If money is no object you can now have a greenhouse or conservatory that looks after itself, with the plants watered automatically. On a more modest scale, garden frames and cloches are extremely useful (andcomparatively inexpensive) pieces of equipment: they are of particular value on the vegetable plot for extending the growing season at either end.
Eight types of greenhouse
Today greenhouses come in all shapes and sizes to suit every need and site.
- Span-roof greenhouse The traditional greenhouse, with pitched roof (each side of equal size and shape) sloping down to the eaves. Normally has straight sides, although there are models with sloping sides which result in better light penetration. The glass-to-ground types are ideal for growing plants at ground level, such as tomatoes in growing-bags; those with solid sides (to about 90cm/3ft) are good for pot plants, as they retain heat better than all-glass houses. Framework available in aluminium alloy or timber (such as western red cedar). You can now obtain span-roof greenhouses with curved eaves.
- Lean-to greenhouse Greenhouse built against a wall, resembling half of a span-roof greenhouse. Has a lighter framework than a conservatory. Available in aluminium or timber. May be glazed to the ground or have solid sides to a height of about 90cm/3ft.
- Mansard greenhouse Also known as a curvilinear greenhouse. The roof panels are set at various angles, giving a curved shape to the roof and ensuring excellent light intensity inside. Glass-to-ground, or solid walls to about 90cm/3ft. Aluminum framework. Lean-to versions also available.
- Round greenhouse Free-standing structure with 6, 9 or 12 sides — actually lantern-shaped, not truly round. Looks attractive in an ornamental setting, such as a large patio. Timber or aluminum framework; glass-to-ground or half-timber sides.
- Mini greenhouse Both free-standing and lean-to types are marketed. Ideal for patios and balconies. Access is by sliding or hinged doors, although a true mini is too small to step inside. Will hold a growing bag for tomatoes; also useful for plant raising. Aluminium or timber framework, usually glazed to ground. Can heat up excessively in warm weather unless you pay careful attention to ventilation.
- Dome-shaped greenhouse Also known as the geodesic greenhouse, this is virtually a glass dome with an aluminium framework. Very light and roomy inside. Available in various sizes. Looks superb in a modern setting and highly recommended for ornamental plant displays.
- Conservatory Built against a wall of the house — ideally a warm sunny wall. Types vary from a simple lean-to style greenhouse (the structure is heavier than a true lean-to) to a more elaborate nineteenth-century style building. Ideally designed as an integral part of the house with access from one of the rooms. Often the building has solid sides to a height of about 90cm/3ft, although models are available with glass to the ground. Conservatories are available as modular systems supplied in sections of timber construction. There are also less expensive kits with a framework of aluminium alloy.
- Polythene tunnel Comparatively cheap structure, ideal for growing vegetables and raising plants, provided that they don’t require very warm conditions — tunnels are not well insulated, so would be quite costly to heat. Framework consists of galvanized tubular-steel hoops inserted in ground and covered with flexible polythene “skin”. A door at each end provides access and ventilation. The polythene will need replacing every 3 years or so, as it becomes brittle and discoloured.
Two types of greenhouse staging
Staging is recommended if you want to grow pot plants andis also convenient for raising plants. It allows maximum use of greenhouse space, especially if arranged in two or more tiers. Tiered staging is particularly useful for lean-to greenhouses and conservatories, as it can be placed against the back wall to create an attractive massed display.
- Aluminium A tubular-aluminium staging framework is the sensible choice for an aluminium-framed greenhouse or conservatory. Some systems are easily dismantled so that you can move out the staging for the summer to accommodate tall crops such as tomatoes. Many systems are extendable.
- Timber Looks best in a greenhouse or conservatory with a timber structure. Available in cedarwood or deal, single-level or tiered. Not so versatile as aluminium staging — generally considered a permanent fixture.
Two types of staging surface
Plant pots can stand on one of two types of surface. Choose the surface to suit the plants you intend to grow.
- Slatted or open-work surface Recommended for pot plants and orchids, which like good air circulation. Also allows surplus water to drain quickly and heat to rise around the plants. There is a choice of timber slats or plastic openwork units.
- Gravel trays Designed to hold gravel, shingle or horticultural aggregate. If these materials are kept moist a humid atmosphere is created around the plants. This is appreciated by many pot plants, especially tropical and sub-tropical kinds. Gravel trays can also be used for capillary watering if filled with sand or lined with capillary matting.
Four ways to heat a greenhouse
A common mistake is to buy a heater too small for thegreenhouse so that the desired minimum temperature is unobtainable during really cold weather. Seek the heater manufacturer’s advice on the size of the unit, quoting the dimensions of your greenhouse and the desired minimum temperature. As a general guide, the following examples are instructive. To maintain a 2.4 x I.8m/8 x 6ft greenhouse at 5°C/40°F when the outside temperature is -7°C/20°F, you will need a .heater with an output of 2kw/6,000 BTUs. To maintain a minimum of 10°C/50°F in the same greenhouse you will need a heater with an output of 3kw/9,000 BTUs. A heated greenhouse should be insulated to keep fuel bills at a realistic level. The most popular method is to line the inside of the greenhouse with bubble plastic insulation (sold specifically for greenhouse use). This can be taken down in summer when the heater is switched off.
- Electric heater Electricity is efficient, reliable, automatic, convenient and clean. It’s also quite expensive compared with other fuels, although thermostatic control (considered essential) will keep down costs. The most popular type of electric greenhouse heater is the fan heater, which blows out warm air. Tubular heaters are hollow tubes containing heating elements; they are generally installed in “banks” along the sides of the greenhouse or under the staging. Neat and compact.
- Gas heater Basically a warm-air cabinet: air is warmed inside and rises out of the top. Natural-gas types are cheaper to run than bottled-gas models. Both are thermostatically controlled. Minimum attention.
- Paraffin heater Portable and highly popular but it needs frequent attention (regular filling and cleaning). Some ventilation must be provided at all times. Often thought to be cheaper to run than other types of heater but not thermostatic — so can be more expensive. Two types available: blue-flame models, with less risk of fumes (these are harmful to plants); and yellow- flame models, which are almost as good. Always use high- grade paraffin.
- Soil-warming cablesElectric warming cables laid underneath a soil bed or in a sand bed on the greenhouse staging. They provide bottom heat, which is especially useful for propagation. Cheap to run, especially with thermostatic control. They do not heat the air, so a greenhouse heater will be needed to maintain air temperatures.
Three ways to ventilate and shade a greenhouse
Very hot sun can severely scorch or damage plants underglass, so some form of shading is needed between mid- spring and early autumn. A greenhouse must also be well ventilated to prevent a damp, stuffy, stagnant atmosphere and to keep temperatures down in hot weather.
- Automatic ventilator openersAll ventilators can be fitted with automatic ventilator arms, which open and close them according to temperature. These are reasonably priced; no power source is needed.
- External blinds To provide shade you can fit roller blinds to the outside of the greenhouse. Most are manually operated but there are automatic systems (expensive, naturally!). Available in various materials, including wooden laths, plastic reeds and shading netting.
- Liquid shading materials Proprietary products which are “painted” on to the outside of the glass to provide shade. Apply in spring and wash off in early autumn.
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