I just started a thread that sounds alot like this one. I am assuming you are putting them out for a spring harvest? I am very interested in this topic as I am trying to determine the best photoperiod to veg my ladies untill this summer, when I can put them out into the greenhouse. Personally I am torn between a 14.5 hour and an 18 hour day. With the 14.5 hour, it will sync with the photoperiod during the last week in May (when I anticipate managable night temps to set out plants in the g-house), and then continue to veg through the solstice and naturally begin flowering sometime around Aug 1st.
If the time of "set out" was after the solstice and I was using a longer photoperiod I think the ladies would transition smoothly to flower, but I need to get them in the tunnel end of May to achieve a larger individual plant size by flowering (medical grow). The tricky part is deciding weather vegging at 18/6 and cutting supplamental light at solstice (with a 14.7 hour photoperiod here at 38.5 N, ) will be enough to initiate and sustain flowering without the much feared "re-veg".
I am leaning towards the 14.5 hour day from seed (indoor) and not using any supplamental lighting in the g-house by "setting out" when the natty light is about 14.5 hours at the end of May. It will keep increasing till 14.7 hours on solstice then start winding back.
This is a site that allows the user to estimate day length based on lattitude on any day of the year. Very helpful and easy to use...
http://astro.unl.edu/classaction/animations/coordsmotion/daylighthoursexplorer.html
Also, a fellow named silverback over at icmag has a fine thread on light dep for outdoors. Interesting read that got me thinking about photoperiod manipulation and the importance of timing with an outdoor grow. I am at 8000 ft. and am considering tarping the g-house to finish in Sept instead of late Oct.
I don't know how going from 14.5 hour days, to 12 hour days to force flower for a couple of weeks, and back to the natty cycle which at that point would be about 14.25 hours would work. May cause re veg, and may need a longer initial veg photoperiod (18/6) to sustain flower (after a couple of weeks of 12/12?) with the 14 hour days of late July early Aug. This is only going to save a couple of weeks but time is critical to my mountain location. I really am not trying to steal your thread, but am also in dire need of some professional advice on the best way to veg indoor for an outdoor (greenhouse) crop.
PS. Make sure to check that silverback thread @ icmag. It really opened my eyes to what can be done with light manipulation even though it may not be the most applicable to my situation. Thanks to all.