If light levels can be controlled by the use of whitewash or shading screens to give partial shade in summer, and by substitute lamps to make up a deficit, then plants of compact habit will be obtained.
The level that suits cyclamen is 40,000 lux.
We should point out that a lack of light at the heart of the plant is often due to packing the plants too close or not spacing them out early enough.
We should first of all observe that cyclamen do not respond to day-length as such: the formation of the flower buds is not influenced by changes in the hours of daylight. The increase in the overall quantity of light, however (length of day and light intensity) does encourage bud stalks to grow and flowers to develop.
Cyclamen are plants that need a lot of light. Any deficiency shows up in the plant as over-long stalks, over-large leaves of a soft texture, and a delay in flowering. The plants become too large, lose their resistance and keep less well.
Increasing quantities of light, on the other hand, encourages a tendency to compact plants. During certain periods of the year, though, especially in summer, it is essential to shade the plants, in order to ensure that the leaves do not receive too much sunshine and above all to avoid too high a glasshouse temperature (cf. § on the control of summer temperatures under glass). Shading is thus an essential factor in producing good plants.
Shading can be done by temporary use of screens, or by whitewashing the glasshouse.
Shading is important when spells of strong sunshine follow straight after spells of weak sunshine. The same applies after such operations as pricking out, potting up or setting out at full distance. Before the final spacing, the plants are close-packed and their lower leaves are therefore kept in a microclimate which is moist and shaded by the upper leaves. When they are first spaced out, then, they are all the more sensitive to sunlight.
It may be desirable at times to decrease the incident sunlight in order to keep glasshouse temperatures within the right range for the plants; but there are also times when the sunlight needs supplementing artificially, when it is so deficient that the quality of the plants might suffer. If there is too little light in winter for good cyclamen growing, this is supplemented with extra lighting, known as artificial sunlight. It improves the vegetative growth of the plants and the number of flowers, and extends the flowering season when natural light is very scarce. Thanks to this practice it is possible to have plants in full flower for the Easter festival or Mothering Sunday without the whole cycle taking more than 8 months. However the practice does have the drawback that it produces more elongated plants, and weaker plant tissue.
To provide this kind of light, SON-T lamps with sideways diffusing reflectors (30-35 W/sq.m. at about 3000 lux at plant level) are adapted.
This supplementary lighting is applied from November until February/March, depending on natural lighting conditions; it is also brought into play when outside lighting falls below 5000 lux. The aim is to provide a quantity of light normal in March or October.
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