Materials
Access
Siting
Interior design
Heating
Ventilation
Shading
Watering
Hygiene
Frames
Cloches
Publications
Addresses

This is a guide on what to consider when thinking about buying a greenhouse or other protected growing structure in glass, polythene or plastic. (The term greenhouse is used here as a term for all forms of protection.)

A greenhouse provides a protected environment for growing more delicate plants, for starting off your seeds or for simply lengthening the growing season to raise that extra range of vegetables or flowers.

Choosing from today's range of products, you can have almost any size of cover, from cloches for individual or small groups of plants, through small greenhouses, to very large ones. The choice is up to you - to suit what you can afford and the space you have available.

Whatever your choice, you should remember that the cost of the greenhouse itself is only part of the outlay you need to consider. There are also the installation costs, in addition to equipping the greenhouse with electricity or gas, water, paving, benches, etc.

As a brief comparison between glass and polythene, glass is more expensive but lasts much longer. It is easier to control the environment under glass. A polythene tunnel can be clad in netting (rather than polythene) to provide a shade house.

A greenhouse can be glazed with twin (or even triple) wall polycarbonate. Although this is more expensive than glass, it is almost unbreakable and has better insulation qualities.

Whatever your choice, the following points should be remembered.

Materials

Polythene tunnels are supplied with metal tubular frames that are put together to form a semi-circle. A newer form - the Solar Tunnel - has straight sides and is therefore easier to fit with benches.

Greenhouse frames are either aluminium (sometimes coated in UPVC) or wood. Aluminium is maintenance free, while wood requires attention either with painting or coating with wood preservative. If you need extra insulation, bubble plastic sheeting can be fixed to the inside of the greenhouse; this is easier to fix to wooden frames.

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Make sure that the greenhouse is easy to get into and to move around in. Most greenhouses have a built-in threshold on which the door slides; this might be a problem if you use a wheelchair. Look for a greenhouse that is built without a threshold so that you can wheel straight in. The doorway should be at least 850mm (2' 6") wide and sliding doors are easier to manage.

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Siting the greenhouse is very important. It needs to be in a position that receives maximum sunshine for as much of the day as possible. Ideally it should be near the house - both for easy access in unpleasant weather and to cut down the cost of laying on electricity and water. Avoid overhanging trees as these will shade the greenhouse.

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The path inside the greenhouse needs to 850mm (2' 6") wide if you use a wheelchair or walking frame. The path should be level, non-slip and firm. Make sure you can reach the back of any staging or shelves - this might mean having staging only 600cm (2') wide. Also ensure that the staging is the right height for you.

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The options for heating are butane gas, electricity, paraffin or none at all. As well as for heating, an electricity supply to the greenhouse can be used for automatic ventilation and soil warming. Even without heat, the greenhouse has many uses.

A minimum of 7-8'C (45~47'F) is sufficient to grow many plants over winter, although the more tropical species will not tolerate this low temperature.

The two popular types of electric heaters are tubular and fan styles. Fan heaters tend to use more electricity but do circulate the air and can be used to help ventilation in the summer. Most manufacturers will offer advice on the size of heater required for your greenhouse.

Paraffin heaters are a very good source of heat and are relatively cheap to use. However, they do produce a lot of water vapour and condensation can be a problem, especially under polythene. They can also smoke unless the wick is kept trim and well adjusted. The heater should always be filled outside the greenhouse as spilt paraffin contaminates the soil and gives off harmful fumes.

Gas heaters are also readily available and should be chosen to suit the size of your greenhouse.

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Most greenhouses will have roof windows for ventilation. Warm air rises and escapes through these vents and cooler air is drawn in from the door or side windows. Several firms market a. simple expanding tube which can be fitted to the roof ventilators to open and shut automatically, depending on the temperature. A pole with a hook on the end can put manually operated ventilators within reach. The most effective type of ventilator is the thermostatically controlled extractor fan. These are easy to install and cheap to run, providing a continuous movement of air and reducing condensation. They are normally fitted to the gable end of the greenhouse. For polythene tunnels, ventilation is usually provided by opening the end doors.

