Seamaiden's statements are absolutely correct. You have to acclimate clones to outside very carefully and slowly if you want a 100% success rate, or close to it. The light cycle should not be manipulated more than 15/min per week, which mimics the natural light cycle. It takes me 3 months to go from an 18 hour cycle down to the 15 hours needed by the end of May to acclimate my clones to an outdoor environment. Trying to drop them from a 24 hours light cycle to go outside in a single season rarely works, even the mother plants used to produced the clones should never, ever be under 24 hour light if those clones are intended to go outside.
It is the build-up of auxin and chromatin that signals the plant it is time to flower. There is actually a 'chromatin switch' that has been discovered, which the auxin triggers, and that in turn unravels the chromatin, telling the plant to flower. Different plants and different strains have different levels of auxin and chromatin required to flip the chromatin switch so it is not a one size fits all sort of technique. The larger the change in the light cycle, the bigger the shock to the plant, and the more likely the plant is to flip that chromatin switch, as it is a survival mechanism. So dropping from say 24 hours of light to 23 hours of light won't be enough to trigger it, but going form 24 hours of light to 18 hours of light certainly can, and going from 18 to 15 absolutely will on most strains.
Longer flowering strains, like Sour Diesel, are more easily transitioned to outside because of this, as it takes a higher level of hormones to trigger flowering. I have seen some people take Sour D from 18 hours of light and put them outside at 15 hours and it did sometimes work but I would not recommend it. Shorter flowering strains required much lower levels of hormones to flower, and therefore are more sensitive to changes in their light cycle. The Old Betsy (aka Mango Kush) cut I have for example, which can finish in 6.5-7 weeks, will start to trigger once the light drops below 16.5 hours, making it not suitable for outdoors.
There are other factors that can influence flowering as well, such as stress. Cold weather and allowing plants to get root bound will also contribute to auxin and chromatin build up. Again that Old Besty cut I have will start to flower under any amount of light if it gets too root bound or stressed out. However, the main factor which determines when the plant will flower is still ultimately the light cycle.
