Use of Artificial Light in the Greenhouse
Having seen how plants react to light the next step is to make use of this knowledge to determine how much irradiation greenhouse plants need and the most effective ways of providing it.
The commercial grower uses light to grow plants to a certain stage in the quickest possible time. He is, therefore, prepared to meet the cost of providing the high level of irradiance (the energy emitted by the lamp — including light) and temperatures necessary to give him optimum growth response. Furthermore he grows, in many cases, tens of thousands of plants at a time, which enables him to install the lamps in blocks at least four lamps square — making a minimum of 16 lamps. He can also double-batch — treat two batches of plants at the same time by moving the lamps every 12 hours from one batch to the other.
The grower uses artificial light to start plants early; for example he will propagate tomatoes in the middle of winter ready for planting out in the greenhouse from late winter to mid spring (early January to mid March), while cucumbers are planted out before this and lettuce are cropped from late autumn to mid spring (October right through to March). He also uses light to extend daylight — by turning the lamps on at dusk or before dawn, in the middle of the night or from dusk to dawn.
Home greenhouse uses
The amateur gardener may be interested in these techniques but will also want to provide extra light during dull winter days to ensure that plants (such as saintpaulia, or other houseplants) continue growing and flowering during the months when the average natural light is not strong enough to promote normal plant development.
The amateur gardener, therefore, does not need to be too concerned about the commercial growers’ exacting growing conditions. You can, however, use artificial light effectively by starting later in the season, say at the beginning of spring (early to mid February), so that once the propagating stage has been completed the plants can be set out in their growing positions at a time of the year when natural light is improving and outside temperatures are rising, and when it won’t be too costly to maintain a minimum temperature of 13°C (55°F) throughout the night.
However, during the period from seedto planting out you will have to keep temperatures up to around 18°C (65°F) during the day, dropping down to 16°C (60°F) at night. To heat the whole greenhouse to these temperatures would be very expensive, but fortunately there are ways of keeping the heating bills under control. The first step is to reduce the heat losses from the house to the outside air. A 250-gauge polyethylene sheet attached to the inside of the glazing bars will effectively save up to 40 per cent heat loss. Next, as only a few plants will be irradiated, use a plastic curtain (500-gauge polyethylene) to partition off the propagating area. This area must have some means of ventilation, but if the greenhouse has fan ventilation do not include it in the propagating area — a single roof vent should be adequate for this. You can, however, use a small fan to circulate air among the young plants, and help deter grey mould (botrytis). But if you are already using a fan heater to boost the air temperature, then an additional fan will not be necessary to provide ventilation.
Before choosing and installing one of the lamps readily available, there is one more factor that needs to be considered — the evenness of illuminance that the lamp provides. How this evenness varies can be seen from isolux diagrams; an isolux is a line joining equal levels of illuminance. These figures vary appreciably according to the type of lamp and luminaire (reflector) used. The diagram (right) shows the illuminance distribution from one 2400mm (8 ft) 125 watt fluorescent tube mounted at a height of 400mm (16 in). Observe that the lines are more or less oval and that the illuminance falls more rapidly as the distance from the lamp increases. As commercial growers must have illuminance that is as even as possible across the growing area (to ensure uniform plant growth), the aim is to provide an illuminance within 10 per cent plus or minus of the recommended illuminance. For example: if an illuminance of 5000 lx is stipulated then the illuminance across the growing area should not be more than 5500 lx or less than 4500 lx; the latter illuminance level is generally referred to as the cut-off figure.
It can be clearly seen that the cut-off for a fluorescent tube is only a few centimetres, so to irradiate an area more than 150mm (6 in) in width it will be necessary to use two or more tubes. You can see that immediately under the lamp the illuminance is 2450 lx but at a distance of 1m (3-1/4 ft) on either side it has fallen to about 200 lx — less than one-tenth. Note also that the light output at each end of the tube drops by nearly one half (to 1250 lx), so you could not expect the same plant response beneath the area 150mm (6 in) wide at either end where the illuminance is less, as you would from the remaining 2130mm (7 ft) length of tube.
To take an example: supposing you wish to provide 2450 lx at plant level with two 2400mm (8 ft) tubes fixed 400mm (16 in) above the growing area, it would then be necessary to install the tubes approximately 300mm (12 in) apart — the distance being measured from the centre line of each lamp. If you then decided to increase the illuminance to 5000 lx, the distance from the centre line of each lamp would be reduced to 50mm (2 in).
