This study was conducted to compare the growth and flowering characters of a dwarf purple loosestrife and purple loosestrife (Lythrum salicaria L.), and to determine the optimal photoperiod of dwarf purple loosestrife for forcing cultivation. Dwarf purple loosestrife was grown under natural sunlight (from 08:30 to18:30 HR, 10 h) and additional artificial lighting [0, 2, 3, 4, 6, or 14 h of continual light, or night interruption of 4 h (NI)] in a greenhouse. Growth and development of dwarf purple loosestrife were promoted by 14, 16, or 24 h or NI. The number of lateral shoots and plant height were higher as daylength was extended. However, the number of nodes was not influenced by photoperiod. The number of inflorescences was 0.6, 2.9, 5.0, or 4.2 under 14, 16, or 24 h, or NI, respectively. Also, plants under NI showed more uniform flowering (86%) than those under 14 h (48%). However, plants under 10, 12, or 13 h never produced inflorescences and remained as rosettes throughout the experiment. Therefore, we concluded that dwarf purple loosestrife has a critical photoperiod, 14-h daylength and NI is the effective method to accelerate growth and flowering for forcing cultivation.