To attain the higher temperatures for propagation it is necessary to construct a propagating bench or case. This merely consists of a section of the staging supplied with extra heat and covered with some moisture-retaining material, such as sand, peat, pearlite, or vermiculite, and preferably enclosed by a glass frame to retain warmth and a moist atmosphere.
The propagation of many greenhouse subjects from seed, cuttings, off-sets, and so forth, requires temperatures considerably higher than is needed for the mature plants. In some cases it is possible to take advantage of the natural warmth of summer for propagation; but summer in the British Isles is unreliable and, in any case, the temperatures required for some forms of propagation may be higher than we could expect to get naturally for any length of time.
In greenhouses heated by hot water pipes the staging could be lowered in one place to be nearer the pipes, or supplementary oil or electric heating put underneath. In some pipe-heated houses it may be worth running some extra pipes under a section of the staging at one point. In houses heated by paraffin oil lamp it may be possible to place the lamp under the staging and utilise the staging space directly overhead for propagation.
Some electric heaters may be similarly adapted (not fan heaters), but with large heaters too much heat may be produced and it will be necessary to obtain an extra smaller heater to warm the propagating area.
Where there is electricity, propagation is easy. Excellent electric thermostatically-controlled propagators are available commercially. Alternatively, an area of the staging can be heated with a soil-warming cable.
With a soil-warming cable it is an easy matter to make your own excellent and effective propagating bench or case. In many cases it is preferable to buying a commercial propagator. A soil warming cable is embedded in a layer of sand spread out on the bench, the sand is then covered with moist peat, and the pot and pans or boxes of cuttings etc., are then immersed in the peat. A rod-type thermostat inserted in the peat layer is ideal for controlling the temperature. When it is desired to propagate a wide range of plants requiring different temperatures, this can often be done on the same warming bench by varying the depth of immersion of the pots or boxes in the peat. Glass laboratory thermometers are useful for checking temperatures.
A soil-Warming cable bench can be covered with a glass frame or sheets of glass with advantage. Such an enclosure can be used for accommodating small tropical plants if desired. It can be used for all the usual methods of propagation too, and when a covering is supplied the peat layer can often be dispensed with, the pots and containers being placed on the sand over the warming cables (which is more convenient). In this case the thermostat should be given the job of controlling the air temperature inside the propagator.
In my experience a greenhouse propagating bench needs at least 12 to 15 watts per square foot if it is to reach temperatures in the 70 to 80°F (21 to 27°C) range. Not all greenhouse plants will need this for propagation by any means, but it is advisable to be able to attain this level if necessary. As already explained, with a thermostat that can be adjusted, no heat is wasted even if the source of heat is over-rated and capable of an excess.
Unfortunately a number of commercial propagators do not reach a high enough temperature, and this matter must be checked before purchase.
It should be borne in mind that the temperature reached by a propagator or bench depends on the surrounding air temperature. For example, a non-thermostatically-controlled propagator that has been reaching about 75°F (24°C) in winter in a 45°F (7 °C) greenhouse may, during spring spells of sunshine, become far too hot when the greenhouse temperature rises into the 60 to 70°F (16 to 21°C) range. This is why it is always wise to fit a thermostat to save fuel (apart from its value as a safeguard for the plants).
Warming cables are available to work directly from the mains or from low voltage transformers. The former are usually the most convenient for propagators. Common ratings are 75, 120, 150 and 300 watts. Two or more can be used if necessary, but for a small greenhouse a 120-150 watt cable will heat a bench about 5 feet by 2 feet. When laying the cable see that no part touches another or overheating which may damage the insulation may occur. Always keep the material in which the cable is immersed in the moist condition. This gives better conduction of warmth as well as maintaining humidity often important to propagation. Do not immerse electric warming cables in dry peat, or peat that is likely to dry out, or serious overheating of the cable is likely owing to the heat insulating properties of dry peat.