The flowering of chrysanthemum is governed by the length of the day.
Day length is defined as the duration of the light period from sunrise to sunset.
Chrysanthemums initiate flower buds when the day length is less than 14.5 hours and develop these buds when day length is less than 13.5 hours. It is the length of the dark period and not the length of the light period which causes the plant to initiate flower buds. Thus night lengths exceeding 9.5 hours initiate flower buds and night lengths of 10.5 hours develop them. The most effective method of preventing flower bud initiation is by the use of artificial lights during the night to produce two dark periods shorter than 9.5 hours. Generally lights are given at 10 pm to 2 am.