Journal of Horticulture and Cottage Gardener, April 12, 1883. 291-292, 368-369
“Single Handed”

When treating of bottom heat it is usual to begin by quoting figures which prove that during the growing season the heat of the soil is higher than the air. As this fact is now perfectly well known, it is not necessary to take up space proving what is regarded as a truism. We will, therefore, at once plunge into the subject of bottom heat, and the effect of soil-warmth in promoting the growth of plants.

In hot countries the heat of the soil frequently becomes so high as to be absolutely destructive. Under such circumstances cultivators have to keep down earth heat by irrigation, and on a small scale by mulching, in order to rescue the land from utter sterility. Unless during exceptionally hot seasons and on thin soils with a hard or rocky substratum underneath, a too high earth temperature is seldom experienced by British gardeners, unless in the cultivation of some few alpines or plants from northern climes. Generally speaking, it is want of heat that we in our northern latitude suffer from, and especially so in those districts where clouds prevail and the sun seldom reaches through to warm the cold earth below.

Increase of bottom heat is equal to increase of air heat; but while glass houses or sheltering walls and plantations have to be erected at considerable cost in order to secure a higher air temperature, higher earth temperature may be secured by applying the facts which science has discovered, and which cost us little. It is well known that dark surfaces absorb heat, while white surfaces reflect it. Acting on this, we in 1882 darkened the surface of half of the space wherein our earliest batch of Potatoes grew, and also part of our Onion beds. The Potatoes in our darkened section were fully a week earlier than those grown on the undarkened part. The soil was a brown loam. On ordinary garden soil, deeply darkened because much humus is present, such artificial darkening would not, of course, have an equal effect. In the case of the Onions the crop was very superior, not so much in weight as in ripeness and quality, to the portion grown on the undarkened part. But other means were taken to keep up the earth heat, which will presently be mentioned. Had the season been at all an average one the results would have doubtless been even more marked. As it was there was remarkably little sunshine after June.

The last time that Tomatoes were grown out of doors here was in 1878. The climate is against their doing well. In that year all were grown on a border raised 1 foot by means of bricks—experiments the year before having shown the advantage of raised soil. Half of this border was darkened by impalpable charcoal dust, and half left au naturel. Charcoal dust (moulders' black used in iron-founding) was used instead of soot to guard against error; the use of soot, though securing the absorption of sun heat, having proved too stimulating and causing a luxuriance that was likely to be attributed to a wrong cause. Even common coal dust we have found effective. The plants grown in the darkened soil ripened fruit eleven days earlier than those under natural circumstances, and produced nearly double the crop, for the others failed to swell all they set—the early autumn frosts stopped them.

The year before, on the same place, half were planted on the level ground, half on soil raised 1 foot. The year 1877 was extremely wet and sunless in this locality, but even the little sun we had warmed the raised border so that the plants benefited considerably, although no ripe fruit was got from either batch. The extra warmth, however, caused a healthier growth, and the plants produced a few green fruits, those on the flat remaining stunted and fruitless.

On cold clays advanced gardeners have long practised planting fruit trees on raised mounds, experience having proved that better ripened wood and increased quantities of better fruit are thereby secured. The reason is that such mounds acquire a higher temperature than the soil on the flat. "We think this fact has not been recognised in all its bearings, or raised borders would be more frequently seen. In these days, when root- cultivation is well understood and generally practised, it would be well worth while planting wall trees at least on borders raised out of the cold bottom to where the sun would early supply the earth with much-needed heat. Especially in the case of trees on dwarfing stocks or as cordons could this be done, as such require borders of no great dimensions.

