Consequently, it has been the subject of considerable research effort in recent years.
The review cited below examines this recent research and concludes that it confirms there are two distinct kinds of raspberry: annualand biennial-fruiting types. Both form new shoots in spring, but the annual-flowering types stop growing in summer as flowers are formed, and these flowers open immediately.
By contrast, shoots of the biennial-flowering types continue growing until the cooler temperatures of autumn slowly bring their growth to a halt. Flowers then form and the plant becomes dormant - a dormancy that has to be broken by chilling at low winter temperatures before the flowers can open and set fruit. Hence, these plants are biennials.
The differences between the two types seem to depend entirely upon whether the newly-initiated flower buds enter dormancy or develop immediately into open flowers. By selecting suitable types and manipulating the temperature in which they are growing, and by cold-storage of canes that are in a ready-to-flower condition, it is feasible to produce fruit all-year-round under protection. Clearly, if warmer winters were to occur in areas where biennial-flowering types are traditionally grown, inadequate winter chill could become a serious problem.
Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick
Physiology of Flowering and Dormancy Regulation in Annualand Biennial-fruiting Red Raspberry (Rubus idaeus L.) - a review by Heide and Sonsteby (2011). Journal of Horticultural Science & Biotechnology 86 (5): 433-442. The authors' abstract of their manuscript can be seen in full at www.jhortscib.com.