It isn't intensity of light which regulates most seasonal changes in plants, but duration of light, aka photoperiod. Consider natural daylength through a normal growing season. The first day of spring is called the vernal equinox, vernal referring to spring and equinox which refers to a photoperiod of approximately equal periods of day and night. As the spring goes into summer the days get longer, up to the first day of summer, also called the summer solstice, the longest day of the year. How long the day is is dependent upon the distance from the equator. The further north of the equator one goes now, the longer the current length of the day. From the solstice into the fall the days shorten until on the first day of fall, aka the autumnal equinox, day and night are about equal again. Then as things head towards the end of the year the days shorten even further, until the first day of winter, the winter solstice. This is the shortest day of the year, the further from the equator the shorter the day. From there days lengthen up to the rough equality of the vernal equinox, and it all begins again. Temperature does play a part, and in some plants changes in intensity of light caused by seasonal changes such as growth of an overhead canopy of deciduous trees. But these things are greatly variable, and make for a bad clock and calendar. Photoperiod at a given latitude is however highly consistent year to year. When you grow indoors a temperate or subtropical plant which has seasonally different stages of growth, if you want the stage which occurs in the fall, be it blooms or in this case gemmae, cutting the light down to 12 hours a day will normally stimulate the desired growth. One consideration with this is ambient light in the grow area. It's hard to make a plant think it's the end of September if the room it's in has a window and it's the middle of June. Or if the room is lit at night. For this reason I run my indoor lighting for periods of time roughly mimicking the actual daylength. My Pygmies get their long days from the sun. They went out as soon as I felt they wouldn't take a frost, which was around mid April. Prior to that they were under 12 hours of light daily. I started them from gemmae in February I think. They'll stay out until after or at worst right before the first day of fall, so when they do come in they'll be under 12 hours. I'll drop that to 10 around Thanksgiving, and back up to 12 after Valentine's Day. Hope this helps!