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Shading has the dual functions of at least limiting the sun's rays - thus avoiding scorching to leaves, fruit, etc - and of preventing too much radiant heat from building up, so keeping the greenhouse to a reasonable temperature. Some shading can also be used to insulate the house and thereby keep in the heat at night. As shading reduces the amount of light into the greenhouse it should not be used on dull days. 'Paint on' shading powder, that is mixed with water and painted on the outside of the glass, remains until wiped off (usually in autumn). If you prefer this method, be aware of the limitations posed by the lower light levels. All forms of shading should only be used on the south side of the greenhouse. Roller blinds can be fitted either to the inside or outside of the house and rolled down when required. However, these are expensive.

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Correct watering of plants is one of the most difficult things to achieve in a greenhouse. If you lack strength or mobility in your arms, but need to water manually, choose a light plastic can with a long spout. Alternatively, tying the end of a hose to a cane or broom handle, or using a lance attachment, will extend the reach. Never try squirting the water by pinching the end of the hose, as this can lead to inconsistent watering and severe erosion of compost from the pots. It also tends to knock over dry (and therefore light) pots. Lances with control triggers are useful to avoid splashes which can spread fungal and bacterial spores.

There are many self-watering systems now available that are either completely automatic, or semi-automatic. They can also cost quite a lot and this needs to be balanced with the likely amount of use.

Perhaps the most efficient form of watering is by capillary method where the pots and seed trays stand on special matting or a sand bed and take up only the water they need. A seep hose laid along the mats or sand bed takes most of the work out of watering.

Trickle irrigation is another method of supplying water directly to the plant or pot at a predetermined rate. A popular form is the spaghetti tubing system in which one supply pipe has a large number of individual smaller tubes leading off it, so that each pot receives a constant dripped supply.

Whatever method you use for watering, remember that the plants nearest the windows will dry out faster than those near the centre of the greenhouse. They are also the pots that are most difficult to reach and so might become neglected during hand watering.

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Good hygiene is vital in the greenhouse, to avoid the build up of diseases and the overwintering of pests. All areas of the greenhouse should be thoroughly cleaned with a disinfectant at least once a year. Care should be taken to get the disinfectant into all the cracks and crevices, used pots and seed trays, etc, where all the nasties try to hide. Regular checking of the condition of plants will ensure that you find pests and diseases early and can then take action to prevent damage. There is a whole range of organic and inorganic controls available for the greenhouse. Take care in following the manufacturers' instructions at all times. Listed are some manufacturers of greenhouses, polythene tunnels and greenhouse equipment. Any good sized garden centre will have a selection of products, so try your local one first.

If you don't have space for a greenhouse you might wish to consider frames or cloches.

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Frames are used for increasing the growing season and for hardening-off plants. They usually comprise a frame of either wood or plastic, with a top which rises or slides up to provide access and ventilation. Lean-to frames are available, as are frames which have a sloping front with sliding doors, making access easier. Frames can use laminated glass for safety or rigid plastic with an aluminium frame. Ensure that frames are stable and will not move in high winds.

For many people, frames may be inaccessible at ground level; in this instance a frame could be raised up on a suitable and stable base. Accessories for frames can include heating and mist watering.

Cloches come in a variety of shapes and sizes - to protect individual plants or rows of plants from harsh weather, to extend the growing season and to keep out pests.

When choosing a cloche consider whether or not it is self-assembly, easy to store, lightweight (and if so, will it be likely to blow around if not well anchored) and that it has ventilation. Some cloches take the form of glass tents which, it may be decided, present too great a safety hazard.

Additional equipment for greenhouses

Staging, heating, automatic vent openers, shading and lighting are additional items which may be required at some point. Many large garden centres sell such items and they are also available by mail order.

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