Advantages of fluorescent tubes
Despite the apparent complications in setting up a lighting installation with fluorescent tubes, this light source has several advantages. First, being a linear source of light, an even illuminance can be expected over the growing area. Secondly, fluorescent tubes need very little headroom compared with mercury and sodium discharge lamps and so are ideal for the modern small greenhouse, where the height of the eaves level above the bench is only 760mm (21 ft), and also for use under the staging.
As a general rule the area under greenhouse benches is too dark during the winter months for the majority of plants to grow normally, even if temperatures higher than 7°C (45°F) are maintained. However, two fluorescent tubes under a 760mm (21 ft) high bench will transform this normally unproductive area into one that will grow many plants, even in the middle of winter. But extra precautions will be needed to ensure all the electrical fittings under the bench are protected from water seeping through from the bench above.
The third advantage of the fluorescent tube is that it radiates much less heat than would mercury and sodium discharge lamps used to irradiate the same amount of bench area.
Other kinds of lamp
To appreciate the advantages of the fluorescent lamp, compare it with other available sources of artificial light, for instance the MBFR/u (mercury fluorescent reflector), sox (low pressure sodium) and SON (high pressure sodium) lamps.
The first to consider is the 400 watt MBFR/U. Instead of the light being distributed evenly along a 2400mm (8 ft) length, as in the case of a 125 watt fluorescent tube, it is concentrated at the bottom of a lamp that has a diameter of only 180mm (7 in). Unlike the oval isolux of the fluorescent tube, the MBFR/U isolux are circular. Again the illuminance falls off rapidly so that to cover any sort of area at all a minimum of two lamps would be required; to achieve the ideal, you would need four lamps, which would emit heat equal to that of a 1720 watt electric fire (1600 watts from the lamps and another 120 watts being generated by the control gear). Two lamps would effectively cover an area of 1250 x 760mm (4 ft 2 in x 21 ft). To overcome any problems with cut-off near the edges of the bench where plants will not be receiving the desired amount of light, these plants can be changed over — with those in the centre of the bench — halfway through the irradiation period. This also applies to sox and SON lamps. Two lamps should successfully irradiate the average small greenhouse bench 760mm (21 ft) wide if they are installed over the bench centre line. Four lamps would be required to cover a bench 1070mm (31 ft) wide, the centre of the lamps being suspended 150mm (6 in) from each bench edge. Similar results can be expected from the 310 watt SONR (high pressure sodium reflector lamp) in comparable conditions.
The 180 watt sox lamp is a linear source. The lamp is very efficient and will produce an illuminance of 6000 lx when hung 1200mm (4 ft) above the growing area. The lamp’s cut-off point is again marked, in fact light output falls appreciably towards each end of the tube; nevertheless two lamps should effectively irradiate a bench 1070mm (31 ft) long, 760mm (21 ft) wide. Again, due to the narrow cut-off, it is necessary to install the lamps parallel to the centre line of the bench, and suspended about 150mm (6 in) in from each edge of the bench.
The 400 watt soN/T (high pressure sodium tubular lamp) with the Camplex luminaire produces a quite different isolux pattern. In this case ‘tubular’ indicates the fact that although the lamp is tubular in shape it is only 338mm (13 in) long, so it can be considered to produce a spot light source. The lamp is usually suspended horizontally and, with the luminaire, produces a dumb-bell shaped isolux pattern.
This is an efficient light source that even at a height of 1500mm (4 ft 11 in) gives an illuminance of 4500 lx at bench level. However, the cut-off is quite marked, there being a 25 per cent fall in illuminance at 500mm (191 in) from a point immediately below the centre of the lamp. Compared with the other types of lamp the 400 watt soN/T produces a very high illuminance (45,000 lumens at 2000 hours) so this lamp should be suspended 1500mm (4 ft 11 in) above the growing area, which would make it impractical for the small greenhouse unless the plants being irradiated were positioned on the floor. One lamp of this type should be sufficient for most gardeners’ requirements.
Protecting other plants
Plants have a tendency to grow towards a strong light. As an example, if you were to place a plant on a south-facing window-ledge you would observe that after a period of time the stems would be bending over towards the light. This property is known as phototropism. In a small greenhouse or, in fact, in any greenhouse where plants are grown in the vicinity of a powerful lamp, phototropism will occur, especially during winter months. To prevent this it is advisable to hang a sheet of aluminium foil between the lamp and the plants being grown in natural light only.
Similar precautions should be taken when long-day treatment is given in one section of the greenhouse, as even this low level of overspill (less than 50 lx) could have an adverse effect on the untreated plants. You can imagine what would happen if the light from lamps used for plants receiving long-day treatment was to spill over to plants that you wanted kept in short days.