Barren cankered trees have been cured of their disease, and their barrenness replaced by fertility, by simply having their roots lifted out of the cold under soil and placed near the warmer surface. Dr. Lindley, quoting from the Memoirs of the Caledonian Horticultural Society, in his "Theory of Horticulture," mentions a case of this kind. Mr. Reid of Balcarras cured cankered trees by taking their roots out of soil which at 8 feet deep was during the summer months 44°, at 18 inches 50°, at 9 inches 57°, and at 6 inches 61°. In a raised border where the soil was dark we, ten years ago, found a thermometer indicate 67° 6 inches from the surface; near by, on the level, it was only 59°. The border, it should be added, had a slope of 25° or thereby. Such figures show that it is throwing away natural heat to make vinery or other borders on the ground level, flat, and 8 feet deep. Bather would we recommend raised borders with a slope to the sun, shallow, and roots close to the surface. Such would cost much less, for excavation would be saved, and less soil necessary. Indeed, as we shall show further on, it would enable us to keep cooler atmospheres, and that means less coal, less labour, and fewer insects.

Earth heat may be considerably increased by cutting off the cold under soil from the warmer upper. In our district it is labour lost in ordinary seasons to put Vegetable Marrows out as is done with success in the sunnier south. Even on sun-warmed raised borders they seldom do well; but when a foot of fresh stable- yard manure is put under the upper foot of soil success, even in such woeful years as last (1882), is nearly certain. Six plants of Muir's New Hybrid were last year planted on the level and nursed for a month under handglasses. They never bore a fruit, though pinched and trained as carefully as Melons. The earth heat was never over 57°. Six on a raised vinery border with a good slope bore twenty-nine fruits, although the spot was more exposed than in the first instance. The earth heat reached and remained some time at 61°, but was not over 59° till August (June and July were very wet), and none ripened. Only nine reached full size. In a position more exposed than either of the other two, six other plants were placed out on a border raised 1 foot by that thickness of very fresh hot stable litter under the soil. This material was laid flat 3 feet wide, 1 foot under the surface. Four days after—the weather was very bright then and also hot (18th May)— the plunged thermometer stood at 72°, a month after it was 67°, and at that it remained till the end of August. What it was in September I was too ill to ascertain, but in October it was still 60°, and that after a peculiarly sunless season. But, in fact, by July the heat was protected by the covering foliage. The produce was, up till September, forty-five fully grown fruits, many of which in a very bad year and on an exposed spot in a peculiarly exposed late locality, ripened. After August the gathered produce was as my man expressed it, "a big barrowful," some of which were being used in February, and all this because of extra bottom heat alone; this heat being secured for only a little trouble, the manure remaining being as good as at the beginning, for it was never wet enough to lose its virtue. No doubt the raising of the border contributed to the result, and also the heat in the manure when first buried. This raised the heat at once. The non-conducting nature of the material and the thick leafy covering preserved it, and this made all the difference between a moderately good crop in a season and in a locality where without such assistance the plants only cumbered the ground. The use of blacking might have secured a degree or two more, for when this was tried on French Beans the plants produced a moderate crop of beans by the third week in August; without it they only produced half-formed pods, which never came to be of any use at all, they were so late, so stunted, and tough.

Very much may be done to secure a higher earth temperature than natural by the use of coverings. By covering at night the surface of a 14-inch pot, plunged in ashes with dried sphagnum at one time and sawdust at another, the soil maintained a temperature of 63° through July and August. One exposed at night was barely 60°. Mr. Thomson of Drumlanrig mentions the following striking instance of how earth heat may be preserved. In October he covered a vinery border with 18 inches of wheat straw laid on like thatch. At midwinter the earth heat was 60°. An uncovered border was as low as 40°. The difference to the plants, supposing both to be forced into growth, would be very considerable. The sap feeding the one would be 20° below that of the other. Under such conditions the one would do well, the other might fail—and failure from such a cause has resulted before now.

Watering tends to lower earth temperatures: rain has the opposite effect. This may seem a paradox. Watering is only resorted to during drought. But under such conditions the water applied evaporates rapidly, and there is nothing that lowers the temperature of the soil so rapidly as evaporation. But when rain falls the atmosphere is saturated, and evaporation proceeds slowly or even not at all. Then we must remember that heat descends slowly. It is this fact which accounts for the coldness of the under soil. Heat inclines to ascend. But when rains falls in the summer it carries heat down into the soil. It changes the air in the soil, too, by displacing it. As the rain afterwards leaves the soil, warm air follows. Thus rain warms, and artificial watering cools, the soil. This fact is taken advantage of by those who find their climate too hot for the particular plants they cultivate.

