In the experiments reported here, rooted cuttings of K. glaucescens 'Freeling's Sensation', K. manginii and K. uniflora were all grown in pots in a glasshouse and allowed 18 hours of light each day to keep them vegetative.
After two weeks, they were transferred to growth rooms where the main light period was eight hours and 55 minutes long. This was extended using low-intensity light from incandescent lamps to produce day lengths ranging from nine to 15 hours.
Day/night temperatures were 22/18 degsC. All three species flowered rapidly under day lengths ranging from nine to 12 hours. However, no plants of K. manginii and fewer than 30 per cent of K. uniflora plants flowered in a day-length of 13 hours, and no species flowered in a 15-hour day.
In another experiment, plants were grown in eight-hour days for different lengths of time. All plants of K. glaucescens eventually flowered after receiving just one week of short-day treatment, but K. manginii needed three weeks and K. uniflora needed six weeks. A day-length of at least 15 hours is needed to keep these species vegetative but all of them will flower when given 12-hour days.
Variation Among Kalanchoe Species in their Flowering Responses to Photoperiod and Short-Day Cycle Number by Currey and Erwin (2010). Journal of Horticultural Science & Biotechnology 85 (4): 350-354. The authors' abstract of their manuscript can be seen in full at www.jhortscib.com