I flipped to 12/12. Why hasn’t it started flowering?

The short answer

How many days has it been?

That is the question almost nobody includes when they ask, and it decides whether there is anything to discuss. Flowering is not a switch you throw; it is a process that takes weeks to become obvious, and a great many “it isn’t flowering” posts turn out to be a plant that is a few days in and behaving normally.

If the days do add up, the next thing to check is not your nutrients. It is whether the dark half of the cycle is genuinely dark.

What the plant is actually responding to

The common shorthand is that you give the plant twelve hours of light. What matters is the other twelve.

A photoperiod plant measures the length of the uninterrupted night. That is why a light leak is disproportionately expensive: a plant that gets six hours of dark, a flash of light, and then six more hours of dark has not had a twelve-hour night. It has had two six-hour nights. Nothing about your timer’s on-hours changed, and the plant read the cycle completely differently.

The consequences growers worry about here are real and well recognised — delayed flowering and slower development of the flowers themselves. That is precisely why the leak is worth hunting rather than shrugging at.

Where the light is getting in

Worth walking the room during the dark period, with your own eyes adjusted, before assuming the plant is at fault:

• Equipment indicator LEDs inside the tent — controllers, power strips, chargers, fans.
• Zips, corner seams, and cable pass-throughs that gape once the tent is loaded.
• A timer that is not doing what you think it is doing. Check it against a clock rather than against your memory of setting it.
• Room light, if the tent vents into a space that has its own lighting on a different schedule.

Opening the tent during dark to look is not itself the disaster it is sometimes made out to be — but do it in a room that is dark, or you have just performed the interruption you were trying to detect.

How long it should take, honestly

I am not going to give you a day count for when preflowers should appear, because I have not found a source I trust for it and I would rather leave a gap than fill it with a number that sounds authoritative.

What I can give you is the surrounding scale. The reference text for the field treats twelve hours on and twelve off as the standard flowering photoperiod, and eighteen hours as the standard for vegetative growth. And flowering as a whole — flip to harvest — commonly runs somewhere in the region of forty-five to seventy days depending on cultivar, which is a wide enough spread to be worth knowing before you decide your plant is late.

Against a process measured in weeks with that much cultivar variation, a few days of apparently nothing is not evidence of a problem.

What would show that I have this wrong

If it has been well over two weeks of genuinely uninterrupted twelve-hour nights and there is still no sign of flowering at the growing tips, then something other than photoperiod is going on and this page has not helped you. The first thing I would want to know then is whether the plant is an autoflower — in which case the light schedule was never what was driving it, and the flip did nothing at all.

Where my device fits, stated narrowly

Croplock has a spectral sensor in the canopy, so it can notice light present during a scheduled dark window and tell you. Stated at its real strength: it is a gross leak detector. It will catch lights physically on when they should be off, or a genuine spill into the tent; it is not a claim to detect every stray photon, and I would not want it sold as one. The walk-the-room check above finds things a sensor in one position cannot.

Sources

• Zheng, Y. & Llewellyn, D. “Lighting and CO₂.” Ch. 6 in Cannabis in Controlled Environment Production (CRC Press). DOI 10.1201/9781003150442 — twelve hours on and twelve off as the flowering reference photoperiod, eighteen hours as the vegetative one.
• The forty-five to seventy day flowering-duration range is the cultivar-dependent prior used across these pages.
• That an interrupted dark period delays flowering is the field’s general understanding of photoperiod response; the leak-hunting checklist is first-party practice.

More of these at Answers. The device is at croplock.com.