Greenhouses: Everything You Need to Know to Buy a Greenhouse
In a well-equipped greenhouse, the choice of plants and produce extends from the half-hardy all the way to the near-exotic types.
Choosing the right site is vital to successful greenhouse cultivation. Even if space is so limited that the eventual choice of site is less than ideal, consider adapting the garden — by moving a border, for example — to counteract obvious drawbacks.
Choose an open sunny site so that the greenhouse gets as much light and warmth as possible — ideally it should be in full sunlight for most of the day and artificial shading can be applied when it is wanted. If only a shaded place is available, it is still possible to grow a wide range of plants though the choice is more limited.
If possible, site the greenhouse well away from large buildings, tall hedges, screens, fences or big trees so that it is not overshadowed. Avoid also sites prone to waterlogging and frost pockets and any exposed to strong cold winds.
Remember that shadows cast in winter are much longer than summer shadows. Also, a wall or tree on the north side of the greenhouse will cast a smaller shadow than a similar plant or structure on the east, west or south side.
Even if a nearby tree does not directly overshadow the greenhouse, twigs and branches may break the glass during a gale, while the roots can damage the foundations. However, small trees up to about 3 5mm (12ft) high should cause little trouble and can help to provide shelter.
The orientation of the greenhouse — the direction in which the ridge runs — affects the amount of light and heat available at different times of the year, and the types of plants which will grow in it.
Most gardeners prefer the ridge of a free-standing greenhouse to run on an east-west axis. This provides the best light in winter, making it suitable for winter and early spring crops, and plants such as orchids, flowering annuals, winter bulbs and alpine plants. It is also useful for raising batches of half-hardy annuals from seed. However, temperatures may rise too high at midday during the summer
A free-standing greenhouse with the ridge running north-south is best for plants which crop in summer and autumn. In summer, it heats up very quickly in the morning and remains warm well into the evening or night.
If space is scarce, consider a lean-to greenhouse against a wall of the house. It will get less light than a free-standing one, but bricks store heat, helping to keep the greenhouse warm after dark. It is also easier and cheaper to extend existing services, such as water,and .
A lean-to greenhouse with an east-west ridge on a south-facing wall ensures maximum heat in summer and maximum light throughout the year, but it might be too hot for many plants. Tropical fruit, such as melons, and exotic vegetables like aubergines and peppers will do well.
A lean-to greenhouse running east-west on a north-facing wall will be in shade for much of the time, even in summer, making it a haven for many shade-loving foliage plants and ferns.
A north-south lean-to on a west-facing wall will be shaded in the morning, but warmth will linger after nightfall, making it useful between autumn and spring. However, on winter mornings it will be chilly and shaded.
With a north-south axis, a lean-to will receive morning light, but will otherwise be shaded for much of the day, especially in winter. It also retains little natural heat.
Although an open site is important to let light and warmth into the greenhouse, shelter from winds — particularly north and east winds — is essential. Strong, cold winds lead to rapid heat loss and hence large heating bills, while even gentle breezes can cause chilling . A gale may rip apart a plastic-sheet greenhouse, and break panes in a conventional one.
If the garden does not have a suitable sheltered site, plant a hedge or a row of shrubs to filter the wind, or build a wall or fence in its path to act as a windbreak. Site the windbreak at a sufficient distance so that it does not cast shade over the greenhouse.
Plants grown under glass need to be inspected and cared for daily, so make sure that the greenhouse is easy to get to, especially in cold or wet weather when it is tempting to stay indoors.
Site it as close to the house as practicable, and provide a hard access path wide enough for a wheelbarrow or a person carrying heavy loads.
Greenhouse plants need constant watering, particularly at the height of the growing season, so site the house close to a source of mains water — automatic watering systems require mains pressure in order to operate successfully. Alternatively, install a standpipe. For easier watering, run a hose off the water source.
If the mains water is hard, consider installing a tank near the greenhouse to collect rain-water, or a water-softening system.
Electricity and Greenhouse Heating
A supply of electricity close to the greenhouse can be used to provide heating in your greenhouse — allowing the greenhouse to be used through the year — fan ventilation, and lighting for evening work or to supplement natural daylight.
If your garden is on a sloping site, stand the greenhouse on a levelled foundation — it is impractical to have sloping benches inside, or doors which don’t hang straight.
