Now is the time when cuttings begin to arrive in greenhouses across North America and Europe. There are a few species, such as dahlias, tuberous begonias, iresine, and alternanthera, that require long day conditions in order to prevent premature flowering and stalled plant growth.
Flower formation in these plants is a short day response. Vegetative growth occurs under long day conditions. From September 21st until March 21stin the northern hemisphere is the time to provide day length extension so the days are 14 hours long or a night interruption to avoid flowering. A typical tactic is to place the plants under lights from 10 pm to 2 am. If the plants are grown with other light sensitive plants, black cloth will need to be pulled to avoid light pollution to those sensitive plants.
In summary, these species need long days to avoid stalled growth and flower formation. Lighting should be provided so that the day is extended to 14 hours or provide a night lighting interruption to avoid flowers.