Environmental manipulation for establishing high yield potential of strawberry forcing plants

The production of ready-to-flower strawberry forcing plants with high yield potential has been studied by manipulation of temperature and fertilization during the natural transition to inductive SD conditions at a South Norwegian locality (60°40′ N; 10°52′ E, 250 m altitude). The cultivars 'Korona', Polka' and 'Sonata' were used. It was demonstrated that, in this environment, sub-optimal temperature is a limiting factor for adequate floral induction in these strawberry cultivars under natural out-door conditions, while the prevailing natural photoperiod is not a limiting factor. Elevated temperature (>15 °C) during the month of September increased flowering and fruit yield, and this effect was significantly enhanced when elevated temperature was combined with a pulse of extra fertilization. The highest flowering and yield potential was obtained when fertilization was applied for a three-week period starting shortly after the photoperiod had declined to the inductive length. It is concluded that, in the cool Nordic environment, strawberry forcing plants with high yield potential can only be produced on a regular basis by greenhouse cultivation at elevated temperature (>15 °C). Under these conditions flowering and yield potentials of the plants are further significantly enhanced by timely application of a pulse of autumn fertilization. While excessive flowering with reduced fruit size was often experienced with 'Korona', this did not occur in the large-fruited 'Sonata'.