ERECTING A GLASSHOUSE. 2. THE EQUIPMENT by Brian Milligan
In Part 1 I touched upon the basics of erecting an aluminium-framed glasshouse. In this article I describe some of the equipment that can be installed to maintain a reasonable environment for the orchids housed in that structure.
Good Air Circulation is essential in any enclosed growing space. It helps to dry any water remaining on the foliage after watering, as well as to provide the leaves with a supply of fresh air with its miniscule but essential content of carbon dioxide. It also discourages attack by various types of fungi and insect pests, such as red spider and scale. Large overhead fans are probably best but they are unsuitable for most small glasshouses, because they tend to decapitate tall orchids and their growers! Oscillating fans are the next best option. They should be fitted near the roof so that they are less likely to be affected by the spray from misting systems etc. The switches should be sealed under plastic film to minimise corrosion and the possibility of electrical shorting etc. I strongly recommend that you get an electrician to install a safety switch in your meter box to minimise the chances of electric shocks during mishaps. Glasshouse fans should operate continuously; they usually last for several years under these conditions.
Heating . If you intend to confine your collection to cool-growing orchids, then you don’t need a heater. But if you can provide some heat in winter, in order to maintain a minimum temperature of 8˚C, you will find that many of these orchids will grow better and you will also be able to grow many orchids that struggle without heat. By maintaining a minimum temperature of 12˚C, you will be able to grow most of that large group of orchids described in orchid books as requiring ‘intermediate’ conditions.
There are three basic types of glasshouse heater available in Melbourne. The Bato gas heater has the advantage that it is cheaper to run than electric heaters but gas heaters do have some disadvantages. I’ve heard of cases in which the pilot light has blown out during windstorms and others in which the flue has developed a leak, resulting in loss of buds and poor growth because of the plant-toxic flue gases emitted by old gas heaters.
Electric fan heaters are widely used because they are easy to install. However, they are expensive to operate and the dry air they emit can dry out adjacent plants very quickly. Specially designed glasshouse fans, such as the Gem heater, are recommended because their electric elements are sealed in a waterproof casing, so that they can withstand the occasional misdirected squirt of water. Ordinary domestic fan heaters usual blow up when hit by a jet of water!
Probably best of all is a ducted heating system in which hot water is circulated beneath the benches via finned pipes. A domestic hot water system is often used to heat the water, which is circulated by a thermostatically controlled electric water pump. This type of system has a far less drying effect than a fan heater. Whatever type of heating you use, a maximum/minimum thermometer is an essential piece of equipment you will need to monitor your heater’s effectiveness.
Cooling . Glasshouses function by trapping the sun’s rays – a highly desirable property in winter but not at other times, when glasshouses can rapidly overheat. On a sunny summer day, the air in a glasshouse will soon reach 40˚C or more, even if a protective layer of shade-cloth has been fitted. Most orchids suffer from heat stress and cease to grow at temperatures above 30˚C, so it is important to limit the temperature in a glasshouse to this figure, or preferably a little less.
There are several ways to keep a glasshouse cool. One is to fit a shade cloth cover, which most growers fit in late September and remove at the end of April. I fit a second layer of shade cloth during the hottest months (January and February). But shade-cloth by itself is not enough. Roofing vents should also be opened in hot weather to allow hot air to escape. Corresponding vents near floor level are desirable, as they will admit cool air, which can then flow through the house before escaping through the roof vents. Some glasshouses have automatic openers for the roof vents. These can be adjusted so that they begin to open the vents when the temperature reaches a preset figure, usually about 20-25˚C.
When the ambient temperature reaches 30˚C neither of the above measures will provide sufficient cooling and an evaporative cooler is then needed. It’s best fitted at the far end of the glasshouse, where it can draw in outside air, which is eventually expelled through the partially opened door. This allows air, cooled by evaporation of water in the cooler, to pass over and around the plants before escaping through the doorway. Evaporative coolers use a lot of water on hot days and their reservoir may need refilling one or more times daily. Some growers fit a ball valve in the reservoir to maintain a constant water level automatically.
Under-bench sprinklers, automatic misters and foggers are other useful aids that help to provide cooling and to increase the level of humidity in a glasshouse. Most orchids grow best at relative humidity levels between 50 and 70%. Cheap hygrometers readily allow you to check relative humidity and it’s $5 well spent to find out whether your glasshouse environment approaches this ideal. The simplest way to increase the humidity in the glasshouse is to spread scoria or pine bark on the floor and keep it continually wet. While this can be done manually if you’re no longer a member of the working class, it’s fairly easy to rig up (under the benches) a simple DIY misting system, coupled to a solenoid valve and an electronic timer. My system, operating for only one minute at 9.00 am and again at 2.00 pm, provides enough moisture so that the floor remains continually moist.
On warm, sunny days the relative humidity in my glasshouse still falls below 50% and I have therefore rigged up a system comprising sixteen fogging nozzles (in four groups) controlled by a timer so that they emit a fine mist of water for five seconds every ten minutes (a humidistat would provide even better control, especially when the sun pops in and out during partly cloudy weather). By now you will have realised that there is much more to setting up a fully functional glasshouse than just erecting the structure. Providing water, electricity, heating, cooling, air circulation, automated misting, fogging etc. usually costs more than the glasshouse itself, not to mention the benches! It’s just as well those sashes won at Showtime seem to make it all worthwhile.