A Greenhouse is Your Garden Laboratory
The Garden Laboratory
The purpose of a greenhouse is to give the gardener much greater control of the environment for plants than is possible in the open. Plants too tender to be cultivated outdoors can be grown there and brought to flower or fruit at an earlier season. Greenhouses also make plant propagation easier : seeds can be sown earlier and will germinate with greater certainty, and manywill root there that would have little or no chance of success out of doors. Frames can provide similar conditions and so fulfil some of these purposes but they contain only a small volume of air, making it more difficult to control temperature accurately, and cannot offer sufficient headroom for big plants. Nor do they provide any protection for the gardener. One of the great plusses about owning a greenhouse is that you can continue to work in it in comfort even when it is raining or freezing outside.
Greenhouses are of three main types: span-roof, lean-to and ‘circular’ (the last is usually hexagonal or octagonal rather than truly round). Each has its advantages and drawbacks. Span-roof houses are the most versatile in that they can be of any size and can be placed almost anywhere. Lean-to greenhouses need to be placed against a wall or building. They are often used as conservatories with direct access from a house and so virtually become an additional room. When used in this way they can draw quite a lot of warmth from the house, and with any lean-to built against a dwelling-house there is the possibility of combining both on the same central-heating system.
Circular greenhouses are compact and often very pleasant to look at. They can sometimes be used as features in the garden design, whereas other types, unless elaborately made, tend to be less decorative and more obviously utilitarian. Most circular greenhouses are small, however, and this, as with frames, not only means restricted space for plants but also makes it harder to control temperature and humidity, both of which fluctuate more rapidly in a small volume of air.
Unless required for some special purpose such as the cultivation of, greenhouses should always be sited in as sunny a place as possible.
Greenhouse Heating Systems
Some means of heating a greenhouse artificially greatly increases its value since it increases the gardener’s control over the climate. Except for tropical plants, few require any additional warmth from late May to early October, but in mid-winter it is usually impossible to keep the temperature above freezing point all the time and that can be critical for many tender plants. Most seeds only germinate freely at temperatures between 13-20°C (55-68°F) and without artificial heat it will probably be impossible to maintain any such level until April or even May. If early are required from sowings made in February and March, some heating is essential.
The most economical way to apply heat is in a propagator placed inside the greenhouse — really a little frame with its own heating system. Because of the double protection and the small area to be heated, the cost of running this can be negligible. However the seedlings or cuttings will soon outgrow the propagator and will have to be brought out into the greenhouse itself. Without some separate method of heating, the change of temperature may be too sudden and too great and the little plants will be killed or seriously checked.
The cost of heating a greenhouse roughly doubles for every 3°C (5°F) that the minimum winter temperature is raised. This is because, in addition to the extra heat needed to reach that temperature, there is the extra time during which it will be required. For this reason many privately owned greenhouses are now run at a winter temperature of 7°C (43°F) — enough to keep most plants alive if not always completely happy. A propagator can then be used in February and March to obtain the 16-18°C (61-65°F) which ensures rapid seed germination, and in April the minimum greenhouse temperature is raised to 13°C (55°F) to accommodate seedlings and young plants coming out of the propagator. This combines economy with a reasonable degree of safety for most popular greenhouse and bedding plants.
There are many ways of heating greenhouses. Electric fan heaters are convenient and most offer the additional advantage that they can be used without the heating element switched on to keep the air moving in warm, humid weather. Gas heaters burning natural gas, including bottled propane, can be used without flues inside greenhouses since the main by-products of combustion—water and carbon dioxide — are not harmful. However, damage can occur from very small quantities of other gases, mainly ethylene, and these heaters should never be used without a little ventilation despite the fact that this lowers their efficiency. Oil heaters suffer the same drawback and may become lethal if they are dirty, or do not get enough air, or if their combustion is disturbed by draughts. Hot-water pipes are safe but small boilers outside the house can require a lot of attention.
The ideal heating solution is to connect your pipes to the domestic central heating — provided this is not automatically cut off at night by a time switch. Wherever possible, heating should be controlled by a thermostat so that it is only used when necessary. Most electrical and gas heaters made for greenhouses have built-in thermostats and this can be very convenient though probably not quite as efficient as a thermostat placed well away from the source of heat.
Greenhouse Air and Water Supplies
Automation in the greenhouse is possible in other ways. Ventilators can be fitted with piston-type openers which are very sensitive to temperature changes. Another possibility is to fit an electrical extractor fan controlled by a thermostat, but this should be shielded from direct sunshine which can warm it unduly before the air temperature reaches the level set on the thermostat.
Watering can also be made automatic in a variety of ways, the simplest being the capillary bench. Any waterproof greenhouse staging can be employed as the base, covered with a thin layer of sand and small gravel or one of the special plastic mats made for the purpose. This is kept constantly wet by drip feed, float chamber or some other device, and the pots placed on it draw water through their drainage holes by capillary action. A variety of kits are available for this purpose. Capillary benches work best with plastic pots since these are sufficiently thin to allow theinside them to come into direct contact with the wet mat or sand. If thicker clay pots are used, wicks of glass fibre should be passed through the holes to draw in the moisture.
Despite the labour and time-saving attractions of automatic watering, there is really nothing to beat individual hand-watering provided it is done regularly and with understanding, and freshly potted plants should anyway be watered by hand for a week or so. Common faults are over-and under-watering, but the former is not likely to have serious effects if the pottingused is adequately porous since it will allow surplus water to drain away quickly. Under-watering is much more serious and a common source of trouble. It can only be avoided by examining plants daily, watering any that appear dry and giving sufficient to soak right through the pot and trickle out the bottom.