Coreanomecon hylomeconoides Nakai, also known as the Korean forest poppy or 매미꽃, is an endemic species in Korea with potential ornamental value as a potted or gardening plant. This species blooms from June to July, with one or several flowers blooming at the end of the peduncle. In order to manipulate the flowering timing, we investigated the effect of cold storage duration and photoperiodic manipulation on the flowering response of this species. Five-year-old plants of C. hylomeconoides, previously overwintered, were stored at 1℃ for 0, 4, 8, or 12 weeks to suppress sprouting and manipulate the flowering season. And then, the plants were transferred to environmentally controlled growth chambers for photoperiodic manipulation: 9/15, 12/12, 14/10, 18/8, or 24/0 (day/night) hours (h). For all treatments, 200 μmol･m-2･ s-1 PPFD was maintained for 9 hours, and day extension with 3~4 μmol·m-2·s-1 was delivered to adjust the photoperiods of each treatment. Under long-day conditions over 14/10 h photoperiods, C. hylomeconoides continued differentiating floral buds and blooming, irrespective of cold storage durations. On the other hand, under short-day conditions of less than 12/12 h, the initially generated floral buds bloomed, but additional floral bud differentiation did not occur and the flowering was delayed. Cold storage successfully delayed the flowering date by 4 weeks for suppression of sprouting, but durations of more than 8 weeks had negative effects, such as decreased percentage of flowering and poor growth characteristics. These results indicate that C. hylomeconoides can be classified as a facultative long-day plant, and a short duration of cold storage can be used to manipulate the flowering season.