12/12 Isn't the Only Way to Flower

The Mistake

Twelve hours on, twelve hours off. That’s what you learn on day one. Cannabis needs twelve hours of darkness to flower. Don’t mess with it. It’s the rule.

Nobody told me where the number twelve came from. It’s not a law of biology — it’s an approximation that works and happens to be easy to remember. Australian researchers tested what happens when you give flowering cannabis more light than 12/12 — up to 14 hours on, 10 hours off.

The plants still flowered. The buds were heavier. And nobody had bothered to check because 12/12 was always “the rule.”

Why This Matters to You

What triggers flowering isn’t the amount of light — it’s the length of the dark period. Most cannabis cultivars need about 10-11 hours of uninterrupted darkness to flower. At 10 hours dark (14/10), many cultivars still get that signal and flower normally. The extra two hours of light means more photons per day, which means more energy for bud production.

There’s a catch, though. In the high-THC cultivars tested, the longer photoperiod increased yield but reduced THC concentration. The buds got bigger but weaker. For the CBD cultivar, both yield and CBD concentration held up — total CBD output roughly doubled.

So this isn’t a free lunch for THC growers. It’s a significant opportunity for CBD growers, and an interesting trade-off for anyone else.

What To Do

• If you’re growing CBD cultivars, testing 13.5/10.5 or even 14/10 in flower could meaningfully increase your harvest.
• If you’re growing THC cultivars and potency matters, stick with 12/12. The concentration drop is real and significant.
• If you’re growing for extraction (edibles, tinctures), where total cannabinoid output matters more than concentration per gram, a longer photoperiod might make sense even with THC cultivars. More material at lower concentration can still mean more total cannabinoid.
• Don’t go beyond 14/10 without caution. Some cultivars will revert to vegetative growth or produce weird single-blade leaves if the dark period gets too short.

The Deeper Science

The cultivar-specific data, the mechanism behind why different chemotypes respond differently, and calculations on whether the electricity cost is worth the yield gain — that’s all in Module 2.3a (Skilled Grower tier).

FAQ

Does this work with autoflowers? No — autoflowers don’t respond to photoperiod for flowering. They flower based on age. Most auto growers already run 18/6 or 20/4 through the entire cycle.

If I add two hours of light, will my electricity bill go up? Yes. Two extra hours of LED at 400W is about 0.8 kWh per day, or roughly 67 kWh over a 12-week flower cycle. Whether the extra yield covers that cost depends on your electricity rate and what you’re growing.

Could I just increase light intensity at 12/12 instead? Yes, and that might be the better option for THC growers. More photons per hour at 12/12 gives yield gains without the potency trade-off. Module 2.1b covers this in detail.