Greenhouses and Cold-frames

A greenhouse allows you to grow plants in conditions of enhanced solar radiation, so extending both the seasons and the range of types that can be grown. But with careful planning, your greenhouse can offer you numerous options: a facility for raising hardy and half-hardy plants to transplant outdoors, space for overwintering tender plants either in a dormant or an actively growing state, an unrivalled opportunity to produce both summer and winter salad crops, protection for a grapevine, peach or other marginally hardy fruit plant, scope for raising and maintaining house plants or even, if you wish, an area that can be converted into conditions suitable for tender orchids and other tropical species.

Initially, you have three main considerations: the style, size and siting of your greenhouse.

Greenhouse styles

a dome shaped greenhouse is very efficient at light capture but inefficient in its use of spaceWhen choosing a greenhouse, there are two main factors to consider besides appearance: the influence the shape has on the usable internal space and the effect that the angle of the translucent surfaces has on light transmission. While time of day and season of the year are obvious variables, a sheet of glass at right-angles to the suns rays will always allow through approximately 90 per cent of the light striking it whereas, at an angle of incidence of more than 40 degrees, the percentage transmission falls off rapidly. So the most efficient greenhouse shape is in fact a dome; but this is scarcely practicable other than on a small novelty scale. Next best is a square house with four roofs of equal size, or house with a semi-circular roof and vertical ends. Although ideally such a house should be as high as it is wide, the design is made more practical by flattening the profile and by using plastic instead of glass; the principle behind the ‘polythene tunnel’. The next preference is a structure with a straight roof and sides 60-70 degrees from the horizontal. While the least effective is the commonest shape: a straight roof and vertical sides. A lean-to greenhouse is usually half the latter pattern built against a vertical house wall, often painted white. There is merit here in that heat from the adjoining building will help retain warmth in winter but there will be a lack of light from one side. Nonetheless, a lean-to can be an attractive option for a small garden.

the span house with or without glass to ground level is the most popular and practical typeMost modern greenhouse frames are aluminium although wooden ones (either teak or the much more durable softwood, western red cedar) are still available. Maintenance is certainly greater with a wooden frame; deal should be painted or treated with preservative annually and even red cedar should be treated every two or three years. While aluminium will, therefore, be the more popular option, I must point out the advantages that a wooden greenhouse confers. Heat loss is less through a wooden frame. The fixing of insulation and other attachments within is usually easier, the structure is generally more resilient when subjected to gales and, in many gardens, especially of old houses, a wooden building blends in in a way that an aluminium one never can. But whatever the material, every greenhouse should be firmly anchored to the ground and preferably erected on 20cm (8in) deep concrete foundations. Easily the best floor for a greenhouse is gravel over firmed soil; this permits water to drain away freely and an annual treatment with a garden disinfectant will eliminate any pests, disease organisms or weeds.

Although most conventional greenhouses are made with the glass held in frames, much cheaper structures are available using one or other forms of clear plastic. While providing useful additional space for protecting plants, they have several drawbacks and cannot really be considered a substitute for a glazed greenhouse. All plastic sheet has a limited life and will need replacing after two or three years as it becomes torn by the wind, attracts dirt by static electricity and is rendered more or less brittle by the ultra-violet radiation in sunlight. Moreover, while plastic more readily transmits light and heat than glass, this is a two-edged attribute, for as a plastic structure will warm up more quickly, so it will cool down more quickly too. The surfaces need to be kept clean, even a moderate amount of dirt can impair the transmission of light by up to 40 per cent. Plastics can be harder to clean than glass, they often leave gaps where they join the frame, and are prone to tears or damage. Plastics can be degraded by ultra-violet light, although for garden use they should have been treated. Glass still has a lot to offer but the safety aspects should be considered carefully. This applies not only to young children but older people as well.

Greenhouse size

Larger greenhouses cost more to buy and to heat; a 3 x 2.5m (10 x 8ft) structure will be about 25 per cent more costly to heat than a house of 1.8 x 2.5 m (6 x 8 ft). For a garden up to about one-third of an acre, I suggest you choose a 3.6 x 2.5m (12 x 8ft) greenhouse with a rigid internal frame partition and a door, dividing it into two 1.8 x 2.4m (6 x 8ft) units. One, with removable staging, can be used for summer tomatoes, overwintering fuchsias, Pelargoniums and other tender perennials or growing winter lettuce. The other unit has permanent staging with a propagating sand bench. It is also heated to overwinter tender subjects and for early and late-season plant raising.

Greenhouse siting

Few gardens have a spot that satisfies all the desirable criteria for siting a greenhouse. It should be close to electricity and water supplies, in a level, open yet sheltered position, away from deciduous trees if possible. Ideally, an oblong greenhouse should be positioned with its long axis east-west for the most uniform illumination.

Greenhouse staging

Most staging is either wood or aluminium and is usually purchased from the greenhouse manufacturer. A mixture of fixed and removable staging allows you to adapt the greenhouse to the needs of different plants through the growing season. The working height of the staging is usually dictated by the manufacturer and by the height and form of the greenhouse sides, but I’ve found it worth installing a short length of staging at a height of 75cm (30in) for potting and pricking-out from a sitting or standing position.

