Growing Crops under Glass

This section is concerned with some of the general principles underlying protected cultivation. With the aid of some form of protective structure it is possible to extend the production season for a number of fruits and vegetables. These structures include greenhouses both heated and cold, walk-in polythene tunnels, cloches, low polythene coverings, cold frames and bell jars. The extension of the season may be caused by earlier growth in the spring or by protection later into the autumn.


Various designs of garden greenhouses are available but most have either a wooden or an extruded alloy framework. The latter type is probably easier to put up and has better light transmission but it looks less attractive than the wooden variety. The size will be influenced by the space available and you must be careful to erect your greenhouse in a sheltered but un-shaded part of the garden. If you can align its ridge so that it runs as near as possible to east/west, you will get more winter light on to your plants. The amount and quality of light has an important effect on crop growth. Clean the glass regularly, for dirty glass reduces light transmission and therefore growth.

The other design feature to which you should pay great attention is the ventilation. Many small greenhouses have insufficient ridge and side ventilation capacity so that occasionally the door must be left open to reduce both temperature and humidity. This is often inconvenient and it is better to provide adequate ventilation from the outset. When all vents are fully open the apertures should be equivalent to about one sixth of the floor area of the greenhouse. Heat loss can be very rapid from greenhouses so make sure that there are no cracked or broken panes and consider putting up a polythene inner skin during the winter.

How can you best use your greenhouse?

If it is heated, a wide range of crops can be grown. It is worth while partitioning off one end as a propagation area for plants for the greenhouse itself or for early outdoor planting. You may prefer to use a purpose built propagator where the temperature is controlled thermostatically. Early tomatoes, cucumbers, melons, sweet peppers and aubergines will all do well in heated houses. Be careful, however, not to mix too many crops together, especially if they have different environmental requirements as in the case of tomatoes and cucumbers. During the autumn and winter the heated greenhouse may be used for crops of lettuce and for forcing chicory and rhubarb, provided that black out facilities are available. Early crops of dwarf French beans can also be produced in heated houses by sowing six to seven seeds in 22cm (8in) pots during February; picking should begin in May.

The cropping potential of an unheated greenhouse is more limited but tomatoes, cucumbers, sweet peppers and aubergines will still do well, particularly in southern districts. It is seldom worth planting tomatoes in unheated greenhouses before late April, while the other crops mentioned should wait another three or four weeks. Planting too early can cause a severe check to the plants and greatly reduce overall production. Spring-maturing lettuce is a useful crop for a cold greenhouse, grown either from a planting in November or February. January sowings of radish or carrots will produce roots in about eight and 14 weeks respectively; plantings of potatoes made at the same time — whether in ridged border soil, in large containers or under black polythene — will produce early tubers.

Walk-in polythene tunnels

The expense of glass covered structures— and the ease with which breakages occur — may deter you from buying a glasshouse. An alternative protected structure is a walk-in polythene tunnel. This consists of a framework of semi-circular metal hoops joined together at the apex with another metal tube. The tubing should be of 1.5cm (0.5m) internal diameter. The ends of the semi-circular hoops are pushed into foundation tubes which have been driven into the ground. These tubes should be 60cm (24m) in length with an internal diameter of 2.5cm (1in); they should be 1.5m (5ft) apart and in two parallel rows about 4m (13ft) apart. If the hooped tubes have an area of between 2.0 and 2.5m (7ft) radius, the structure framework will have a basic width of about 4m (13ft). This framework should be covered with a sheet of 500 gauge ultraviolet inhibited clear polythene which, for a 4m (13ft) wide structure, will need to be 7.0 to 7.5m (24ft) wide. The sheet must be tightly stretched over the framework and this can best be done in warm, still weather conditions. The edges of the sheet are buried in a 15cm (6in) deep trench on the outsides of the two rows of foundation tubes. Roll-up or hinged doors made out of the same polythene sheet and attached to a wooden framework are satisfactory for polythene tunnels. Polythene sheet of the type indicated will last for about three growing seasons by which time it will probably have become brittle and should be replaced.

