Canes were cut down and chilled at 2°C in the dark for six weeks. As new shoots emerged from the roots, they were removed and grown on at 21°C with 10 hours of daylight each day. After two weeks, they were then moved to constant temperatures ranging from 12°C to 30°C in either 10-hour or 24-hour days (produced using tungsten lamps).
The effect of a three-hour night-break treatment was also tested and yet other plants were given more chilling at 6°C for seven weeks before being grown on at 24°C. The earliest flowering occurred at 24°C but the longest shoots were found at 27°C in continuous light. The most flowers were produced at 27°C and under continuous light, but plants in the night-break treatment also performed better than those in short-days.
Flowering of 'Polka' was evidently promoted by "long-day" treatments, as well as by warm temperatures. For protected cropping in the UK, it is suggested that air temperatures should be kept above 20°C and the crop be grown in natural long days or with a night-break treatment.
Effects of Photoperiod and Temperature on Growth and Flowering in the Annual (Primocane) Fruiting Raspberry (Rubus idaeus L.) Cultivar 'Polka' by Sonsteby and Heide (2009). Journal of Horticultural Science & Biotechnology 84 (4): 439-446. The authors' abstract of their manuscript can be seen in full at www.jhortscib.com.