Greenhouse Construction and Greenhouse Accessories

A greenhouse, however small, opens up new vistas and offers the gardener endless possibilities and delights. In a greenhouse, whatever the weather, even the more tender plants are well protected, and pot plants and bedding plants will flourish. It is possible to pick tomatoes and cucumbers at a time when they are expensive to buy, while other vegetables that are started in the greenhouse, such as cauliflowers, onions and leeks, will produce a heavier yield when planted out-of-doors later on. Alpine flowers in winter, carnations or cacti and succulents are but a few of the choices open to the greenhouse owner. Never erect a greenhouse without the approval of the local authority and, if a tenant, the landlord. If a greenhouse is built against the wall of a house, the local council may insist that the base be built of bricks on a suitable foundation, and that the structure does not interfere with the external ventilation of a living-room. If the greenhouse seems likely to increase the rental value of the property, there will probably be a small additional rate assessment.


There are four principal types of greenhouse: the span-roofed, the lean-to, the three-quarter span and the Dutch light. The span-roofed house is ideal, for it can be erected in an open position and admits more light than either the lean-to or three-quarter span types. It also lends itself to interior staging, which can be adapted to suit many different types of plant. Staging may be erected on one or both sides to accommodate pot plants, or removed for growing crops in the soil at ground level.

The Dutch light type of house, although unsuitable for staging, also admits the maximum amount of light, and its height particularly assists the cultivation of plants that grow in the ground soil, such as tomatoes in summer and chrysanthemums in winter.

A three-quarter span house is preferable to a lean-to, as it naturally admits more light. A lean-to house should be built against a south wall, so that it receives direct sunshine for most of the day, and it should never be overshadowed by trees or hemmed in by buildings.

Nothing can compensate for natural sunlight, although it is possible to use special electric lamps to supplement daylight under certain conditions.

If the greenhouse is to be used mainly during the summer then a north-south setting is the best, but one that is used during the winter months as well will receive the maximum amount of light if placed from east to west.


All greenhouses will last longer if built on a low brick wall laid on a good concrete foundation.

A wall that is too high reduces the amount of sunlight admitted to the house. A 3-ft. wall is generally suitable for span-roofed and three-quarter span greenhouses. For the lean-to type of house an outside wall of 2 ft. is sufficient, the vertical glass framework above being about 3 ft. high. The sloping sides of the Dutch light house should rest on an even lower wall, about 1 ft. high.

Wood is much used for greenhouse construction, for it retains warmth, and shelving is easily fixed to it.

Cedar wood is extremely durable and need not be painted outside, though if red cedar wood is used, it should be oiled with linseed oil every four or five years. All wood inside the greenhouse should be painted regularly with glossy white lead paint, both as protection and also to reflect light.

Special types of aluminium are also used for greenhouse construction, for it is possible to make the supporting bars narrower and therefore admit more light. Also, metal does not harbour pests. Aluminium, however, does not retain heat as well as wood, nor is it so easy to attach supports for plants to it.


Provide good guttering all round the greenhouse so that rain water is carried away quickly, either into drains or into a storage tank, with an overflow to a drain. Water tanks are best kept outside the greenhouse, for they are apt to become contaminated if kept inside.


Before building a greenhouse, it is important to decide which of the many different methods of heating is to be installed, although heating is unnecessary if the house is to be used only for raising annuals, such as sweet peas, for planting out in the spring. The most common is a boiler for heating water, which then circulates through pipes built underneath the staging round three sides of the greenhouse. The water will remain hot for a long time, and is said to create a better climate for plants than dry electric heat. Take the advice of engineers when installing hot-water pipes, to ensure a correct flow of water and to make the best use of the space. Four-inch pipes are usually considered the most efficient.

Only a full-time gardener should use a boiler that burns coke, Phurnacite or anthracite, for it needs stoking regularly night and morning, and sometimes in the middle of the day as well. The magazine-type boiler requires less attention, because the fuel trickles down automatically, and the draught regulator can be fitted with thermostatic control. This also applies to the automatic gas boiler, which should, however, be installed with the assistance of the local gas company, to be sure that no gas fumes escape into the greenhouse.

Oil heating is effective, and an oil burner can be fitted into an existing solid fuel boiler. It is advisable to discuss the installation of such a burner with an oil-healing firm. The water in the pipes may also be heated with electricity, by using an immersion heater.

