Any discussion on illumination must be prefaced by the reminder of the precise role of light in plant growth. The intensity or level of light has a vital effect in the velocity of the photosynthesis process which is essential to all green plants; the duration of light, or day length, also has profound photoperiodic influences; and sources of artificial light, as distinct from sunlight, have differing stimulative effects on plants.
Types of artificial lighting
Tungsten filament lamps
These lamps (the type frequently used for home lighting) produce light from a superheated filament which contains a good deal of the far red in its spectrum and this will, in many plants, produce intense elongation. The intensity of light they produce is low but is sufficient to produce photoperiodic and photosynthetic reaction in extremely sensitive plants such as, pelargoniums and many others. Tungsten filament lamps are also satisfactory for bringing many bulbs into flower.
The introduction of Sungro-Lites has brought a new dimension to the use of artificial lights for plant growth in the home and greenhouse. With their low heat emission and excellent light spectrum, with specially designed fittings, they can now be used at relatively low cost for both supplementary and replacement units in convenient places. Other lights of this type are available.
These produce light by the discharge of electricity through a gas, and the colour varies according to the gas employed. Neon gives a largely red light, vaporized sodium a yellow light, and mercury a greeny blue. Neon is at present not thought to be a great deal of use for horticultural purposes, but sodium (SOX) has proved very effective. Mercury vapour lamps (MB) on the other hand are excellent for the production of high intensity lighting and are therefore used to supplement natural daylight where this is too low for optimum photosynthesis activity. The spectrum of light supplied by mercury vapour lamps appears to be adequately balanced for most plants, although their use is not recommended for growing room cultivation entirely under artificial light as they produce a lot of heat. A 400W mercury vapour lamp is usually suspended at least 90cm 3ft), above the bench or floor area being illuminated, to give a high level of illumination compatible with maximum coverage of plants.
Types MBF or HLRG have a larger enclosed glass area than mercury vapour lamps and are coated entirely with fluorescent powder, so that some of the ultraviolet radiation is converted to visible light. This is an extremely useful light source for a wide variety of activities, including night-break or growing room techniques.
Tubular fluorescent lights (MCF) operate with mercury vapour at a much lower pressure than fluorescent lamps, and much of the ultra-violet radiation is successfully extracted and converted to useful light by the use of powders. By varying the powder coating, white, natural or warm white light is produced, the warm white being most suitable for horticultural purposes. A further advantage is that they do not produce much heat, having a low surface temperature.
Effective reflection ensures that the maximum amount of light is concentrated on the plants being treated, without at the same time directing too much heat upon them. Reflectors should not unduly restrict the entry of natural light, unless of course under growing room conditions.
Aluminium foil plates are used for tungsten filament lamps, whereas mercury vapour lamps need large reflectors. MBFR/U fluorescent lamps have a coating of reflective titanium oxide, and special fluorescent tubes with a small reflector so that maximum illumination is concentrated through a 130° arc.
Four main roles for lighting in
1. To allow working on short winter days or in the evenings tungsten filament (the domestic type) lights or fluorescent strip lighting can be used, care being taken to position the lights so that standing at the potting or working bench on dull days or in the evening does not cast a shadow. Light fittings and control switches should be of waterproofed design.
2. To provide day-length manipulation or night-break techniques by the use of low intensity tungsten filament lights, suspended above the plants.
3. To supplement the natural daylight when it falls below a level sufficient to maintain optimum photosynthetic activity, and to improve vegetative development and flower truss initiation a high intensity light source is used. This was mainly supplied by mercury vapour lamps in the early days, but fluorescent tubes are now widely used, for a set number of hours per day over a limited period when the plants are at an early stage of development. Recently, however, more prolonged use of supplementary lighting has been made on more mature plants, with spectacular results in poor light areas.
4. To illuminate growing rooms.
The term applies to a variety of techniques whereby artificial lighting supplants natural light. In most cases the waste heat generated by the lighting is used to provide a heat source. Low-intensity lights at 100 watts for every square metre of growing areas are used in a building or cupboard of high thermal insulation. This is particularly suitable for bringing bulbs into flower: the tungsten filament lamps are kept suspended about 45cm (18in) above the leaf tips of the bulbs, light being given for 12 out of 24 hours, preferably by time switch. In a well-insulated building, heat from the lamps may be sufficient to keep the temperature sufficiently high, while during dark periods electric heaters are used.
Details of the construction of a growing room can be obtained by application to local electricity boards who have plans of standard growing rooms, although many different and varied designs of growing rooms are now in use, the basic objective being to increase the actual area available for the plants and at the same time reduce capital costs. The basic formula is a chamber within a building of high thermal insulation. The sides of the chamber are of pegboard to allow the passage of fan-circulated air, which avoids the local build up of heat from the lights. A thermostatically operated ventilation vane allows entry of cooler air from outside when the temperature rises above the desirable level; when heat falls below the desirable level, a heating bank then comes into operation. These have in many cases now been superseded by linear growing rigs in commercial spheres.
Some very rudimentary growing rooms involving banks of fluorescent lamps above benches have been installed, surrounded by black/white polythene, with suitable air outlets, in basements or rooms in older buildings and have been used successfully for many years. Here again Sungro-Lites and similar types have a useful application at low cost and without complicated installation. More recently growing rooms have been developed in black (insulated) plastic greenhouses. The basis of the technique is the utilization of light and heat in combination with low heat loss, thus avoiding the cost of heating a traditional greenhouse with its high heat loss.
The many technical issues which surround the construction of growing rooms and their use are beyond the scope of this site and are best taken up either directly with the Electricity Council or the agricultural/horticultural officers of the respective electricity boards, although reference will be made to their use in connection with crops such asand bedding plants, which lend themselves admirably to growing room production.