Crop production in polythene-covered structures is generally similar to that in greenhouses but there are a number of different considerations. The framework of a walk-in tunnel is less substantial than that of a greenhouse and this may present problems with crops like tomatoes, cucumbers and melons which need vertical supports. Polythene and glass have different heat transmission and retention properties which will affect crops. Both materials allow radiant heat to enter during the daytime but polythene allows the heat to pass out again at night so that on a clear starlit night very little heat indeed is retained by the structure. Radiation frosts are most likely to occur in the late spring, when particular care must be taken. Polythene-covered structures are more difficult to ventilate than greenhouses which have ridge or side ventilators but the humidity and temperature can be controlled by opening the doors.


Garden frames can, for many purposes, be used instead of a greenhouse or polythene tunnel. Again, maximum light transmission is vital and your frame should, therefore, be built on a sunny, southerly site. The base may be constructed of bricks or wood and the covering may either be old English lights or the more versatile and less heavy Dutch lights. Dutch lights allow more light into the frame and can also be used for protecting tender vegetables which are planted on patios or against walls. It is possible to buy frames with an aluminium skeleton which is glazed right down to the soil and which, therefore, admits more light. Cold frames are most commonly used but thermostatically controlled soil-warming cables may be incorporated to provide some heating.

Finally, frames can be used for drying off and ripening potatoes, bulb onions or root crops which have been harvested while wet and which are to be stored.

Cloches and low polythene coverings

Several varieties of cloche are available. They may be constructed either of glass or plastic but are more draughty than frames. They are easily moved and invaluable for warming up the soil prior to sowing or planting, raising seedlings, extending the growing season, producing low-growing crops during the summer and drying off harvested crops. If you draw up a cropping programme beforehand you will be surprised at the amount of produce which can be raised from efficiently used cloches.

The earliest outdoor sowings or plantings of peas, French beans, runner beans, carrots, radishes, lettuce, marrows, courgettes and’ sweet corn can be protected by cloches, while late maturing crops of peas, dwarf French beans and endive can also be protected well into the autumn.

Melons must be covered throughout the growing season. Other fruiting vegetables, such as tomatoes, peppers and aubergines, can be protected by surrounding them with barn cloches stood on end.

Low polythene coverings are constructed by stretching thin gauge clear polythene film over semi-circular metal hoops covering a 60cm (2ft) wide strip of ground. The polythene is held on to the hoops by lengths of string or wires over the top. The ends of the tunnels are formed by burying the polythene in the ground or securing it around wooden stakes. The polythene will degrade rapidly and can be used only for one season. These tunnels are useful for strawberries, spring-planted lettuce and early sowings or plantings of peas, dwarf or runner beans. Ventilation is particularly necessary on warm, sunny days for, unlike glass or plastic-covered cloches, the polythene film produces an airtight seal. Lifting up the polythene between the metal hoops and the retaining strings will allow sufficient air movement.

Frames can be used to raise vegetable plants for transplanting. Early summer cauliflowers may be sown in late September and over-wintered prior to planting in the spring. Early plants of spring lettuce may be raised in the same way. Summer planted vegetables such as cabbage, cauliflower, Brussels sprouts, calabrese and leeks can also be raised in your cold frame for transplanting. Another valuable use is hardening off heated greenhouse raised plants before they are planted outside.

Cold glass lettuce can be planted in your frame in November or February and will mature at about Easter time. Radish and carrots sown in January will produce very early roots. During the summer use the frames to start off early crops of marrows or courgettes, removing the glass after the danger of frost has passed. Melons are a useful summer frame crop but they will need protecting throughout their life.

16. April 2013 by admin
Categories: Fruit Gardening, Greenhouse Gardening, Tips and Advice, Vegetable Gardening | Tags: , | Comments Off on Growing Crops under Glass


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