Hints on greenhouse construction

Too often it happens that we are the victims of circumstance and have to make the best of what falls to our lot, but when we can be the architects of our own fortunes in greenhouse matters it is well to know beforehand exactly what we want. Without pretending to enter too much into details, which must be regulated by individual cases, let me set down a few points of construction of special importance to the well-doing of the unheated greenhouse which have come under my own notice.

Aspect

To begin with, a few words on aspect, which, though they may apply equally well to all plant-houses, may not be out of place here. It is generally admitted that for the successful culture of plants a span house, wherever it is possible, is far and away the best form of glass structure. It is warmer and lighter, and in it a genial atmosphere can be more easily maintained. An open position, away from high walls and trees, should be chosen for its site, with a run, by preference, from east to west which, on the whole, gives the best results.

But a lean-to house is often a necessity, and, in such circumstances, should be placed, if possible, against a south wall, especially when it is required for the bringing on of early flowering shrubs or bulbs, and general working purposes. For a conservatory the case is different, and east, west, or even north, should be chosen rather than a southern aspect, which is generally far too scorching in spring and summer for the well being of plants in flower, though necessarily colder in winter.

It has happened to me to enter into possession of a conservatory so badly planned at the outset, though intended to be imposing, and such a veritable suntrap, that it was hopeless to try to keep it well arranged with fresh-looking foliage or flowers. Ultimately it was turned into an abode for Cacti and succulent plants, for which it was passably well adapted. For strictly decorative purposes, therefore, a north aspect may be not without its advantages. Otherwise it is to be avoided, as it can only be used successfully for certain shade-loving plants, of which Lapageria may be given as a notable example.

It is well to remember that under any aspect-favourable or otherwise-something in the way of plant life may be found to which the special circumstances are exactly suited, and so, when these are adverse, to take courage. It sometimes happens that a most unpromising greenhouse may turn out to be a blessing in disguise. A lean-to house in a north aspect, for example, has been found before now to be specially well adapted for the culture of certain fine Orchids, such as Disas, and for New Zealand Filmy Ferns.
Site and aspect being chosen, the special needs of the unheated greenhouse must be considered. These are dryness, ventilation, and shading.

Dryness

It may seem absurd to insist upon dryness as an essential point in a plant-house, which, from its very purpose, must be more or less damp ; but there is always a point when a blessing may become a curse, and of all worries and annoyances in a cold greenhouse there are none greater than drip. The evil is not so urgent in summer, though even then there are bulbs put aside to ripen, or succulent plants on no account to be over-watered, which the drip is sure to visit in preference to the moisture-loving plant a foot or two distant ; but in winter, when heavy rain is often speedily followed by hard frost, to have pot plants soaked with water and then frozen is simple ruin, and means many a heart-break.

It is by no means a very easy matter to avoid this trouble, and carpenters inexperienced in horticultural building have sometimes to be employed, especially in the country, and they are apt to make mistakes. It is probably safer to put all such work into the hands of some well-known and established firm, but it is a very general practice in these days to obtain woodwork and glass all ready to put together and to do the fitting on the spot with such assistance as is at hand. When this is done care should be taken to insist on well-seasoned wood, the lack of which is a fruitful source of mischief.

Forewarned is forearmed, and a few hints to those who have to overlook an inexpert workman or to do the fitting themselves may be helpful. The slope of the roof, the quality and lap of the glass, and the shooting to carry off the surplus rainfall, are all matters of moment which cannot be provided for at haphazard with impunity. It is an axiom well understood by gardeners, but not so well by amateurs, that a plant-house should be no higher than is actually necessary for the plants grown in it, allowing, of course, comfortable head-room for the cultivator. In other words, the plants should be as near the light as possible, a lofty house in most cases being positively detrimental to them.

On the other hand, a low-pitched house will, as gardeners say, " keep the moisture down," which means a damp-laden atmosphere. Now this is just what is wanted in some cases-e.g., in a Cucumber pit-where strong heat and heavy moisture are essential factors in successful culture ; but in an unheated house moist air, which in winter is liable to be stagnant and must be chill, is exactly what we have to avoid. To put it more accurately, we must have it in our power to regulate the degree of atmospheric damp, supplying moisture when required in hot, dry weather, but keeping mostly on the side of moderate dryness both in summer and winter. Where the roof is steep, however, hot sunshine will cause a rapid upward current, which dries the air so thoroughly that plants will quickly droop, and unless constantly refreshed they will soon hang out signals of supreme distress in the way of blight. Not only so, but a steep roof takes more glass and presents a greater surface to frost.

A happy mean must therefore be struck, and it lies between the two extremes of 25° and 35°, or at most 40°, with the proviso that the house be kept as low as it reasonably can be in accordance with its proposed purpose, whether it be for dwarf-growing alpines or flowering shrubs or Roses. For a small house 20 ft. by 12 ft., a good average slope would be 37°, for if the width be narrow a higher pitch is required to give standing room ; but in planning a house it is much better to allow for as much breadth as possible, as it will be found more convenient in every way.

Careful glazing is essential. Glass of 21 oz. quality should always be used, to save breakage amongst other reasons, and a good average size for the panes is 22 in. by 14 in.

