Artificial Lighting

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Plant growth depends on a number of factors, two of which arc light and heat. In the simplest of terms, plants convert water and carbon dioxide into complex carbohydrates. In order to enable the plant to carry out this synthesis it has to be supplied with energy. The plant absorbs light through its leaves every day, and by a process known as photosynthesis converts it into energy.

As some of the processes which take place in the plant are heat-sensitive, the ambient temperature is also a factor in the plant's growth.

It can easily be seen, therefore, that for optimum conditions for growth, a plant needs both heat and light in sufficient intensities. During the summer months these conditions are fulfilled naturally, but in the winter there is both insufficient heat and light.

During the period six weeks before and six weeks after the shortest day (the winter solstice) there are many days when the light intensity is insufficient to provide the plant with the energy required for the chemical synthesis to proceed, and consequently growth ceases.

It is therefore necessary to provide artificial light, as well as heat, during this period, if you wish to keep your plants growing.

All plants benefit when daylight is augmented by suitable artificial light at times when the light intensity is below average. If plants are given extra illumination on the numerous dull days the increase in growth will be considerable.

To use artificial light indiscriminately is merely incurring an unnecessary expense, but there are occasions when the benefits achieved are more than worth the cost. An illustration of this is in the growing ofa cyclamen from a corm. After flowering, a cyclamen is rested by withholding water, which causcs the corm to become dormant. It is usual to restart the corm into growth in early summer, and if it is given a minimum temperature of 55°F (13°C) it will usually flower early in the winter. Few amateur gardeners can afford to provide such a high temperature in their greenhouses, but the same result can be achieved by providing 3 minimum temperature of 450F(7°C) and artificial illumination for about ten hours a day during late autumn and early winter. They will remain in flower until late spring, if kept in a cool atmosphere without any further artificial light. If grown at 45°F (7°C) without any artificial light cyclamen will not usually flower before early spring.

Should you wish to use artificial light for any purpose other than augmenting daylight, it is necessary to ascertain the plant's light requirements, because plants vary in their requirements and the wrong application of artificial light might prove harmful. There are three main types of plants, as follows.

  1. Short-dayplants.
  2. Long-day plants.
  3. Day-neutral plants.

An example of a short-day plant is the chrysanthemum, which flowers in the autumn and winter. Until the days shorten to a certain number of hours, the flower buds do not form. By keeping a chrysanthemum in complete darkness for 14 hours in each 24 during the lighter months ofthc year, it can be induced to initiate flower buds and come into flower at any time of the year. In contrast, by exposing it to daylight plus artificial light for 14 hours a day it would never form any flower buds;

thus you can appreciate that it is necessary to know a plant's light requirements before extending the daylight period.

Some plants, such as the tomato plant, are day-neutral which means that they are not alfected one way or the other by the length of time they are exposed to light, from the point of view of flower bud formation. They do, of course, benefit from being given extra light.

I-ong-day plants are plants which will not initiate flower buds until the day is beyond a critical length. Examples of this are certain varieties of fuchsia which will only form buds when the days are longer than 12 hours.

No doubt in future years all gardening books will include this kind of information in the same way as they give the plant's heat requirements.

To use artificial lighting it is not necessary to know all the technical terms, but merely to appreciate that the quality and the quantity of the light is important. As to the quality, the best source of light for the amateur gardener is the fluorescent tube. Of the different types available, the 'warm white' is the most suitable for general purposes if you are only using one tube; but a mixture of light, from one 'warm white' and one 'daylight', gives the best illumination if you have a two-tube fixture.

The quantity of light is governed by the length of time the plants are exposed to the artificial light and how close the fluorescent light tubes are positioned above the plants.

For use with the propagator already described, which is approximately 5Vj ft (1.7 m) in length by 2'/j ft (75 cm) in width, a unit consisting of two 5 ft (1.6 m) (80 watts each tube) fluorescent tubes is ideal, and units of this type are specially made for greenhouse use. The lighting unit should be suspended from the roof of the greenhouse in such a way that it can be lowered to within 2-3 in (5-7 cm) of the glass top of the propagator when in use, and when not in use can be secured close to the roof, out of the way and not casting a shadow on the propagator. It should be positioned centrally over the propagator to ensure that the light falls evenly over the whole area.

The intensity recommended for supplementary tight is 10- 18 watts per square foot (930 cm-). The intensity is measured by dividing the wattage of the source of light by the area covered.

It will be obvious that, when artificial lighting is used, this necessitates switching the lights on and off at the correct times, and although this can be done by hand reasonably satisfactorily it is much more convenient and more flexible to incorporate a time switch in the lighting circuit, in the form of a clock which can be set to the precise times at which you require the lights to function. You will find it well worth the extra expense incurred in fitting an automatic time switch.

The advantage of using artificial light is well established and it is widely used by commercial growers. No doubt more and more amateur enthusiasts will make use of it as time goes on.


