BIOLOGICAL EFFICIENCY OF SUPPLEMENTARY LIGHTING ON CUT ROSES THE YEAR-ROUND

The average monthly supplementary photosynthetic photon flux density (PPFD) effects on yield and energy consumption were studied for two cut rose cultivars using results from 2 years experiments with supplementary lighting. The effect of supplementary PPFD on rose yield lasted all through the year, and the extra yield due to extra PPFD tended to be lower around the equinox for both cultivars. The biological efficiency (marketable) yield divided by kilowatt-hours or moles) of supplementary PPFD persisted year-round. However, for both cultivars the biological efficiency of supplementary lighting was higher in the summer than in the winter; e.g. in June and July the biological efficiency was about twice as high as in December and January. Since the photoperiods were equal, the increased light sum, which caused elevated average air and leaf temperatures, enhanced leaf expansion and increased light interception, and thereby increased growth vigour, may account for the higher summer efficiency of the supplementary PPFD. For the cut rose industry in northern countries, year-round use may prove advantageous when added to the general increase in yield and quality resulting from the use of supplementary lighting.