Lavandula × intermedia is cultivated as an ornamental plant in Japan but its fiowering characteristics have not been elucidated. So, a series of experiments were conducted to determine the infiuence of cold treatment and day length on fiowering. An exposure to a 14-week cold treatment (CT) under natural winter condition induced fiowering. An increasing duration of day length was associated with an increase in fiowering rates and with a decrease in days to the first visible bud (VB) and the first open fiower (FLW). This suggests that L. × intermedia may be an essential quantitative long-day plant. When the plants were treated with a low temperature in a cold room at 5°C, a 10-week CT resulted in perfect fiowering. Plants without CT and plants after a 6-week CT did not initiate fioral buds. An increasing duration of CT was also associated with an increase in the rate of fiowering shoots and with a decrease in days to VB and FLW. Thus, exposure to a period of low temperature is the primary factor promoting fiowering in L. × intermedia. Apical dissections and scanning electron microscope (SEM) observations showed vegetative meristems at the end of a 10-week CT at 5°C. Then, dome formation and sepal initiation of the first fioret of each shoot apex were observed at 7 and 21 days after CT, respectively. This suggests that L. × intermedia is a so-called “after effect” fiowering plant. Thus, we conclude that L. × intermedia is a typical vernalization type plant.