Regulation of temperature in a greenhouse

It must be always borne in mind that shelter, not coddling, is the aim and end of the unheated greenhouse, and the grand difficulty in face of cold-house gardening is how to maintain a fairly equable temperature. When the selection is limited to strictly hardy plants, the regulation of cold is, probably, of less importance than that of heat, but when half-hardy plants and bulbs come within the range of our desires (and it is well that they should) the greenhouse thermometer in winter must not be allowed at any time to fall below 35° Fahr., which is the lowest temperature at which frost can safely be kept at bay. But the owner of an unheated plant-house aims at reaching a point beyond mere safety.

He is ambitious enough to hope that his winter conservatory may rival, if it may not in some degree even surpass, the hot-house devoted to tropical plants, in its wealth of flower and greenery gathered together from distant quarters of the temperate zone.

Emulation, it is true, strikes but a low note in the scale of ethics, but human nature needs a healthy stimulus to rouse it into action, and so we are none the worse for a friendly contest with a neighbour. But there does come a time when winter is at its darkest and coldest, from the shortest day onwards for a few, though only a few, hard weeks, when it must be confessed that it is almost impossible to keep up a bright display of flowers in a greenhouse that is totally unheated.

Not quite impossible, however, for we are by no means at the end, but rather at the beginning of our possibilities in this direction; yet even very hardy plants have a trick of looking self-conscious and rueful in a black frost, and it is the very motive of the cold greenhouse to make hardy plants comfortable and to coax them into earlier flowering.

Personally I have always been quite content with a small, portable apparatus, called a Heat Radiator, the invention of an exceedingly able mechanician, Mr. Gillingham, of Chard, and constructed on scientific principles. This has been found to answer well in unheated greenhouses of moderate dimensions-say from 15 ft. to 25 ft. in length.

The special advantage of the little brass or copper cylinder which goes by the above name, in addition to its neat appearance, is its portability. When it is out of use, it need take up no room in the greenhouse, but may stand in any convenient hall or passage. Should a change of weather give warning of danger, which sometimes happens very suddenly, five minutes will suffice to place it in position and light the lamp, when a rise of several degrees of temperature will very speedily be shown on the thermometer, which should be in every plant-house.

On rare occasions it may smoke, but this calamity-for calamity it is-is generally due to want of care in trimming the wick, and can be avoided by the most ordinary attention. Few things are absolutely perfect, but the principle of the Heat Radiator is excellent-it is not unsightly, and as far as I know, though by no means a new invention, has never been surpassed for simplicity and efficiency, as a temporary means of keeping out frost.

During those few dreary weeks to which reference has been made it was always a simple matter to keep the air in circulation on chill, foggy days, or a night temperature, according to weather, of between 35° and 40°, which was all that was wanted to make Daffodils and winter Iris and a host of charming spring flowers hold up their heads in happy contrast to their fellows in the garden outside.
There are several other inventions for the purpose, none of them perhaps without some drawback, and every season sees new additions to the list. A demand seriously maintained for a perfect removable apparatus of sufficient power to keep out frost will surely call forth exactly what is required.

It may be worth while to make a note of a very rough-and-ready contrivance which has been successfully extemporised on occasion for keeping out frost. An ordinary lamp is lighted and placed on the floor, with a pillar of bricks on each side to form a support for a very large flower-pot, which is inverted over the chimney. Upon this a pyramid of inverted pots, each a little smaller than the last, is built up, and the hot air rising through the holes of the pots heats them through and through, and thus a stove may be devised on the spur of the moment which will diffuse an atmosphere genial enough to keep the enemy, for the nonce, out of a house of moderate dimensions.


By some strange perversity the proper ventilation of the amateur's greenhouse is often one of the last details of construction to be considered, which would never happen if experience could only be bought ready-made. Plenty of air is as necessary to plant life as to human beings, and it would seem to be as unreasonable to plan a greenhouse without ventilation as to build a cottage with windows not made to open ; yet it is by no means an unheard of case for a greenhouse (mostly of the smaller sort) to have no other means of ventilation than the door, while, even in more important structures, roof ventilation is the exception rather than the rule. For perfect efficiency there should be not ventilators merely but a system of ventilation.

Free circulation of air is indispensable, but it is not always recognised on the other hand that draught is most injurious to plant growth. The shrivelling of tender foliage, especially of Fern fronds, generally set down to want of water, is due, more often than is suspected, to exposure for a few hours to cross-currents of air. It should be possible, therefore, in all greenhouses to "put on" and "take off" air, in gardening phrase, according to the way of the wind, and in order that this may be done effectively ventilators fitting closely and firmly must be provided both at the top and at the sides of a plant-house.

A span roof 20 ft. in length should have four ventilators placed alternately at each side, and over these, upon the ridge, it is well to have a wooden cap as an additional safeguard against drip. Strong hinges of the kind called " water joints " are to be recommended, as being more durable than the ordinary make. In a long house it is a great saving of labour to be able to open and shut these top ventilators simultaneously by an iron winch. Side ventilation is provided for, either by having the lights made to open or by wooden shutters let into the brick walls.