It was pleaded not long ago in a monthly serial of high artistic authority that greenhouses should be built "as of old, with small sheets of glass laid thickly overlapping, and more proof against scorching and freezing than the neat, big-sheeted, modern kinds," and from an artistic point of view this advice cannot be gainsaid. There is truth, too, in the argument that thick overlapping may give more shade in summer and greater protection in winter, though it does not counterbalance other disadvantages ; but greenhouses, unhappily, are not picturesque, and we must be content meanwhile with the practical side of the question.

So let the gardener court all the light that is so essential a factor in the well-being of plants. An overlap of half an inch is quite enough. When it is broader green mould collects, which is not even picturesque in its ugliness, while in hard frosts the moisture which lodges becomes frozen, and very often cracks the glass. Smoothly ground edges to fit closely without any lap are sometimes recommended, but the panes are liable to slip and occasion needless trouble. In any case this plan is better left unattempted by unskilled hands. As a hint to an inexperienced glazier, and to ensure a water-tight roof, the squares should be well bedded in putty, which must be neatly trimmed off within and without-no outside putty being required-and it is well to use, besides, four brass-tacks to keep each pane in place. If these small details are not overlooked, and the woodwork kept at all times thoroughly well painted inside and out, there ought to be no fear of annoyance from drip. An excellent method adopted by a thoroughly practical horticultural architect is slightly to round off the lower edge of each pane of glass, which attracts the wet to the middle point and greatly lessens the risk of drip.

No less important is it to avoid ground-damp, and a good brick or concrete path in a working greenhouse is a great boon. In a conservatory opening out of rooms a flooring of tiles that are non-porous is very desirable. Flags are commonly used for this purpose, especially in the country, and perhaps look better, but they " hold the wet," and if quite plain tiles are chosen there need be nothing to offend the eye, while the gain is great. In hot weather the floor can be sprinkled several times a day, and the moisture will quickly find its way into the air and leave no puddles behind it. Borders are not to be recommended, as has been said before, or any kind of planting out in a conservatory, especially in a small one, partly, though not wholly, on account of ground-damp, but this must be a matter for individual taste to decide.

Delightful as it is to look out upon a garden picture of bright flowers close at hand when all beyond is shrouded in fog or iron-bound in frost, it must be granted that a good deal of damp, very undesirable but easily communicated to a sitting-room, is a not infrequent sequence of the privilege, especially when the conservatory is unheated. It can be mitigated, however, if not altogether avoided, overhead by the thoroughly sufficient system of glazing that has been here recommended for the prevention of drip, and under foot by a floor tiled throughout, which effectually precludes the rising of moisture from the ground.

Shooting

An abundant supply of rain-water close at hand is important, no less for the convenience of the cultivator than for the welfare of his plants, and no working greenhouse should be built without a tank of some sort to hold the rainfall from the roof and proper gutters to convey it into the right receptacle. The waste of water in this land of ours is enormous, and it is a subject which calls for much more serious consideration by experts than has hitherto been given to it. In a small way-e.g., from the roofs of glass-houses in private gardens-much good, pure water is allowed to run away for want of proper shooting, soaking into the ground, where it makes a swamp and doing harm where it should be an unmixed good.

A well-cemented covered tank below the ground level with a pump is perhaps the most convenient form of reservoir, as its holding capacity may be as great, in reason, as space and circumstance will permit, and certainly greater than that of any cistern that can be placed in the greenhouse itself. An open tank possesses this advantage, however, that the water it contains is tempered and never so cold as that which is drawn from below ; but it takes up room, and, unless very well planned, is ugly.

For some positions a barrel half-hidden by Ivy answers very well, and may be made a pretty feature ; but, in truth, standing water in an open tank is better outside than within an unheated greenhouse, so long as there is a supply close at hand. The shooting in most general use is 3 in. "half round " iron guttering, held up by brackets ; but 3 in. " three-quarter round " spouting is better for heavy rainfall, as it is a great nuisance to have an overflow running down the glass and finding its way into the greenhouse instead of into the tank. In fixing the gutters a fall of from 3 in. to 4 in. should be allowed in a length of 50 ft.


Before passing on to other conditions of temperature dependent on ventilation and shading, it may be well to suggest here that much vexation and trouble will, generally speaking, be saved if the plan of the greenhouse or conservatory be made on the simplest lines. As far as my experience goes most round or octagonal houses with a lantern-shaped addition at the top are less adapted to answer the needs of the average amateur, as they certainly are of the professional gardener, than the more usual straight-lined forms, which also have the advantage of being less costly.


One more item. It is becoming more and more the custom to add coloured glass in some shape or form to plant-houses. Sometimes it is intended as an embellishment, sometimes as a screen from too neighbouring houses. Be that as it may, it can scarcely be too forcibly stated that the plant-grower who is content with plain, clear panes will find himself the happier man in the long run. The question of taste is another matter. Here we are only concerned with the best methods of growing plants under certain conditions and enjoying them to the utmost, and my earnest advice to any one who has it in his mind to build a greenhouse is to eschew all and every sort of stained glass in its construction.

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