Greenhouse heating

Although all greenhouses are warmed by the sun, they are usually called ‘unheated’ unless additional heating has been supplied artificially. Extra heating will enable you to grow a wider range of plants and possibly to grow crops out of season, but heating costs must be taken into account.

bottled gas heaters are efficient but don't offer the useful air movement that an electric fan providesThere are four main ways of heating a greenhouse: electricity, paraffin, solid fuel and gas. There are advantages and disadvantages for each heat source but I believe the benefits of electricity with its versatility, reliability and lack of maintenance make it easily the first choice. A thermostatically-controlled, electric fan heater is a cheap, simple heat source with the bonus of encouraging air circulation, which can help minimise diseases. The fans can also be used in summer to help cooling. Check the temperature inside the greenhouse with a maximum and minimum thermometer. Every 2.8°C (5°F) rise in temperature roughly doubles the amount of heat required (and hence the cost), so choose very carefully the minimum temperature necessary for your plants. For example, a night-time minimum of 2°C (36°F) will keep overwintered plants, like pelargoniums, fuchsias and dahlia tubers alive, but nothing will grow. A night-time minimum of 7°C (45°F) will allow you to grow winter lettuce as well as cuttings and young plants. A minimum temperature of 15°C (59°F) and additional lighting will be needed to grow winter salad crops like tomatoes and cucumber.

On balance, I recommend a winter minimum of 7°C (48°F); then, from late winter onwards the warming effect of the sun during the day should raise the temperature, so sowing and pricking-out can commence. And of course, the greenhouse makes a welcome winter retreat for a gardener too.

Temperature management in greenhouses

Insulating the greenhouse will help cut fuel costs. Double skin, bubble polythene film is popular, but do choose one specifically for greenhouse use, as apparently similar materials sold for packaging may not be ultra-violet stable. It is possible to cut down heat loss by 40 per cent using insulation; there will be some-loss of light transmission but this makes little difference to crop yields in practice. Insulating film is usually removed in spring, but I find it more convenient to leave it in place and so dispense with the need for summer shading.

In summer, greenhouses become very hot and ventilation is essential. Many greenhouses are sold with insufficient vents but it is worth choosing a greenhouse with additional ones at the time of purchase; they are much more tricky to add later. There are three functions of ventilation: to restrict the rises in air temperature as a result of the sun’s heat, to restrict the rise in humidity, and to admit air that can supply carbon dioxide needed for photosynthesis. You should aim to have vents both in the roof and in the sides and they should be equal to at least 15 per cent of the floor area to work effectively. .Automatic-venting systems, which require no electricity are extremely valuable, particularly if, like most gardeners, you are out for much of the day.


Between the removal of summer crops and preparation of the greenhouse for winter, the whole interior should be cleared of plants and scrubbed inside with a garden disinfectant. Finish by washing surfaces down with clean tap water and fumigate with a fungicide and insecticide ‘smoke’.

Greenhouse watering

Watering in a greenhouse is a perpetual problem and much time will be saved in the summer months by planning, at the outset, how your plants can be watered most efficiently. The biggest consumers of water in most gardeners greenhouses in summer are tomatoes.

Ring Culture: It is appropriate to discuss here the ring culture technique as this can be set up when the greenhouse is constructed. A trench, 30-35cm (12-14in) deep and 35-40cm (14-16in) wide, should be dug along one side of the greenhouse. The trench should then be lined with heavy-gauge plastic-sheet and filled with pea gravel. Bottomless pots containing tomato plants in soil-based compost stand on this gravel. Liquid feed is applied to the compost in the pot but the plants’ water needs come from a second root system that develops in the gravel bed. The gravel reservoir need only be filled once a week, compared with the need for more frequent, if not daily, watering of growing bags or conventional pots.

Greenhouse lighting

greenhouse lighting is not essential but if you do use it, be sure that its output really is adequate to make a significant improvementMost gardeners manage without using supplementary greenhouse lighting, but it has its uses if your greenhouse is heated to high levels to grow plants over the winter or you wish to obtain good seedling growth in late winter or early spring. There are a number of important considerations in choosing a lighting system but essentially you require one that offers the maximum amount of light output within the wavelengths required for photosynthesis while having low installation and running costs. Much the most efficient are low-pressure sodium lamps but they are among the most expensive to install and most gardeners opt for the relatively inefficient but cheap low-pressure mercury discharge fluorescent tubes. Whichever system you select, however, do check carefully that its output really will be cost-effective.

Propagating benches

Not to use a greenhouse for plant propagation is a fearful waste of resources and I consider a sand propagation bench to be essential. It is an important consideration when you are planning a greenhouse and deciding on the type of staging.


Anyone who has a greenhouse for raising young plants for use outdoors will need a cold-frame as a halfway-house for hardening them off. A cold-frame is also required for raising hardy seedlings and cuttings, it can be used for winter salads and during the summer it can be used for growing cucumbers, melons or other tender crops. A well-made, insulated and correctly-sited cold-frame should elevate the temperature by at least 7°C (12°F) above the outside ambient.

Cold-frames are available in the same materials as greenhouses, but being smaller structures, the better heat-retaining properties of glass and wood are even more important. Where young children use the garden, how-ever, a covering of plastic will be safer. The commonest size of frame is about 60cm x 1.2m (2ft x 4ft), which should hold about eight standard-sized seed trays full of young plants, but is too small for crop raising. A height of at least 30cm (12in) at the front and 45cm (18 in) at the back is essential for hardening-off taller bedding plants. The lid should be easily removable and able to provide ventilation, although the whole structure should be draught-proof when required. Damage from winds is a serious problem so it should be sited in a sheltered position and anchored firmly.

There is much to be gained from making your own from a wooden window frame, or buying a large commercial wooden Dutch-light pattern frame (with glazed covers but solid sides). Site the frame near the greenhouse, in a sunny sheltered aspect if possible. Fit bubble insulation to the frame in winter. In summer, shading material may be needed if you use it for rooting cuttings.

31. July 2013 by Dave Pinkney
Categories: Greenhouse Equipment, Greenhouse Gardening | Tags: , , | Comments Off on Greenhouses and Cold-frames


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