The latter alternative is in use, mainly, when the span roof rests upon the wall-plate-a form of glass-house rarely seen in any but professional gardens, though such " pits " are admirably adapted for plant culture.

It is difficult to lay down precise rules for ventilation, but as a general principle it must be understood that the treatment of plants in an unheated greenhouse is in strong contrast to that given in the stove, where a close humid atmosphere does no harm. There are not many days in the year, even in heavy fog, when at least a chink may not be left open to allow free circulation of air, for though it is a common practice to keep greenhouses closely shut in foggy weather, a little dampness from outside is actually less harmful than stagnant damp within. In winter it is always safer to open the upper ventilators rather than those at the side. The chief trouble comes in the early days of spring, especially if we are ill-provided with blinds.

Then the thermometer is apt to rush up to 8o° or so, on the slightest hint of bright sunshine on the glass, while a keen wind may be blowing from north and east. It is often hard enough, under circumstances like these, to know how to regulate the temperature. The temptation is to open side-lights as well as roof ventilators and to let in all the air we can, for scorching heat is as bad for hardy plants as frost for those that are half-hardy, yet even hardy plants under artificial treatment easily " catch a chill " at the roots in this way, from which they may never recover.

At such a moment we find ourselves on the horns of a dilemma, and very much inclined to sympathise with the pitiable case, near akin to our own, of the gardener of whom Mr. E. V. Lucas tells us, who wrote to his employer : " I'm varry sorry to tell you that I cant do enaything with the greenhouse. I think he will kill every plant I have sometimes he will get varry hot and another time I cant get enay heat in him and we cant stope him from smoking so I doant know what to do with him " ! Happily the troubles of the cold-house gardener are in great measure simplified, and if the ample provision which is so essential be made for ventilation, experience will soon teach the necessity of avoiding draught. Only let it be remembered at the same time that a stuffy stagnant atmosphere, persisted in for a few days only, will surely set up an invasion of that very infectious complaint " damping off."
For eight months of the year it is scarcely possible to give too much air, and where it may chance that a glasshouse is chiefly used for the shelter during winter of half-hardy shrubs, or Roses planted out, it is an excellent plan to have it so built that the lights can be altogether removed when required. Of late, portable houses on rails for forcing Lily-of-the-Valley and other temporary crops have come somewhat into vogue, and some adaptation of the idea might be of value for the glass-garden. In any case spare lights can always be used to good purpose in various ways-e.g., for Tomatoes during the summer-and need not lie idle.


It is from February onwards that shading, no less than the ventilation, of the unheated greenhouse becomes a serious question. The shelter of glass from storm and stress is heartily welcome, as we know, to hardy and half-hardy plants which flower at inclement seasons, yet no sooner does the sun begin to shine in his strength than the glazed roof is apt to become a funeral pall. If the scorching rays are not by some means tempered, a sudden farewell will have to be taken of faded and withered flowers. Tulips open wide and pale, beneath the ardent sunbeams.

Narcissi lie prostrate and can never more be persuaded to lift their languid heads. Rhododendrons and Azaleas hopelessly cast off their flowers and dangle them in an aggravating way by their long pistils. Camellias put on brown edges in token of mourning, and every flower seems to pant out a feeble protest against the cruel fate that took them away from the fresh open air and consigned them to a furnace-so soon are benefits forgotten when times are changed !

But blinds of tiffany or thin strong netting-materials which give sufficient but not too dense shade-fixed on rollers, so that they may be raised or lowered at pleasure, will save all this disaster. Sometimes the sunshine lasts but an hour or two, yet every gardener knows that more mischief can be done by an hour of neglect than can be repaired in a season, for it is not only the flowers that suffer, but serious and lasting injury is done by the scorching and scalding of tender leaves, which are in truth the lungs of the plants.

Permanent shading is sometimes given by washing over the inner surface of the glass in spring with " Summer Cloud " or some similar composition, and not removing it until the autumn, but it is only rarely expedient. Plants subjected to constant shade soon get drawn up in a vain attempt to reach the light, and their leaves grow pale and sickly. Occasionally, however, shading of this kind may be used with advantage in a Fern-house or for a conservatory built in the eye of the sun, the tenants of which are continually shifting their quarters. In such cases permanent shading-i.e., for the summer-may be an actual gain, but no clouding of the glass looks well, and it is at best but a poor makeshift.

For many years, during the bright months, I used fixed blinds with the best success in a sunny conservatory which had no established occupants. These were made of very thin strips of wood painted green, through which the light passed freely but with a softened shade as of sunshine in a Hazel coppice, full of restful harmony. I never saw such as these elsewhere, but in some positions nothing could serve the purpose better, and many comments of approval were bestowed upon them. In most plant-houses of the kind under consideration, however, roller-blinds are undoubtedly the best.

Where these cannot for any reason conveniently be fixed, tiffany may be fastened to the rafters by hooks and rings, though at the expense of a good deal of time and trouble in putting up and taking down. It should be hooked to the rafters rather than to the sash bars, to allow the hot air to circulate freely between the shading and the glass.

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