Watering, for this and other reasons, should not be indulged in unless absolutely necessary. Many check all growth by their perpetual dribblings. After watering, as soon as the soil is dry enough, means should be taken to preserve not only the water but the earth heat by checking evaporation. In hot seasons short grass, manure, leaf soil, cocoa-cut fibre, &c., may be employed as a mulching. But even a hoeing, in order to produce a loose layer of surface soil, will do much to check evaporation, while it will allow sun heat to pass on. In the case of many tender plants this is a very important matter.

Draining raises earth temperature. Soil that is wet is also cold. Instead of using sun heat to raise its temperature it employs it to evaporate the water. Wet soils are always cold, and a plant on a cold soil draws up cold sap, which keeps the plant cold no matter what the air heat may be. Nay, a watery diet produces even a greater degree of cold, for, in order to get rid of the superfluous water in the over-diluted food, the plant must evaporate more than is necessary, and thereby renders itself colder still. It is this that makes corn on wet land late. Draining, then, is of paramount importance, for thereby the soil is warmed, the sap plants draw up warmed, and this is seen to be as good as an increase of air heat for the plants. Corn, vegetables, fruit mature sooner and improve in quality as they do when transplanted to a better climate.

We spoke of the better Onions that we raised by darkening the soil, and hinted that another method was used to promote a higher ground temperature. That other method was to keep the surface soil constantly stirred with the hoe, especially after being battered by rain. Even in such a sunless year as last the results were markedly superior; but in sunny years we have even seen a greater difference.

In the application of bottom heat by means of hot water or fermenting material sometimes too much is given. It is necessary to guard against this. Often Seakale and Asparagus are "drawn" so much as to be useless by too much bottom heat. A safe rule is to keep within 5° of the earth heat in the warmest summer month outside for most hardy plants. But "drawing" is not always an evil. In the case of forced Lily of the Valley, for instance, drawing is systematically practised.

The earth is a vast storehouse of heat. By coverings much of the heat thus stored might be utilised. To some extent this is already done. By mulchings of manure Roses are protected. Many tender herbaceous and lowly alpines are kept at a temperature much above that of the air by coverings of cocoa-nut fibre, leaf mould, or even ashes. These are all, more or less, good non-conductors, especially when kept dry; but it is here many protecting materials fail. Perhaps the best article ever heard of is the waste of flax—pob it is called. For mulching, covering frames, Potato pits, water pipes, few things equal it, for it is hardly possible to make it wet. Such acts by keeping the comparatively warmer earth separated from the much colder air, just as the layer of litter in summer kept the different temperatures of the upper and under soil apart, and prevented the one neutralising the other. All act as protectives by conserving the earth heat.—Single-Handed.

(To be continued.)


Continuing this subject from page 291, the object at present is not to teach what degree of earth heat this or that plant is benefited by, but rather to direct attention to the principles that ought to guide the cultivator in the application of bottom heat. Were the object merely to teach the degree of heat required by Melons, Pines, Cucumbers, and other plants to which artificial bottom heat is applied, the space at our disposal would be wholly filled up with facts and figures to be found in every book treating on such subjects as we have named, and in every calendar of gardening operations. Our purpose is different, and is rather to show to what an extent bottom heat might be utilised, and the principles that ought to guide us in our application of it.

Apart from such plants as those named, which are natives of tropical climes where the earth heat ranges high, and which we are all agreed are benefited by high, artificially raised, earth temperatures, there are many others grown at unnatural seasons —in other words forced—that also need bottom heat in order to do them justice.

Not very long ago some gardeners denied the utility of bottom heat for plants treated a; midwinter to midsummer heat. Vines were said not to require it. This need hardly be discussed. Failures have often occurred by men trying to force tops and allowing bottoms to remain dormant; and were gardening failures recorded as faithfully as gardening successes, the black list would be so appalling as to effectually frighten young practitioners from doing things upside down—from reversing natural operations.

