Images: Tim Sandall
For raising tender plants and seedlings, glasshouses can provide an invaluable protected growing environment. However, with many different kinds to choose from, careful selection and fitting out is vital if your needs, both currently and in the future, are to be met.
The value of a glasshouse
A glasshouse may represent considerable investments of both time and money, but if chosen and used properly, it soon becomes an essential part of the garden.
Growing under glass provides a protected environment away from the extremes experienced by plants outside and is ideal for raising seedlings, overwintering tender perennials, growing crops such as tomatoes,
or even cultivating plants that need protection year round. It is important to remember that plants under glass are dependent on gardeners for their needs, so before investing in a glasshouse, ensure you are able to devote the time and attention that will be required.
A good glasshouse lets in as much light as possible, retaining heat in cold weather but not overheating in sunny conditions. Choosing appropriately is important not only for the best results, but also to make caring for plants a joy rather than a chore.
Larger glasshouses are easier to manage than small ones, as large volumes of air fluctuate less widely in temperature. Reduced shade is cast proportionately by the structure and air flow is improved. Spacious conditions are also easier to work in; it is surprising how quickly a glasshouse fills with plants, so purchase the largest possible.
Choosing structural material
Aluminium is usually the material of choice for a glasshouse, whether in natural metal or a coloured finish (left). A coloured frame may fit into the garden better especially if the structure is to be viewed from inside the home. Aluminium needs no upkeep, and the structure casts little shade.
Wood is an attractive, traditional building material and better suited to some garden situations; however it will need periodic upkeep unless you specify more expensive, low-maintenance, cedar timber. Wooden frames tend to be bulkier than aluminium and can cast excessive shade inside.
Polythene tunnels (polytunnels) (right) are a highly cost-effective means of growing summer tender crops, such as tomatoes or cucumbers, and hardy winter plants such as lettuces, but are rather unsightly and not usually economical to heat.
When choosing a glasshouse, look out for cross braces and sturdy glazing bars in the roof; these help prevent storm damage and can be used to anchor supports for climbing plants - such brackets may be missing from some cheaper houses. Capturing rainwater runoff with guttering helps reduce winter dankness, which can cause plant problems such as rots and moulds. It also allows the collection of valuable rainwater using water butts set up outside.
Shape and size selection
Domes and other odd-shaped glasshouses can prove more difficult to manage, especially when trying to ventilate efficiently, and they tend to be more expensive than the traditional shape (left).
Mini glasshouses are popular but they have such a small volume of air that great care has to be taken to avoid harmful fluctuations in temperatures - however, they can be useful, especially in smaller gardens.
The taller a glasshouse is at the eaves, the better the light transmission and therefore the wider the range of plants that may be grown, providing greater ease of use. At least 1.5m (5ft) is required, and ideally 1.8m (6ft) or more. The ridge should be at least 60cm (2ft) above the eaves to allow a door that gives easy access and sufficient slope to shed rain quickly and encourage loss of excess heat. Glazing to ground level gives the greatest flexibility but heat loss is reduced by using half walling, whether masonry or timber.
A sturdy base to support the structure above ground level is required,with ready-made metal ones eliminating the chore of laying masonry.
Ventilation and shading
Sufficient provision for ventilation is often neglected, but is important, especially in summer. Roof vents are the most useful, and ideally should be on both sides of the ridge and equivalent to 15-20 percent of the floor area.
Side vents are no substitute for roof ventilation, and while louvered vents (left) allow regulation of air flow, they are hard to draught-proof in winter. Automatic openers that open or close as a cylinder of wax expands and contracts (right) are useful but slow to respond and need supplementing with manual control, such as opening some windows and the door each morning. Motorised vents activated by heat sensors are efficient but may be too costly for home use.
Ventilation alone is unlikely to be sufficient to keep temperatures below damaging levels. Shading will be needed, and installation of screens (ideally external ones) that can be deployed in hot weather and wound in during dull conditions, will facilitate temperature control. Painting glass with white liquid shading is a simple but less satisfactory alternative.
Heating and insulating the glasshouse
While unheated glasshouses have limited use for growing tender plants, they take the edge off winter weather; a little heat to keep out frost increases the range of plants that can be cultivated. Cool glasshouses (heated to 7°C/45°F in winter) can be used to overwinter slightly tender plants, such as Aeonium, Citrus, olive trees, tender rhododendrons, Streptocarpus and other pot plants. Temperate houses (heated to 13°C/55°F) and stove houses (18°C/65°F) are more costly to heat and are used by growers of specialised plants.
Deciding on a heat source
Electric heaters are usually the most economical form of heating, and the provision of an outdoor power supply with a waterproof electricity point is all that is needed (left). Gas, whether mains or bottled, is also good for cool glasshouses, as long as adequate ventilation is provided. However, bottles of gas are heavy and supplying mains gas is difficult. Burning gas also produces fumes and condensation that can harm plants and promote diseases; remember also to fit a carbon-monoxide alarm.
A simple paraffin heater will protect against frost, but will need near-daily attention to top up fuel, adjust the flame and trim the wick. Attempting higher temperatures will also often lead to damaging condensation and fumes. Beware of touching gas or paraffin heaters as they will quickly become hot.
If you are heating your glasshouse, extra winter insulation will dramatically reduce fuel bills, helping to trap the warm air for longer, particularly on cold nights. Choose a glasshouse with glazing bars that accept fittings sold to hold insulating bubble polythene. Lean-to glasshouses cost less to heat as the wall side is well insulated and absorbs heat from daytime sun to be released later.
Glass or plastic glazing?
The best glazing material is glass as it lets 90 percent of light through, does not degrade in sunlight and, unlike plastic materials, reflects heat radiated from within the glasshouse, back into the structure instead of being lost. Toughened glass should be used for doors and anywhere people might fall onto the glass. Double glazing and special glasses that alter the spectral composition of sunlight are often used for conservatories, but are less suitable for cultivating plants as the reduced light quantity and quality affects plant performance.
Plastic glazing materials do have their place; indeed expensive, double-skinned polycarbonate sheets are resistant to breakage being effectively double glazed.
Other features to consider
A glasshouse with a sliding door is sensible
as it will not slam, allowing extra controlled ventilation. If the house is glazed to the ground, a kickboard is an good safety feature.
Flooring depends on use. Paving suits pot-plant cultivation, bare soil suits summer vegetable cultivation. Alternatively, use gravel laid on a weed-suppressing membrane.
Finally, budget for the extra features such as lighting to aid work in winter evenings, shelves and staging (left) to grow plants on. Also consider installing an automatic watering system.
Guy Barter