Raspberries can be produced out-of-season today mainly because past research established the environmental factors that controlled growth and flowering.
Most of that research was done almost 50 years ago and so it was necessary to ensure that the cultivars grown today responded similarly. In the research described here, 'Glen Ample', a modern cultivar widely used for out-of-season production, was raised in pots of compost in naturally lit controlled-environment chambers.
When grown at different temperatures in the natural day-lengths of autumn, growth stopped soon after the plants were transferred to temperatures of 12 degsC or less and flower formation began at the same time. Flowers were formed later at 15 degsC, but none were found at 18 degsC. When grown at 15 degsC but in different day-lengths, flower-bud initiation occurred in day-lengths of 15 hours or less. Thus flower initiation and the cessation of growth require low temperatures together with relatively short days. The plants then require to be exposed to cold for many more weeks before their dormancy is broken.
Finally, canes failed to form flowers if they had 10 or fewer leaves at the start of treatment. This research should allow better out-of-season fruiting schedules to be produced.
Environmental Control of Growth and Flowering of Rubus Idaeus L. Glen Ample by Sonsteby and Heide (2008). Scientia Horticulturae 117: 240-256. The contents of issues of Scientia Horticulturae and abstracts of papers are provided at www.elsevier.com/locate/scihorti.