Temperature considerably influences vegetative growth and flowering ; a large amount of research has been conducted on temperature control in greenhouse cultivations in order to obtain high-quality flowers. In particular, there is a need for a cost-effective temperature control technique. The plant height control technique, which uses the difference between day and night temperatures (designated "DIF"), was developed about 15 years ago, and since then, considerable research has been conducted using this technique. However, this technique is not feasible for practical production because during the developmental stage of plant growth, the period during which DIF can be used to control plant height is limited ; moreover, this technique requires a lot of energy and the costs involved are high. In order to overcome the drawbacks of this technique, we conducted a study on the effect of short-term heat treatment using African marigold, which is suited for studying physiology of flowering under the controlled environment of an incubator. Similar experiments were also conducted using spray-type chrysanthemum, a major cut flower, both under the controlled atomosphere of an incubator and in greenhouse cultivation. We found that short-term heat treatment at the beginning of the dark period (after sunset) resulted in earlier flowering. Subsequent experiments showed that this did not occur when the plants were exposed to the short-term heat treatment before the end of the dark period (before dawn). Thinking that this new finding can be used to develop a flowering control technique, we conducted a detailed analysis to establish a technique that can be applied to the commercial production of ornamental plants. First, we examined the effects of short-term heat treatment at different time points, for different durations, and with different temperatures in the dark period under controlled weather conditions on the flowering of African marigold. The plants were exposed to 3 different heat treatments : (1) 30℃ for 3 hours at the beginning, in the middle, and at the end of the dark period ; (2) 22℃ in the light period ; and (3) 14℃ in the dark period (control). We found that treatment at the beginning of the dark period accelerated flowering, while treatment at the end of the dark period delayed it. The plants were exposed to heat treatment at 30℃ for 0.5, 1.0, 1.5, 3.0 h at the beginning of the dark period ; flowering was accelerated even in the case of the 0.5-h heat treatment. On exposing the plants to 3 different temperatures-22℃, 26℃, and 30℃-for 3 hours at the beginning of the dark period, we found that flowering was accelerated even at 22℃. Morphological observations were conducted at each stage of flower bud differentiation and development. We found that because of short-term heat treatment at the beginning of the dark period, the developmental stage from flower bud undifferentiation through auxesis to the stage preceding floret formation was accelerated. This result showed that acceleration of flowering through short-term heat treatment at the beginning of the dark period is caused by acceleration of the developmental stage from flower bud undifferentiation to floret formation. The hypothesis was supported by the fact that there was no difference resulting from varying the time zones in the treatment of short-term heat treatment after the flower bud stage. Then, we studied the effect of this treatment on acceleration of flowering in chrysanthemum (Chrysanthemum morifolium Ramat), which is a major cut flower. As was observed in the case of the African marigolds, in the chrysanthemum cultivar 'Sei-roza', flowering was accelerated as a result of short-term heat treatment under a low-temperature environment. We found that this treatment had a considerable effect on the developmental stage of the flower bud and the point of action in the developmental stage from undifferentiation to floret formation ; these observations were again similar to those made for the African marigolds. We studied the effect of short-term heat treatment at the beginning of the dark period on flowering in the context of winter cropping in greenhouse cultivation. We found that in chrysanthemums, flowering was accelerated in response to short-term heat treatment at the beginning of the dark period. This effect was more remarkable when the cultivation temperature was lower. Moreover, we observed improvement in flower formation characteristics such as the length of lateral shoots and their angles ; these characteristics were found to be similar to those observed in conventional cultivation. On the basis of the above results, we demonstrated the possibility of reducing the heating cost by 15% through the use of short-term heat treatment after sunset in a chrysanthemum variety showing low-temperature flowering response at temperatures of around 13℃. This treatment does not cause any remarkable deterioration in the quality of the cut flower.