Greenhouse Growing
Greenhouse Growing: Skills of Greenhouse growing
Plants in a greenhouse need extra care because of the high temperatures generated and the exclusion of natural rainfall.
Watering
Water is a constant demand of greenhouse plants, and in summer, daily watering will often be necessary. Plants in pots need more frequent watering than those growing in the soil. In winter and spring, be careful not to over-water. Wet compost is cold; it stops growth, and can encourage root rot fungi that cause root death.
Cooling
Ventilation on hot days is necessary to keep plants cool. About 25° Celsius is an ideal maximum for most plants. Beyond that, growth slows down and stops. High temperature levels can be reached from late spring onwards.
In summer, ventilation alone might not be enough, so shading can be necessary. Apply Coolglass, or Summer Cloud, in June or early July and remove it at the end of August. A simple way of cooling the house on really hot days is to damp down the floor. Ventilation in winter and spring, on dry, bree days, dries the greenhouse atmosphere, helping disease control.
Feeding
Greenhouse plants must be fed much more often than plants outdoors. The frequent watering that greenhouse plants receive tends to wash the soluble plant nutrients from the restricted reserve of a pot.
Feed little and often - even as often as once a week for large, quick-growing plants early in the growing season. Liquid feeding is simplest to use and most effective. Take care not to feed a dry pot, for fear of scorching the roots.
Hygiene
Pest and disease damage is usually more severe in a greenhouse, where the warm conditions are ideal for insects and fungi, and predators are absent. Remove old and diseased plants, or pest-ridden plants, to break the cycle of infection. Control pests and diseases when they appear.
Wash down the glass, pots, trays and benches in the winter with Jeyes Fluid or Armillatox. Consider soil disinfection with Armillatox, Clean-up or Jeyes Fluid, every few years. To disinfect the soil, dig it over and water on a suitable product. Flood the greenhouse soil in early spring by watering heavily to leach out excessive salts left over from frequent feeding the previous year.
Greenhouse Growing: Heating and Ventilation
A cold greenhouse means that there is no artificial heat provided. A cool greenhouse has a heating system that will provide some artificial heat - usually just enough to protect against frost. A warm greenhouse has a heating system capable of providing an air temperature of at least 10°Celsius.
Heat conservation
In an unheated, cold greenhouse it is important to conserve all available heat, but even if a greenhouse is heated, maximum heat conservation will reduce the running cost. Lean-to greenhouses, by having one solid wall, lose less heat because of the lower area of glass. The solid wall acts as a heat reservoir too, heating up by day and releasing this at night.
A greenhouse exposed to strong winds, apart from risking damage, quickly lose heat especially if there are broken or ill-fitting panes. A layer of polythene inside the greenhouse acts like double glazing to conserve heat but it may have a messy appearance. Covering tender plants with newspaper or cloth on frosty nights can be enough to prevent damage.
Heating systems
If basic frost protection is all that is required, there are two options - paraffin or electricity. A simple paraffin heater is relatively cheap to buy, and to run, but needs to be lit and refuelled, and can give off damaging fumes if not set up correctly.
An electric fan heater is more expensive to buy, but very easy to operate, usually featuring a built-in thermostat. Seek professional advice when installing an electric heater and be sure to set it up so that it does not get wet.
Running costs for a fan-heater are low, if it is used only for frost protection. A two kilowatt fan-heater will keep a 3.6 metres by 2.4 metres greenhouse free of frost on a night when it is minus 8° Celsius outside.
Heating a warm greenhouse to about 10° Celsius is a more difficult proposition. An extra radiator can be taken off the domestic heating system if a conservatory or greenhouse is attached to, or very close, to the house. Most central heating systems are on a time switch that shuts down the system at night - just when the greenhouse needs it most!
However, usually enough heat will have built up earlier to protect plants adequately. A thermostat over-riding the time switch can be installed to prevent very low temperatures.
Electric storage heaters can be used in a conservatory too. These use night-rate electricity. There must be no danger of them getting wet, for safety reasons. They tend to be bulky too. Free-standing greenhouses are difficult to heat economically beyond basic frost protection. A separate hot-water boiler is ideal, but few people would consider this necessary or affordable.
Vents
On a sunny day in summer, greenhouse temperatures could rise above 40° Celsius. Plants dry out quickly and can be scorched or killed at these temperatures. It will be necessary to allow the hot, dry air to escape.
Vents in the roof and sides should be provided. Opening the door helps too. Roof vents are important not only because hot air rises, but also, because it may not be possible to leave side vents or doors open in a conservatory for security reasons.
Polythene tunnels are difficult to ventilate properly, usually relying on leaving the ends open. This is adequate, once the tunnel is not too long.
Greenhouse Growing: Protective Structures
Greenhouses
Greenhouses can be free-standing structures, or 'lean-to' against a solid wall. If the wall is part of the house itself, the greenhouse becomes a 'conservatory'. A free-standing greenhouse is usually cheaper, and traps more light, but it is more difficult to heat - if this is intended.
Frames
A garden frame is just a large, low bottomless box with a translucent lid. The sides of the box can be made of wood, concrete, galvanised iron or any other building material - very often salvaged scrap materials will do.
The frame can be any length but should not be wider than 120 centimetres for comfort and safety. It should be 25 centimetres high in front, and about 50 centimetres high at the back. The 'lid' is a number of 'lights', each consisting of a wooden frame with glass or plastic - on it. Each 'light' matches the width of the garden frame in its own length and should be about 75 centimetres wide, and can be made of suitable timber lengths.
Cloches
A 'cloche' originally was a bell-shaped glass jar placed individually over tender plants. The term was extended to include continuous structures of glass sheets supported by iron brackets. These continuous cloches were normally about 45 centimetres high but are now unusual, because low plastic tunnels and cloches have taken over.
Walk-in tunnels
A 'walk-in' tunnel, or polytunnel, can substitute for a glasshouse. Being relatively cheap to put up, it is possible to cover a larger area economically. A polytunnel can be of any length - using a greater number of the tubular steel supporting hoops.
Tunnels are generally sold as kits and the standard widths commercially available are 4.2 metres and 5.1 metres. Walk-in tunnels can be difficult to ventilate properly, and the plastic will have to be replaced. It usually lasts two or three years, although there are more durable kinds of plastic film that last for five years.
Low tunnels
A low tunnel is a row of wire hoops - 45 centimetres high, 75 centimetres wide - supporting a 1.8 metre wide polythene film. Length of row is variable. At each end of the low tunnel the film is tied firmly to a short stake. The wire hoops are 1.8 metre lengths of strong wire with an 'eye' twisted into them about 20 centimetres from each end.
Strings tied into each 'eye' and stretched across the polythene hold the tunnel film in place. Although both garden frames and low tunnels are limited in use by their size, they are very successful for low crops such as early and late vegetables, early strawberries, and cuttings.
Greenhouse Growing: Glazing Materials
Glass was once the only available glazing material, and it is still the best, letting in light and retaining heat better than any alternative. It is more expensive and not as safe as plastics, but it lasts much longer. Safety glass is available and, though it is more expensive than ordinary glass, it should be considered for a conservatory.
Rigid plastics are not as expensive as glass, and they are safer, but they do not let in as much light; nor do they look as well, except for perspex, but this is expensive, too.
Polythene is cheap and safe, and allows in light well but does not last so well as the other materials. PVC and U-V inhibited films are more expensive than polythene but last longer.