If plants are to be grown in containers, sloping sites provide no particular problem once construction is complete. But remember that if you want to grow plants directly in the greenhouse border, soil drainage on a sloping site will create problems. Take measures to impede the drainage of soil moisture down the slope before building the greenhouse. Remember also that frost pockets are likely at the base of a slope.
A greenhouse can free gardeners from the tyranny of the weather. Under cover of glass plants flourish that would suffer from cold, heavy rain and wind if they were grown in the open. A greenhouse also protects food plants from attacks by birds, animals and some other pests.
An unheated greenhouse will not keep out much winter frost, but it creates warmer conditions for plants in the growing season. This extra heat speeds up the ripening of crops, and improves the flowering of many decorative plants.
Perhaps the most valuable function of a greenhouse is to lengthen the growing season. Plants can be encouraged into growth early in the spring, and either kept under glass or planted out in the garden. In the greenhouse, plants will grow on well into the autumn.
This artificial growing season is achieved by the glass or plastic cladding trapping the heat of the sun, and retaining it in the soil, staging, any brickwork and in the plants themselves.
Unheated greenhouses are widely used to grow a crop of tomatoes during spring and summer, and then for growing late flowering chrysanthemums in autumn, when the tomato plants have been discarded. They can also be used for grapes and melons.
For the gardener who is interested in producing top-quality blooms, an unheated greenhouse is invaluable for growing a range of half-hardy shrubs, fruits, annuals, lilies, gladioli and many other bulbs.
All out-of-season vegetables normally grown under frames or cloches — lettuces, carrots, radishes, potatoes and French beans, for instance — can be grown just as well in an unheated greenhouse.
By installing equipment for ventilation, shading and watering, control of the growing environment can be quite precise — with some of the more expensive systems, control can be semi- or even fully automatic.
Staging and shelving
By installing staging and shelving in your greenhouse, you can at least double the growing area, and at the same time raise the plants to a more workable height. Sun-lovers, including seedlings and young plants, can be given the topmost spot while plants preferring some shade can be put under the staging or on low shelves. Similarly, trailing plants can be placed on high shelves to tumble to eye-level, and tall upright plants can be put at a low level. Where space allows, stepped or tiered staging is useful for displaying plants.
By cramming as many plants as possible into the greenhouse, you will also help to increase the humidity — essential to plant health when the temperature is high in summer
The simplest form of staging is the slatted wooden type. Many timber greenhouses come complete with adequate wooden staging, or you can build your own. Manufacturers of aluminium greenhouses invariably offer a range of aluminium shelving and benching to match, whichor bolts on to the framework. Some shelving has a surface top of stout wire-mesh.
To provide extra humidity in summer, cover the slats with a sheet of polythene, then spread a 2.5cm (1in) layer of moist sand, gravel or vermiculite over the top. Prevent the sand from falling over the edges by pinning a timber framework on to the staging, under the polythene. Alternatively, use special plastic or aluminium trays filled with a moisture-absorbing material.
Pot plants standing on moist sand will draw up water through their drainage holes by capillary action. Keep the sand moist — but not flooded — at all times. You can do this by manual watering or by an automatic system. In winter, when high humidity is not required, the sand should be cleared to allow good air circulation around the plants and better distribution of artificial heat.
Whenever staging or shelving consists of two or more tiers, always use drip trays, sand trays or polythene sheeting to prevent drainage water from splashing on to the plants below.
A greenhouse — whatever its de- sign or size — should have at least two ventilators in the roof and some low-positioned ventilators at opposite sides, staggered so as to avoid through draughts, otherwise it is very likely to become too hot in summer. As a general rule, the total surface area of the roof vents should be at least equal to one-sixth of the floor area — hot air rises, so the roof vents are of prime importance.
Fungus diseases flourish in a warm stagnant atmosphere. When top vents and side vents are open at the same time a rapid change of air takes place, keeping the air as fresh as possible — hot air escaping through the roof vents pulls cooler air in through the low side vents. Additional ventilation can be achieved as necessary simply by propping the door open.
Sliding side vents at ground level allow access to the area under the staging, for cultivation and the addition or removal of pots and boxes.
