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.
By August the Pines were going to rest with a reduced top-heat, and the temperature of the bed stood at 80°; but when the hot weather of September came it rose to 95°, and as it was continuing to rise we had to move the Pines out to cooler quarters, and no farther notice was taken of it at the time. Before returning the Pines to the same house again in December, I however tested the bed, and found the temperature to be 70°, or about 5° higher than the mean top-heat, which at the time was about 65°, pot vines having been in the house for six weeks. When the bed was originally made up more than a year previous, it was only about 2 1/2 feet deep, so that fermentation could not be very active; and no doubt the long-sustained high temperature was due to the insulated position of the bed, for in our ordinary lean-to pits the heat of the beds does not last for half the above period. I have no wish to revive obsolete practices, but were I called upon to erect a range of pine-stoves, I would have the beds enclosed within inner walls; and if a sufficient quantity of leaves was at command to renew the beds partially once a-year, I should not fear the result or grudge the little extra labour entailed, if in the end it would be worth speaking of.
The practice of testing the temperature of hotbeds by means of trial-sticks should be condemned. In experienced hands the plan may be safe, but it is better not to trust to it; and a thermometer should be placed in every bed, and in a position to indicate the temperature of the soil about the roots of the plants. Unless this is attended to, disappointment will often be the result.
The Melon will stand a higher temperature at the root than the Pine, and consequently more exposure to the sun. Under the bright sky of Persia, where Melons are said to attain the greatest perfection, the ground must be heated to a high degree; and as the Melon is a shallow rooter, it must have the full benefit of it. And so we find in forcing that 100° is not too high for its roots in summer, provided the foliage is well exposed to the light and air, and that a high top and bottom temperature is the surest means of preventing damping off, which disease is indeed the result of a sluggish action in the plant or impaired vital energy. In early forcing, however, this practice must be modified considerably. During the dark days of February and March a high bottom-heat will compel the plants to grow; but the pale sickly foliage which tbey make under such circumstances will perish before the first blink of the April sun. In early forcing, therefore, of the Melon, it is much better during dull sunless weather, by a lowered top and bottom temperature, to keep the plants still, and make hay while the sun shines.
Mere elongation of the tissue, the result of hard forcing during the early part of the year, may be growth, but it is not progress, for in the end neither time nor quality will be gained.
With regard to bottom-heat generally, it may be laid down as a maxim, based upon experience, that all plants, from Pines to Potatoes, require a root-temperature equal to the mean temperature necessary for the development of the branches. I do not mean to say, of course, that we cannot succeed partially unless these conditions are strictly fulfilled, but I say that the nearer this is accomplished, our success will be the greater. This is proved by everyday experience; and the question is not whether bottom-heat is necessary, as has been argued at great length in the case of Vines, for instance, but how is it to be applied? That Vines will not only stand, but be immensely benefited by, a high root-temperature, has been proved often enough - witness its effects in the forcing of pot Vines. We started some last November, at the same time as our early vinery; the fruit on these is now ripe, but the early house will not be ready for five weeks. The pot Vines were plunged in a bed of leaves the most of their time; in the other case the roots of the Vines are in an outside border. The inference is plain. Yet we have at various times been astonished with accounts of Grapes being ripened by the beginning of May, though the roots of the Vines had never enjoyed more than a winter temperature.
I am afraid such feats must be put down among the things that are not dreamt of in our philosophy, and be reserved for some future Lindley to unravel.
My convictions regarding the importance of heated borders are unshaken, and have been forced upon me by experience of the most conclusive description, though I am not an advocate of some modes of heating. We have two vineries here, which I will for convenience call No. 1 and No. 2. No. 1 is an old vinery, destined to be taken down in its turn and rebuilt, and which has for the last four years been pressed into service as an early house. The roots of the Vines are all outside. The border is about 4 feet deep, or rather more, and has apparently never been drained in any way underneath - for the reason, perhaps, that the subsoil here is dry and rocky; and in anticipation of having to renew the border entirely before long and replant, we have never disturbed it. No. 2 is a new vinery, but the Vines are about forty or fifty years of age, and in excellent health; their roots are all outside also, but the border in this case, having seemingly been made or renewed at a later period, is thoroughly drained, the bottom being covered with about 9 inches of rubble, and intersected with good drains, 4 feet apart.
