Development of strawberry flower initials: timing and response to nitrogen application and temperature conditions at northern latitudes

The dominant growing system of strawberry in Finland is the perennial cultivation of June-bearing cultivars in the open field. At northern latitudes the period of favourable conditions for floral initiation is limited, and the timing of cultural practices after harvest requires wide understanding of flowering physiology. Three experimental set-ups were conducted in southern Finland to study strawberry flower development in different cultivars, as related to 1) the effect of nitrogen fertilization in controlled day length conditions in the greenhouse, 2) the precise timing of floral initiation, and 3) the effect of nitrogen fertilization and application of fleece cover during the floral initiation in early autumn in the open field. The development of flower initials of early, mid-season and late cultivars were followed by dissecting the crowns under a stereo microscope and recording the number of inflorescences and the developmental stage of the top flowers. The abundance of flowering was observed in spring. Some early-stage flower initials were detected already at the end of August, indicating that flower induction occurred in mid-August onwards. However, the timing of floral development differed between cultivars. Nitrogen fertilization during the floral initiation enhanced flowering in two out of three cultivars when applied in the greenhouse, but the results varied in the open field, where no clear relation between nitrogen treatments and the number of flower initials was observed. The application of fleece in early autumn to cover strawberry canopies did not enhance flowering. Our results show that moderate nitrogen applications can have positive effects on strawberry flower initiation under controlled greenhouse conditions. However, the variations of weather in autumn, together with the differing nutrient reservoirs of plants and soil, form such complicated interactions under open field conditions that it is difficult to show any consistent benefits of nitrogen applications to strawberry flower development on farm level.