Top vents, and most hinged side vents, can be fitted with automatic openers in glass greenhouses. A special liquid compound inside the unit expands or contracts with temperature changes and motivates a piston system of levers. These units are quite expensive, but once installed you can leave the greenhouse unattended even in the most extreme of weather conditions while you are away.
An alternative form of ventilation is by an electric extractor fan controlled by a thermostat. A supply of mains electricity in the greenhouse is, of course, necessary. The fan is best placed in the apex of the roof, at the end of the greenhouse opposite the door. Various sizes of fan are available. As the fan sucks air out of the greenhouse, sufficient air will usually come in under the door and through gaps under the glass. But during very hot weather the door or vents at a distant point from the fan should be left open to give maximum ventilation.
The most efficient method of shading the greenhouse is to use one of the proprietary roller blinds, usually made of wood, cane or aluminium slats, or woven material such as hessian. They can be lowered on sunny days and rolled back on dull days. They can be fitted to the inside or outside of the roof, and should be set on rails 2.5-5cm (1-2in) from the glass.
Automatic blinds are also available but are expensive. These are operated by photo-electric cells which respond to light intensity.
Blinds are usually used only across the greenhouse roof, although they are available to cover the sides as well. A green-house where the ends face east and west should be shaded on the south side of the roof. A greenhouse running north and south should be shaded on both sides of the roof.
An effective, and also cheaper, alternative to blinds is electrostatic shading paint. This type of paint is bought in a concentrated form and must be diluted with water before use. Use either a brush or sprayer to apply the paint to the outside of the glass. If you cannot reach all parts of the roof area easily from a step-ladder, attach the brush to a long pole, or use an old soft broom to apply the paint. Never attempt to climb on a greenhouse roof — you may break the glass, buckle an aluminium frame, or even fall through the glass and suffer severe injury.
Shading paint is waterproof so will not be washed off by rain, but is easily wiped off with a dry cloth at the end of the season. It can be applied to the sides of the greenhouse as well as to the roof. If the house runs north-south paint the south, east and west sides, and both sides of the roof.
Shading — whether blinds or paint — should be used only where plants need to be protected from strong, direct sunlight. Ferns, orchids and tropical jungle plants will definitely need such protection, but other plants generally do not, provided the ventilation is adequate. For a cheap, temporary shade during spells of unusually hot weather, you can pin an ordinary cotton or nylon sheet across the glass rather than using proprietary shading materials.
Unless a greenhouse is being built alongside an existing garden tap, a permanentshould be installed. This is work for a plumber, but the expense is justified by the saving in work and time later. You may need to pay an additional water rate for the use of an extra supply point — check with your local water authority.
Once a tap has been installed, plants can be watered either by hand, with a long-spouted can, a hosepipe or automatically. The amount of water depends on the plants, time of year and the temperature and must be guided by common sense. During active growth, more water is obviously needed than during dormancy. The aim is to keep the compost just moist, never waterlogged or desiccated — on a hot summer’s day, tomatoes, for example, may need watering several times.
Automatic watering has several advantages. It can be more reliable than hand-watering, especially where plants are crammed in and difficult to get at, and it promotes steady growth of plants. It also saves time and enables you to leave the greenhouse unattended for days, or even weeks provided ventilation is controlled automatically.
Capillary sand benches are a popular method of automatic watering. Small units are avilable that can be extended. A unit consists of a plastic tray, a tank or reservoir, and a constant-level float valve.
Place a layer of sand mixed with a proprietary non-toxic algicide to prevent the growth of algae in the tray. An alternative to sand is a proprietary absorbent mat — capillary matting.
The float valve regulates the level of water in the tray and hence the degree of wetness of the sand or matting. Use plastic pots with large drainage holes for capillary-bench work. Do not put crocks in when potting. Press the pots down firmly on the moist sand or matting, so that the compost makes contact with it through the pots’ drainage holes.
Water the pots well. This will ensure continued uptake of water from the capillary bench.
Trickle irrigation is a more sophisticated system of watering which consists of a plastic pipeline with nozzles at intervals which drip water into the pots. The flow of water is controlled from athat siphons automatically. The frequency and amount of water supplied can be controlled by a valve. Water flow can also be regulated by adjusting the nozzle aperture. Nozzles can be held over the pots by wire staples, or by proprietary clip-pegs.