Abstract: In Argentina, southern highbush blueberry (Vaccinium sp.) exhibits two periods of vegetative growth in the same year, the first one in spring (spring growth, SpG), arising from vegetative buds on one-year-old wood, and the second in summer, from vegetative buds formed on spring growth, just after harvest (summer growth, SmG). Histological studies confirmed that flower bud initiation (FBI) occurred at the end of December on SpG and at the end of March on SmG. On SmG, FBI occurred under an 8 h photoperiod, and shortening daylength. However, on SpG, FBI was observed under increasing daylength (up to 15 h) and an average temperature of 22.5 ?C. Basal florets in apical floral buds were always in a more advanced reproductive stage on SpG than on SmG during the season. The two peaks in volume of fruit harvested were likely a result of differences in the ontogeny of buds on SpG and SmG shoots in the previous year.