Johan A. Bakker, T. Blacquière
Supplementary lighting of greenhouse crops is widely applied nowadays in the Netherlands, especially for roses but also for lilies, chrysanthemums, freesias and pot plants. Due to the often dense concentration of greenhouses, a neighbouring greenhouse can be exposed to light overspill which may affect the crops therein. Leakage of light through the side walls of the greenhouse is restricted by screens, but a part of the added light leaves the greenhouse through the roof. This light is reflected by clouds and is diffused over the surroundings. When the clouds are low, light levels of up to 0,20 μmol m-2s-1 may be expected in a neighbouring greenhouse. Levels of 0.10 μmol m-2s-1 have been measured in practice.

For this reason, the effects were studied of 0.05 and 0.20 μmol m-2s-1 (400–700 nm) of HPS-light, either in the first or in the second half of the night, on the growth and development of some greenhouse crops. The natural day was 10 h.; the night, either with or without ‘overspill’ light was 14 h. In the light treatments, a 6 h. period of darkness was maintained.

Elongation growth of Cucumber ‘Farbio’ was reduced by light as compared to dark controls; fruits developed later and from a higher node. Flowering of the short day plant, Chrysanthemum morifolium ‘Daymark Cream’, was delayed by 0.05 μmol in both the first and second half of the night; by 0.20 μmol m-2s-1 in the first half, or fully inhibited by 0.20 μmol m-2s-1 in the second half of the night. In the short day plant, Euphorbia pulcherrima ‘Angelica’ (Poinsettia), all light treatments prevented flowering. The long day plant, Fuchsia ‘Beacon’, was induced to flower by 0.20 μmol m-2s-1, but not by 0.05 μmol. Callistephus chinensis (China aster) ‘Milady’, ‘Kometa’ and ‘Starlight’, responded to all light treatments: 0.20 μmol m-2s-1 induced flowering and caused a marked leaf expansion and elongation of stems and flower stalks. 0.05 μmol induced flowering without extra elongation and leaf expansion. Generally, light in the second half of the night was more effective than light in the first half of the night.

The results indicate that growers should take care with the use of supplementary lighting in areas where there are many greenhouses.

Bakker, Johan A. and Blacquière, T. (1992). REFLECTED SUPPLEMENTARY LIGHTING CAN AFFECT THE CROPS OF NEIGHBOURS2. Acta Hortic. 327, 95-96
DOI: 10.17660/ActaHortic.1992.327.11