The panel type of electric heater is both neat and efficient, and has a water trough incorporated above it. Alternatively, tubular electric heaters may be fixed beneath the staging. In a small greenhouse it is also possible to use a convector heater.

Do not be tempted to use paraffin oil heaters, except in an emergency, as fumes can damage plants.


Plants need fresh air as well as warmth, and all free-standing greenhouses should have a ventilator on each side of the roof. A lean-to house should have ventilators at each end of the roof. Generally, a ventilator should be 4 ft. wide and 2 ft. deep for every 8 ft. of greenhouse. When growing tomatoes, it is an advantage to have ventilators at the ends of the house as well, so that the air can move constantly and there is no danger of the atmosphere becoming stagnant and causing tomato mould disease.

Most ventilators are worked by hand. It is also possible to install automatic ventilators, which operate on a thermostat and open and close according to the heat in the house. Certain firms specialize in automatic ventilation.

The movement of air in a greenhouse is influenced by the position of the sun and the direction of the wind, but generally the hot air rises and flows out of the top ventilators, and cold air enters to take its place. Open the ventilators early in the morning if a rise in temperature is expected; close them partially in the afternoon, and close them fully when the sun goes down, in order to trap the remaining heat of the sun and save fuel. In midsummer it may be necessary to keep the ventilators open all night.

Although ventilators may be opened suddenly, a fully opened ventilator should not be closed suddenly because of the possibility of excessive condensation. On a damp, dull day, when the air in the greenhouse tends to be sluggish, use more heat to encourage the air to circulate, and give a certain amount of ventilation, even if the temperature outside is fairly low. Never open ventilators on the windward side, during high winds, for fear of draughts.


Blinds, already mentioned as a means of conserving heat at night, are even more important to prevent strong sunshine scorching the leaves of plants, and to keep the temperature of the greenhouse within reasonable limits on a hot, sunny day. Never allow a greenhouse to become excessively hot, for the plants will be unable to ‘pump’ enough moisture into their leaves to keep cool, and will suffer in consequence.

The best type of blind is that made of very thin but hard wooden laths 3/4 in. wide. These are linked by metal clips so that a gap of 1/2 in. is left between each lath, fix the blinds to the outside of the roof at the top so that they can be rolled up and down quickly by means of pulleys and cords. If ventilation is needed at the same time as shading, cut away the laths over the roof ventilators and attach the cut-out pieces to the ventilating sashes. Some firms provide wooden lath blinds with the greenhouse.

Another means of shading in summer is to spray the outside of the greenhouse with a special distemper-like solution, a light lime wash or, most adhesive of all, a thin flour paste. The disadvantage of spraying is that if the weather changes, it is difficult to remove the shading quickly to let in the sunlight, and it is also a fairly arduous task to wash it off for the winter.


If it is intended to grow pot plants or raise seeds in boxes, the greenhouse should be equipped with either permanent or temporary staging at a convenient height, usually about 3 ft. above ground level. Staging is usually made of aluminium, or of the same wood as the frame of the greenhouse. In the latter case, it should be painted with at least three coats of good white lead paint.

Fit sheets of corrugated iron over the staging, and cover the sheets with washed gravel, pea shingle or large breeze, to ensure good drainage and to allow for aeration. Stand the pots and boxes on the shingle, which should be raked smooth and level from time to time, and always kept free from moss and algae.

If the staging is to be temporary, slatted bars resting on legs like those of a table usually replace the corrugated iron. The entire staging can then easily be removed at the end of the propagating season, and plants can be grown in the ground soil.


A simple propagating box with a glass or polythene top can be placed on the staging near the boiler. The front of the box should be about 6 in. deep, and the back about 7 in., so that the glass slopes. Place the box so that it faces south, and fill it to within about 1 in. of the glass with a mixture of coarse silver sand and very fine peat, which should be carefully levelled.

Electrically heated frames are also available, the rising heat providing ideal conditions for rooting cuttings easily.


A recent development of great importance to the gardener is the production of pots of various types of plastic, which have several advantages over the clay pot. The principal materials used are extruded polystyrene and Alkathene, which are easy to clean, and can therefore be sterilized or thoroughly cleaned at the end of each season. These pots are extremely light, and fit neatly into each other, so that large numbers can easily be stored or moved from place to place. They are also virtually unbreakable.

