miniature roses should be given for 20 h∙day−1 to maximise light use efficiency. The light dose should be given daily in order to avoid growth reduction [9]. A combination of 150 μmol∙m−2∙day−1 during a 20 h∙day−1 LP gives a PAR of 10.8 mol∙m−2∙day−1. If adding daylight, as in the present experiment, a light sum of about 12 mol∙m−2 day−1 is recommended for miniature roses at an air temperature of about 20˚C. This is in accordance with previous results with miniature roses [9]. In order to reduce the time until flowering from 45 - 50 days to 30 days, however, the temperature should be increased to about 24˚C and the PAR should be increased to 15 to 20 mol∙m−2∙day−1 in order to produce plants with many flowers [10]. This is a light integral within the range found to be optimal for a range of ornamental species [11]-[13]. If we compare this PAR with the natural PAR inside a greenhouse (with 60% transmission) at latitude 59˚ north, it appears that supplementary lighting must be given during five months of the year to reach 12 mol∙m−2∙day−1, and it must be given about seven months to achieve 20 mol∙m−2∙day−1 (Figure 1). In Almeria, South Spain, a region with the highest solar radiation in Europe, 12 mol∙m−2∙day−1 is reached throughout the whole year without supplementary lighting in a greenhouse with 60% transmission (Figure 1). Even in this sunny region of Europe a PAR of 20 m−2∙day−1 is reached only in eight months of the year. Although if a sufficient high PAR is obtained, this light is probably much less efficient than expected due to short photoperiods at 37˚ north (9 to 14 h∙day−1) and due to PFD levels that considerably exceed the light saturation point of 400 - 500 μmol∙m−2∙s−1 in miniature roses, as well as in other pot plants [10]. A positive effect of increasing the LP to 24 h∙day−1 was the relative decrease in powdery mildew, as has been previously shown with some cut rose cultivars [7]. To some extent this could also be obtained with a shorter LP if it was combined with daylight to give continuous light. The reason for the development of powdery mildew when plants are exposed to a dark period is probably a temperature drop that makes a diurnal temperature variation that stimulates the germination and proliferation of the spores [14]. However, a negative effect of a 24 h∙day−1 LP, in addition to decreased light use efficiency, was an increase in water loss from the leaves, indicating reduced stomata closure. This is in accordance with previous results with cut roses, which showed that decreased stomata function caused increased transpiration and flower wilting in indoor conditions [6]. In the present experiment with plants with intact roots this did not cause any reduction in keeping life since the plants were watered frequently. In practice the increased water consumption resulting from reduced stomata closure may cause the pots to dry out and the plants to wilt if they are not frequently watered. It was concluded that 20 h∙day−1 LP including a dark period of 4 h∙day−1 and a PFD of at least 150 μmol∙m−2∙s−1 should be applied to miniature roses during the winter months in order to effectively produce miniature pot roses with a high quality.