Our most successful gardeners maintain the necessity of raising the soil heat as the air is raised. This is only natural, and Nature is generally a safe guide to follow. Nature and successful men are thus far agreed.

The best manner of applying this heat is not, seemingly, agreed upon. Many consider that when, the roots of early Vines are confined to inside borders no further trouble is needed to secure a high-enough earth temperature. When the roots are near the surface, as they should be, those who hold such views are doubtless right, especially when soakings of warm water are applied and a dry surface maintained, for, it should be remembered, a wet surface means evaporation, and evaporation cold. Others, again, build hotbeds inside, which assist to start the Vines by warming the air, and also to keep up a steady moisture; but the heat thus imparted to the border is much less than is generally thought to be the case. Those who have tested the matter in the only satisfactory way—by inserting a thermometer, know this to be so. Applied to outside borders, otherwise drenched with melted snows and ice-cold rains, hotbeds will do much to impart bottom heat to the outside roots; but they exclude air and turn the soil to a puddley inert mass, which is not favourable to the production of roots of the best character. Dry sun heat secures a circulation of air in the soil, and this air produces change of a beneficent nature. Hotbeds have an effect quite the opposite, and are, therefore, an evil, if the least of two, as they undoubtedly are when the choice lies between borders windswept and soddened down to a minimum British winter temperature, while the tops are treated to a French or even Italian summer heat and hotbeds. Still hotbeds are by no means faultless. We leave outside of the question the amount of labour tbeir management involves, and their untidiness, and only discuss their fitness for securing the conditions for which they are employed.

Bottom heat for fruit borders is frequently secured by another method. By coverings of non-conducting material the beat stored during the previous summer and autumn is preserved. Although recommended by the best of gardeners, including one we all look up to—Mr. Thomson, Drumlanrig—it may be questioned whether the plan is so good as seems at first sight. The experiment already quoted proves the possibility of preserving the heat; but, as the proof of the pudding is in the eating, nothing but long successful experience could make us certain that bottom heat during the resting period is not harmful. Vine roots under natural conditions, and others as well, remain dormant till the leaves are matured enough to carry on the functions of leaves; then the leaves begin to act, and then new roots are formed. All growth before this is at the expense of the material stored in the stems the previous autumn. Out of this the newly formed leaves and shoots are formed, but by giving bottom heat enough the roots may be made to move first—nay, they may be made grow considerably before the buds move, as anyone possessing a pot Vine may prove. This is unnatural, and, we are convinced, mischievous. The shoots of Vines so treated have a smaller btore to fall back upon than have those which get their fill before, metaphorically, the roots help themselves. Now, roots in soil, the heat of which has never been allowed to decline, are kept abnormally active. They are deprived of their winter's rest, at least to some extent.

Bottom heat is supplied to Vines by pipes. These are generally covered by flagstones, on which rests the drainage. That, when properly managed, heat thus applied has proved of real benefit is beyond question, but it is not uncommon to see such arrangements taken to pieces after a few years' trial. Heat thus applied has a tendency to cause an excessive dryness exactly where the best roots are, for these travel towards the heat. Even floodings often fail to wet soil once thoroughly parched, and this parching is liable to occur even in good hands. The consequences are shanking, stinted Vines, poor crops. Given the requisite skill and attention, there is no reason why heat applied from below to Vines should not succeed as well as when applied to Melons. Theoretically that is how the case stands. Practically gardeners find it cheaper and more satisfactory to have inside borders only for the very earliest house, and these, when a pit exists for forcing pot Vines, are from one to two months later than was the case only a dozen years ago.

But for the expense, there can be little doubt that the best way to warm outside borders in winter would be to cover them with glazed sashes and to maintain a summer temperature over the border by hot-water pipes. Most persons who have forked over an inside border must have remarked how the roots cluster near the surface in the vicinity of the hot-water pipes. Such a fact proves that heat by heated air from above is not only most natural but most efficacious. Nor need the heat of such a frame be wholly devoted to warming the border. Such a place would prove of more than usual value for forcing many things, especially salads, during winter.