A plastic pot retains moisture better than a clay pot, and the extruded polystyrene also holds the warmth of the greenhouse or room, thus raising the temperature of the soil slightly and promoting better root action. Polythene pots are produced in a range of gay colours, and although a little more expensive than clay pots, they will last much longer.

The type of soil in which a greenhouse plant is potted is of great importance. The simplest and most efficient compost for raising seeds in either boxes or pots is the John Innes seed compost. This consists of two parts good sterilized loam, one part peat and one part coarse sand (all parts by bulk). Add to each bushel of this mixture 1-½ oz. Superphosphate of lime and 4 oz. ground chalk.

When the seedlings are large enough to be repotted, one of a number of John Innes potting composts is used. The standard mixture consists of seven parts fibrous loam, three parts peat and two parts coarse silver sand (all parts by bulk), with the addition of fertilizer: 4 oz. John Innes base and 1 oz. Ground chalk or limestone per bushel. The John Innes base is composed of two parts hoof and horn meal, two parts superphosphate of lime and one part sulphate of potash (all parts by weight), and can be obtained already mixed from horticultural nurseries. The total mixture is known as John Innes potting compost No. 1.

Some plants, particularly at the second or third potting, need a stronger compost than this, and should be given John Innes potting compost No. 2, which simply indicates that double the quantity of fertilizer (8 oz. base and 1-1/2 oz. chalk) is used per bushel. The even stronger John Innes potting compost No. 3 contains three times the amount of fertilizer per bushel.

Instead of the John Innes potting composts, an Eclipse No-Soil compost may be used. This can be bought ready made up, and only needs the addition of the special plant foods that are provided in a separate plastic bag, and which should be used in accordance with the directions on the packet.


Damage can be caused both by under-watering and by over-watering plants under glass. Never give little sprinklings of water, but do the work thoroughly. Water in the evening during the summer and at about 10 am in winter, using rainwater when possible, particularly in districts where the tap-water is hard. Syringeing the plants and watering the paths and walls in summer, a process known as damping down, creates a good moist atmosphere and provides a kind of natural dew, thus preventing the plants from transpiring too much. Do not damp down in winter, except in warmer green-houses, and be careful not to spray inactive or hairy-leaved plants, or those in full flower.

If the ground soil in the greenhouse is used for growing crops, the drainage should be perfect. Water does not drain as easily in a greenhouse as in the open, and the soil should never be allowed to become stagnant.


The greenhouse should be kept perfectly clean. Regularly pick off and burn dead flowers and leaves, and never allow moss and algae to accumulate on the top of the compost in pots. Scrub and white- wash the inside walls of the greenhouse once a year, and clean the outside of the glass thoroughly in late September or early October. Paint wooden greenhouses with a genuine white lead paint at least once every three years.


Greenhouse pests and diseases can be controlled by fumigation, a process usually known as ‘smoking’. Pesticidal smokes control all insect pests, while fungicidal smokes prevent fungus diseases such as botrytis and mildew.

Smoke bombs are available in various sizes, and produce a clean cloud of microscopic particles of insecticide or fungicide, which fills every crevice and covers every surface of the greenhouse. If the correct bomb is used, the smoke cannot taint food or harm crops. Use smoke bombs in calm, warm weather, but never in bright sunshine. First work out the cubic capacity of the greenhouse, and use one small pellet smoke bomb for every 500 cubic ft., or one cone smoke bomb for every 600 cubic ft.

Make the greenhouse as smoke-tight as possible. Just before closing down for the night, light the first bomb at the far end of the house and others at intervals while moving back towards the door. As soon as the last bomb has been lighted, shut the door firmly, and keep it closed overnight. Be sure to put up a warning notice outside: ‘Do not enter; house being fumigated’.

Further bombs may be lighted once a month as a preventive measure, and if infection should occur, light bombs once a fortnight until the trouble is controlled.

When using fungicidal smokes, fumigate 24 hours before planting, so that all fungus spores are killed.

16. February 2012 by Dave Pinkney
Categories: Featured Articles, Garden Management, Gardening Calendar | Tags: , , , | Comments Off on Greenhouse Construction and Greenhouse Accessories


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