I've learned that most modern types are more prone to show preflowers when kept root bound early in. It doesn't work with the landraces necessarily, depending upon the origin and how worked, or wild the line is. But the trick I believe besides this, is knowing the stock your growing and being able to segregate by the size and structures.
Forcing the light cycles like so to induce earlier flowering or "preflowers" isn't going to always be beneficial with today's common cross bred weed, there's a higher odd of hermaphrodites in a lot of today's "strains" and the littlest things can bring these traits to surface.
When I was a kid growing in my closet, I early in my learning curve would trigger seedlings into early flowering with 12/12 being utilized around a week or less. Anything else longer would often induce flowering that would take a while to reverse. Eventually I realized how detrimental this was and ineffective in the long run.
Once you get a really good grasp with identifying the individual sex traits you won't have the time to do things like this, as it is pretty easy to narrow down the two with high accuracy and even a perfect call being made time after time with your seedlings/plants.
A lot of the hybrids out there tend towards having predominant preflowers anywhere from the sixth node to the eleventh, even though most can be found once the fourth node is well developed and branching out. Then the higher nodes tend to be easy to sex utilizing a magnifying lens if necessary.
Best of luck with your endeavors, I have a tray of three week old hybrids that I made and I will be sorting today looking at the boys I'll need to cull alongside any weak unwanted phenotypes
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