Before the invention of heating by hot water, flues were occasionally employed for producing bottom beat. But flues are now nearly obsolete, and would not have been named here but for the fact that there is an idea abroad among unpractical men that economy is observed by carrying the flue used for conveying the smoke from the boiler fire underneath earth beds for growing Melons, Cucumbers, and other crops, in order to get bottom heat for nothing, as is believed. The first objection to such a practice is, that it necessitates deeper stokeholes than would otherwise be necessary. The second is that one end of a flue is hot and the other cold, when of any considerable length. But, worst of all, it is impossible to regulate the bottom heat by such means. When the flues are newly cleaned the heat is too high, when foul too low. On bright days there is no bottom heat, for the fire is allowed to go out. The sun which warms the air indirectly cools the soil. We only know of one such modern erection, and our faith in the wide dissemination of common sense leads us to believe that not many such arrangements exist, but that one is such a failure, and such a waste of labour and capital, as leads us to warn others against adopting the seemingly good theory in their practice.

Bottom heat is of great service in the propagation of plants. This in modern well-appointed pits is supplied by hot water; but well-appointed propagating pits are the luxury of the comparatively few. To the mass many makeshifts have to be resorted to. One we may mention. A sheet of iron placed over hot-water pipes in any early vinery or elsewhere, on which is placed a handlight and a few inches of wet sand, will prove a capital little propagating frame. Inside a common greenhouse or larger frame, or even no frame at all, the little lamp-warmed frames are of much help to the grower with few wants and inadequate appliances. But a good dung frame is still one of the beet propagating appliances. On the management of such room cannot be spared here. But it may serve some good purpose to say here that a mixture of leaves and frequently turned stable litter, so universally recommended and so un-come-at-able by thousands, is not at all necessary for the making of a thoroughly satisfactory hotbed. Tan, which is a waste product, is quite as good as leaves, and a bed made of half stable litter, half tan, will last good for months. Failing tan, half-decayed manure will answer very well; Stableyard litter alone in a fresh state is quite unsuitable—it heats too violently.

A half-spent hotbed affords an amount of bottom heat that is better for the propagation of many things than a new one. On such many hardwooded plants may be rooted that would be excited too much in a warm bed. On an old hotbed under a close frame Rose cuttings of firm wood strike readily in July and August.

The "Pob," already mentioned, when thoroughly moistened gives out a very gentle heat, which lasts for months during winter. Cuttings of a large number of coniferous plants, inserted in pots and plunged in this under a frame from which frost is excluded, will callus and root when they will refuse to do so either without bottom heat or when treated to what is generally considered bottom heat, but is too exciting. It is said that soap waste makes a good and lasting hotbed.

Bottom heat stimulates by the warmth it supplies. It does more. Much of the ordinary manurial matter applied to plants is not available for plant-food till changed. Warmth hastens this. Nothing stimulates rapid growth like nitrates. These form in all soils whenever alkaline bases exist alongside of nitrogenous matter. But when the soil is cold the process proceeds very slowly, when frozen not at all. But the change proceeds very rapidly where there is a bottom heat of from 75° to 90°. It is doubtful if ammonia, or even urea, can be directly utilised by plants. But these substances in a moist fertile soil begin to change as soon as they are fixed; so that what to-day may be regarded as a mineral substance and not fitted for plant food, may, when the soil is warm, be converted by an invisible bacterium into plant food, and by the plant into plant-tissue or starch or sugar by this time to-morrow. Wonderful are the operations of Nature and well worth finding out.

Space has prevented us doing more than merely skimming the surface of a subject that we advise our readers, and especially the younger of them, to dip into deeply. We have touched on all the salient points that have occurred to us, and although much more might be written on it—perhaps with profit—the object we had in view will be served if it directs the attention of our readers to a phase of gardening practice too much neglected. By shelter we may protect our plants from the bellowing blast; we may raise the temperature by raising walls that warm the air by taking in the heat from the sun and giving it to the air; and all this we have endeavoured to do, and then made the most of it. It is doubtful if, to an equal extent, we have utilised earth heat.—Single-handed.