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 might be reasonably supposed - as was formerly argued by Knight, I think - that plants like the Pine or Melon, for instance, should not require a more stimulating root-temperature than would be communicated to the soil in which they grew from the atmosphere of the hothouse, provided the temperature of the air was at all times properly regulated, seeing that in their native climate the heat of the ground is entirely dependent upon the temperature of the air, or solar radiation; and I conceive that but for our cloudy sky we might dispense altogether with hot-water pipes or other artificial means of supplying bottom-heat to some kinds of plants, for the successful culture of which such appliances are considered essential. The hot summer through which we have lately passed proved this; and could we have foreseen it, we might have saved ourselves the trouble of providing hotbeds for many things.
For weeks the Pines here may be said to have grown in the open air, for the lights were taken off the pits altogether during the day, and left half off at night; and with great apparent advantage to the plants, though they required a great deal more water at the root. Such summers are, however, not to be depended upon in a general way, and so we have to look to more reliable sources than the fickle presence of Sol for a supply of bottom-heat. Yet though I fully appreciate the importance of maintaining in all cases a root-temperature corresponding to that required for the healthy development of the branches and maturation of the fruit, I think we are sometimes apt to over-estimate its advantages, and that its application is not unfrequently a cause of failure. In speaking of bottom-heat, I of course use the term in its ordinarily understood sense, and refer to the auxiliary appliances generally in use for such purposes.
Considering the reciprocal action which exists between the roots and the branches, it cannot be doubted that when the top and bottom temperatures are out of due proportion with each other the worst effects must follow, as is proved by experience. It would be proper enough to run the temperature of a vinery up 15° or 20° with sun-heat, but no one would think of doing so with fire-heat in a dull day. Yet it is common enough to see the bottom-heat of Pines and Melons maintained steadily at from 90° to 100° for days and weeks together, during which the maximum top temperature may perhaps never exceed 75° or 80°. No doubt it is sometimes difficult to regulate the temperature of the bed as nicely as could be desired when it is composed of fermenting materials, but when pipes are used it may be controlled easily enough, and, as a rule, it should never be allowed to exceed the mean temperature of the house above 2° or 3°, if healthy foliage and fully-swelled, heavy-weighing fruit, either of Pines or Melons, is an object. This, at all events, is according to my short experience. "Within certain limits, size is no indication of weight in a Pine, as I have proved over and over again. Hurried fruit always weighs light, and is of inferior flavour.
The heaviest Pines, in proportion to the number of pips we have ever cut, were from plants that had never been subjected to higher bottom-heat than 82° or 83°, and that figure we never attempt to go beyond with fire-heat. I saw some time ago, at a horticultural exhibition, a Queen Pine about 24 lb. weight, which had deservedly received the first prize, though much larger fruit were entered against it; but it was one of the most beautifully-developed fruits I ever saw, and on that account it was awarded the prize. I afterwards found that it had been grown on a bed of leaves, about 2 feet deep, in a pit, and that the plant had not been disturbed for six or eight months.
I must confess to a hankering after the old-fashioned hotbed, in preference to hot-water pipes. Though it entails more labour and inconvenience, there is a virtue about it that the hot chamber does not possess; and in conformity with this predilection, and as we happen to have a good command of leaves, we use them rather extensively. We winter our fruiting Pines in a house heated with hot water above and below, because we can graduate the heat better; and we keep them there until they are about showing fruit, when they are moved into another house where a bed of leaves has been prepared, upon which they are just set but not plunged, the pots being filled up between with rotten leaf-mould, the better to guard against a too-violent heat. When our present fruiting plants were moved into this house in December last, the bottom-heat stood at 78° at the bottoms of the pots. When the fruit was fairly visible, the temperature of the house was raised 5°, and the temperature of the bed rose gradually to 84°. It has never exceeded this yet, and the earliest plants are now (March 12) out of bloom; the fruit is well above the foliage, robust, and full of promise.
The heat of this bed will last for twelve months, and after the fruit is cut off the present lot in June, the succession plants will take their place without any additional fermenting material, and be left till they complete their growth, as we fancy the gradual subsidence of the heat of a leaf-bed more congenial to the perfect maturation of the plants.
It is better, certainly, to have a command of bottom-heat by means of pipes when needful. Still, though I never made the experiment purposely, I have seen sufficient to convince me that, were the plunging beds of pine-stoves always properly insulated, so that the plunging material would not come in contact with the cold outer walls of the house, whereby it is robbed of its heat, Pines and other things might be grown during summer at least without the aid of pipes or fermenting material, as heat sufficient to maintain healthy root-action would be communicated to the soil from the air of the house. In confirmation of what I say, I will relate an instance that came under my notice here within the last twelve months. One of our pine-stoves, an old house, is constructed something on the principle I have hinted at; i.e., the pit inside the house is separated from the outside walls by a sunk path which runs all round it. In November 1867 this pit was filled up with leaves, which were turned over and incorporated with some decayed leaf-mould, the remains of a former hotbed. Cucumbers were grown in the house during winter and spring, and after they were over the bed was levelled down and a lot of succession Pines were plunged in it.