is at the same time doing the very best on this point for his crops. It is well known to all experienced cultivators how much better plants thrive when he can command his maximum temperature with a minimum or entire absence of lire-heat. It is equally well known how injurious it is to have these two sources of heat in vigorous activity at the same time, and how, at certain seasons especially, it is most difficult to hold the one in proper abeyance while the other does the work. True, with a well-constructed hothouse and an efficient heating apparatus, heat under certain circumstances can be measured out to the atmosphere almost to a degree; but capricious sunshine outwits the most careful calculations and management of the fireman; and we are far from envying the person who is placed between a hot fire, a hot sun, and a hot-tempered superintendent all of a sudden!
How often, after a cold night and a still colder dawn in April, is it not found necessary to add a little coal and stimulus to the fire, in order to prevent a sudden and injurious depression of temperature! and presently the sun suddenly and unexpectedly throws his power into the scale with hot pipes, and we have a combination the most undesirable and injurious. To counteract the evil effects of a parching heat, the fire and ventilating apparatus call for immediate attention. The one must be smothered up and beaten down firmly, etc, and the other must be opened to keep down the temperature, and to prevent absolute scorching. To be able to manage this firing and air-giving is no small garden-accomplishment. It requires a great amount of watchfulness and "gumption" to work the shovel and ventilators; and so difficult is it at times to secure the conditions required, that we have often wished we could let heat off or on into hothouses the same as water is let into a cistern, by merely turning a tap, or at all events, that means could be devised by which fire-heat could be withdrawn in the presence of sun; but we suppose this is not possible.
It is, however, quite within the limits of the possible to avoid the evils arising from a strong fire and a strong sun more than they frequently are avoided. Our coldest nights in March, April, May, and June, are generally succeeded by the brightest days; and under these conditions we usually have the greatest amount of the evil arising from hot pipes, a bright sun, and fully-opened ventilators combined. Now, to fix a night-temperature, and order it to be worked up to through cold and windy nights, so that at sunrise a certain degree of heat must be indicated, irrespective of violently-heated pipes and a rising sun, cannot be regarded in any other light than a mistake. Much sounder practice do we regard it to fix a maximum and a minimum heat, the one for mild and the other for cold nights; and to avoid high night-temperatures, taking the example so universally afforded us by nature. Management the reverse of this gives more highly heated pipes, just at a time in the morning when such is not only unnecessary, but most injurious when accompanied with sun, and calling for ventilation to an undesirable extent, simply to keep down the heat within reasonable limits. It is from this state of things that the evils we are anxious to impress upon the minds of the unexperienced arise.
Take, for example, a pit of succession Pines in a bright May morning after a cold night, with the pipes thus heated, a bright sun, and a full flow of ventilation, and what are the consequences? Every particle of moisture which rapid currents of dry warm air can lick up is rushing out at the ventilating openings, and the young plants are being subjected to a rush of dry parched air, which is pumping the very life out of them. This state of things is not easily counteracted if there be no access to the pits - no paths nor vacant surfaces that can be frequently sprinkled with water to diminish the aridity. We need not tell experienced cultivators that the plants are thus exposed to an ordeal which, if it is prolonged, is sure to ruin them. They soon begin to look dejected and parched-like. Their leaves assume the half-circle fold, and look wiry and stunted, the end being that they show fruit prematurely, and are lost. In the case of Cucumbers, Melons, or Vines, it soon tells a similar tale, and red-spider pounces on them and helps to complete what a cruel war from the stokehole has been the chief instrument in beginning.
Of course, when such conditions as we have been condemning are forced upon us, as they sometimes are, action must be taken on their first signs of approach. The fire must be as summarily dealt with as possible. The ventilators should be opened a little and a little more at intervals, and before the heat gets very much too high, and not neglected and kept closed too long, and then more extensively opened. The latter practice allows the moisture inside the pit to be taken up by the atmosphere in suspension, and the plants are first subject to a vapour-bath much too hot, and when full ventilation is given all at once, then to a violent reaction, which goes on, as has already been noticed, till they have to contend with a rush of dry killing air. Especially after some successive days of dull cold weather is this state of things most injurious, from the fact that a growth made in a more moist and dull atmosphere is more tender and sensitive. Under such circumstances, it is best to partially shade when suddenly overtaken with brilliant sun, and to give less air than would otherwise be necessary.
In houses which are accessible, every available spot, except the plants, should be damped occasionally, and the plants themselves dewed over at "shutting-up time," to counteract so sudden and violent a change. And by anticipation of such ordeals, all hothouses and pits should have more or less air in the dullest of weather for a short time every day, not only to change the air, but at the same time to prevent as much as possible a thin flabby growth, and to counteract the mere attenuation of tissue which takes place in the comparative absence of light. Under such circumstances most especially, a maximum night-temperature is to be deprecated. A high night-temperature and sunless days cause growth, certainly, but growth of the most useless and undesirable kind, and which is sure to suffer under the circumstances we have been describing. When sun-heat can be stored by shutting up early in the afternoon, evening and night growth is then desirable, but should be so looked upon and encouraged only in proportion to the extent that it can be promoted with as little fire-heat as possible.
Regarding the points we have touched upon as of great importance in horticulture, we should be glad to have the views of our correspondents regarding them, for we are certain few, if any, of the daily operations of the garden rank higher in importance, or play a more important part, either for good or evil, in the culture of tender flowers and fruits.