Abstract: To study the characteristics of floral bud differentiation（FBD）and effects of photoperiod
on growth and development of Pelargonium hortorum‘Horizon’，paraffin section method was used in
identify the process of FBD，and flowering time and flower quality were investigated as the effects of 7
kinds of photoperiod. The results show that：(1) There were 8 phases during FBD of‘Horizon’，and it
lasted about 9 weeks. (2) Significant correlation existed between vegetative growth and FBD phases，and
juvenile phase of‘Horizon’ended at 5-leaf stage according to the regression equations. (3)‘Horizon’
was a facultative short day plant，of which the best photoperiod promoting flowering was 12 h. (4) The
plants tended to have a tall and loose habit under longer days，and an opposite habit under shorter days. It
is suggested that Pelargonium hortorum‘Horizon’planted under long-day of 16 h before 5-leaf stage to be
strong as seedlings，and then under short-day of 12 h to promote branches and flowers.

Key words: Pelargonium hortorum, floral bud differentiation, vegetative growth, photoperiod, flower control