Much has been written about composts for pot plants and few gardeners will not have heard of the John Innes sowing and potting composts, which are widely offered wherever gardening requirements are sold. These composts are so called because they were developed as the result of several years' work at the John Innes Institute. Prior to that time there was no such thing as a standard potting compost and each gardener had his own formulae, and in most instances used different mixes for different plants.

As the result of the experiments carried out by the John Innes Institute it was shown that it is quite unnecessary, for the most part, to make up different composts for different plants.

The standard composts put forward by the JI Institute consist of one seed and cutting compost and three potting composts. The latter are known as John Innes potting composts Nos.l, 2 and 3. The difference between the three lies in the quantity of chemicals in the mix, and these chemicals are known as the John Innes base. No.2 contains twice as much as No.l, and No.3 three times as much. This provides the grower with three different strengths of potting compost. In addition to the John Innes base, a quantity of powdered chalk is added to the mix, except when preparing compost for plants which require an acid soil, such as heaths and rhododendrons, when it is omitted.

Apart from the chemicals and chalk, which are measured by weight, all the other ingredients of the compost, the loam (soil), peat and sand, are always measured by volume. This is important because, if measured by weight, the quantity would vary according to the water content, particularly in the case of the sand, and this would give rise to lack of uniformity.

A convenient method of measuring by volume is to use a standard seed tray which measures 14 x 8Vi x 2 in {36x26x5 cm) and, for all practical purposes, nine such standard seed trays have a volume of approximately I bushel (8 gallons or 36 litres).

J I seed compost

2 parts fibrous loam.

I pan peat,

1 part washed sand.

To each bushel of the above mix add

1 '/a oz (42 g) superphosphate of lime and

1 oz (28 g) powdered chalk.

JI potting compost No. 1

7 parts fibrous loam.

3 parts peat.

2 parts washed sand.

To each bushel of the above mix, add

(21 g) powdered chalk.

The quantities of chemicals and chalk to add to make Nos.2 and 3 are as follows. For No.2 add 8 oz (227 g) John Innes base and 1 x/i oz (42 g) powdered chalk per bushel. For No.3 add 12 oz (340 g) John Innes base and 2lk oz (64 g) powdered chalk per bushel. The fibrous loam most suitable for these composts is turves which have been stacked and allowed to stand for some six to nine months. If this is not available you can use topsoil from a plot which has been kept rich in humus by the addition of garden compost from year to year.

The loam used in making any of these composts should always be sieved through a V*-in (1-cm) riddle. The peat should be one of the fine grades, the sand should be a washed or horticultural grade, not builder's sand, and preferably a coarse sand.

The method I use when making composts is first of all to measure out the loam and put it in a heap. On top of this I put the measured quantity of peat followed by the sand. Before adding the sand to the heap, the JI base and any other chemicals, but not the chalk, are mixed with the sand. This helps to spread the chemicals throughout the mix. Using a spade the heap is turned over a few times and then the powdered chalk is sprinkled over the heap as evenly as possible, after which the heap is turned over until all the ingredients are thoroughly mixed. The compost is then stored in a plastic bag of the type used for packing peat. Usually the loam, peat and sand contain sufficient moisture to be in the ideal state for use, i.e. if a handful is squeezed it just holds together, neither falling apart nor congealing into a solid lump. If the ingredients are too dry it is as well to add sufficient water, using a watering can with a rose, to correct this.

You can if you wish make up your own John Innes base according to the following formula.

By weight:

3 parts hoof and horn meal.

3 parts superphosphate of lime.

lVa parts sulphate of potash.

The main difference between JI composts and the composts of yesteryear is that the soil is sterilised by heating it to a minimum temperature of 180°F (82°C), the object being to destroy weed seeds, fungus and insects which may be present. The JI composts you buy have been treated in this way, but if you decide to make your own composts you will need to obtain a steriliser, if you consider it important to sterilise the soil. Personally I do not consider it necessary to sterilise the soil you use in a compost (neither did the very successful Victorian gardeners) unless there is reason to suppose it is infected in some way, in which case it should not be used.

In my very long experience of plant culture I have never suffered any deleterious effects from using unsterilised loam, and I have always made my own composts; but then gardening is, of course, like this. Some people do all the things which the text books say you should not do, and get excellent results, so it is not wise to be dogmatic when dealing with nature.

Most enthusiastic greenhouse gardeners are keen to fill their greenhouses, and this inevitably means the use of considerable quantities of potting compost; so it is really essential to mix your own composts from the point of view of the expense involved.

As previously mentioned JI composts are suitable for the majority of plants excluding, of course, those which require lime-free composts. When deciding which compost to use, a factor to take into account is the conditions in which the plants are to be grown.

When plants are grown on capillary watering trays, the compost is much damper than it is when the plants are watered at the top, hence it is desirable to have a compost with a more open texture. This is done by increasing the proportion of peat and sand in the mix. A compost which has proved very suitable for plants growing on watering trays is made up as follows.