In addition to this, as the house was intended to be pushed forward for early work while No. 1 was being rebuilt, we, between two and three years ago, cut a wide drain along the front of the border, the lower end of which was made to open into a pine-stove, which is on a lower level than the vinery; and all the old cross-drains under the border were let into the front drain, which was carried forward into a house on a higher level, so that a current of warm air from the pine-stove was made to pass continually along the front of the border, and by shutting the top end of the drain at intervals it was made to circulate also among the rubble under the roots of the Vines. The plan has been in operation for two winters; and now for the result. The great difficulty with No. 1 has always been the getting the Vines to start easily, and with hard forcing to have the Grapes ripe by the middle or end of May, owing, no doubt, to the deep undrained border into which the roots have penetrated. In November 1867 the border of No. 1 was covered with about 18 inches of leaves and litter, and the house started at the same time. The border of No. 2 was covered the same way a month later, the air-drain turned on, and the house started.
Forcing was conducted in both at the same rate, but the Grapes in No. 2 were only a week behind No. 1 in being ripe, and we were cutting from both houses at the same time to send to London. Again, No. 1 was last autumn started at the beginning of October and No. 2 at the beginning of November; the latter broke readily, and was in bloom before No. 1, which has in consequence been held back as second house, and I fully expect we will be able to cut from No. 2 by the end of April. I need not say that the crop in No. 2 is always by far the best, and there is never either shanking or red-spider. I may state that the current of warm air from the pine-stoves is generally moist, and a mulching is left on the border all the summer.
Assuming, therefore, the necessity of bottom-heat for early vineries, the only question to be solved is, what is the safest and most economical method of applying it? Hot-water pipes are most convenient, and, in judicious hands, are no doubt safe enough. I am very far from thinking that the failures which have attended their use in the hands of a few experimentalists are to be taken as an argument against them. Nevertheless, I believe, if all the Vine-borders in the country were heated with pipes in the ordinary way, there would be such destruction of Vines as was never before heard of, and a universal wail from John o' Groats to the Land's End. People's discretion and good sense are not always equal to their faith in some receipts, and I have strong suspicion that this is the case with some in the matter of bottom-heat. I have in my mind's eye a case where, after much sagacious deliberatiou, it was determined to introduce pipes under the border of a late vinery. The house was erected, the border made, and the Vines planted; forcing top and bottom was commenced, and evidently carried on at a rattling rate, in sanguine anticipation of the result, which was soon apparent.
But "the best-laid schemes of mice and men gang aft agley:" the Vines, instead of making the expected rush to the top of the house in rampant vigour, made a feeble attempt to get hold of the bottom wire, and then perished. After much wonder and comment it was eventually concluded that the soil must be bad, or the Vines had some hereditary disease; at all events, it was clear that they were exceedingly stupid and ungrateful, and did not, appreciate the advantageous start in life which they had had; and so fresh soil was procured, and fresh Vines of a different variety, which were planted and another start made. The plants struggled through the summer, but looked as if another season would terminate their existence. They were, however, rescued in time, and as the bottom-heat pipes were arranged in an unsatisfactory manner in connection with the top-heat, they were plugged up, and their use discontinued. In careless or unskilful hands there are the elements of danger about pipes when they are placed in proximity to the roots; if used continually, as some seem to imagine they ought to be, they exert a dx-ying influence in the wrong place, and arrest all capillary action from beneath.
If they were sunk, say, 18 inches in the subsoil, instead of being laid among the drainage, all danger would probably be averted, provided the border was always well watered, sufficiently to soak even the subsoil if it was dry. This plan would entail a little more expense in fuel, but it would be safe and not impracticable. Fire-heat underneath the roots of plants is unnatural, but it is safe, provided the heating power is situated deep enough. We are told on good authority that the effect of the heat on vegetation around hot springs is wonderful; and I know of a coalpit which has been in a candescent state for years, and its track above ground is marked by the earliness of vegetation. The heat is just sufficient to accelerate the maturation of vegetables and fruit, and keep the water in the wells tepid; and I have beard of cottagers located upon this favoured strip, who send their early Potatoes to the market when they are selling by the pound, and realise a handsome profit. This, however, is bottom-heat on a scale which would hardly pay even for Grape-growing, but it indicates the conditions to be arrived at, if we wish to be successful in its application for that purpose.
J. S. W.