Compost E6

3 standard seed trays of fibrous loam.

2 standard seed trays of peat.

1 standard seed tray of coarse horticultural sand.

6oz(l70g) John Innes base,

Vt oz(21 g) hoof and horn meal.

Va oz(21 g)bone meal.

This compost is suitable for most plants for all stages from the first potting up to their final potting.

Two other composts arc recommended for certain plants in the text as follows.

Compost E2

1 standard seed tray of fibrous loam.

1 standard seed tray of peat.

I standard seed tray of leaf mould.

1 standard seed tray of coarse horticultural sand.

No chalk.

Compost E3

2 standard seed trays of fibrous loam.

2 standard seed trays of peat.

2 standard seed trays of leaf mould.

2 standard seed trays of coarse horticultural sand.

8 oz(227 g)John Innes base.

1 oz (28 g) powdered chalk.

In the event of being unable to obtain leaf mould use peat, which will in efTect double the amount of the peat in the mix. The main difference between composts E3 and E6, when no leaf mould is present, is the ratio of loam to peat and sand and the quantity of chalk in the mix. Compost E3 is more open and more suitable for plants which prefer slightly acidic conditions (about pH6), such as gloxinias and fuchsias.

Greenhouse horticulture involves a considerable amount of potting and repotting and a very necessary and useful requirement is a portable potting bench. This can very easily be made by anyone capable of sawing and nailing.

All you require is 4'A ft (1.4 m) of 6 x 1 in (15 x 2,5 cm) planed timber and 4'/i ft (1.4 m) of 6x Vi in (15x 1.3 cm) planed timber. Cut each piece into three lengths of 18 in (45 cm) and, using the I-in (2.5-cm) thick wood for the bottom, make a three-sided open top box, using the !/i-in (1.3-cm) wood for the sides. It is quite satisfactory to use nails, but if you are a perfectionist you may prefer to use screws to fasten the pieces together. To complete the portable potting bench the corners of the side pieces should be sawn off at the open end. This is a most useful piece of equipment as it can be used anywhere both outside and inside. You can, for instance, put it on a greenhouse bench, and instead of getting compost all over the bench it is all contained. When you have finished potting, any compost left over can easily be tipped back into the compost bag.


During the spring and summer months it is essential to protect plants in the greenhouse from the direct rays of the sun. On a cloudless day, when the sun is shining continuously, the temperature in the greenhouse can easily reach 100°F (38°C) and more, and when this happens many plants are losing water, by transpiration, at a faster rate than it is being replaced, with the result that the leaves become limp and the plants droop. In this condition the direct sun scorches the leaves, causing considerable damage to the plants.

By providing the greenhouse with adequate shading the temperature in the house is kept to a lower level, and the plants are also protected from the direct rays of the sun.

When choosing a method of shading it is necessary to bear in mind that there can be as many dull grey days as there are sunny days in an average spring and summer, depending on your local climate. If the shading is too heavy this would result in the plants receiving inadequate light for satisfactory growth on dull days, and if the shading is too light, it would not fulfil its purpose on very sunny days. A material which answers the purpose admirably is a very fine mesh plastic netting, green in colour, often sold in rolls 2'/: ft (75 cm) wide. It is not difficult to devise a method of hanging this on the inside of the greenhouse roof and sides. The method I adopt is to fasten a wire along the inside of the greenhouse eaves and, after securing the netting to the underside of the roof, using spring-type clothes pegs, it is then passed over the top of the wire and down the side of the greenhouse. In a 10 ft (3 m) wide greenhouse the netting is not wide enough to extend from the roof ridge, over the wire running along the eaves, and down the side of the greenhouse to the level of the bench; so I join two pieces together by threading thin coated wire through the holes in the netting at about 3-in (7,5-cm) intervals. This type of plastic netting can be used outside the greenhouse, which might prove more convenient to some greenhouse owners, as it is quite impervious to the weather.

Finc-mesh plain white net curtain material is also very good for shading, and protects the plants adequately while allowing a good light to reach the plants on dull grey days.

The greenhouse should be shaded from early spring until early autumn.


To get the best use of a greenhouse it is necessary to be systematic, which means that the year's work needs to be planned. There are many jobs each year which arc repeated more or less at the same time. What these jobs are depends on the plants you are growing, and this, of course, will vary from one individual to another. If you are growing a fairly wide range of plants it is really a necessity to write out a programme showing the dates for sowing seeds, taking cuttings, starting tubers and bulbs etc., repotting and pruning. This not only ensures that these jobs are done at the correct times, but it also enables you to plan your work week by week.

Mediocre plants do not give much pleasure either to the grower or the onlooker, but a really well-grown plant is admired by all and gives the grower immense satisfaction. Pot plants require carc and attention during the whole of their growing season and if they do not receive this at the time it is needed you will not get the best results.

Best Plants For Pots

Chvia mtnuta ii jntlcgani plant whith flowers in (hc spring

Artificial Light For Plants

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