Is 12/12 Really Best for Flowering Cannabis?

Why

Twelve-hour nights have been recommended in growing manuals for decades. Robert Clarke's Marijuana Botany, published in nineteen eighty-one, described using twelve hours of darkness to encourage flowering.

Yeah, forty-five years ago. It’s fair to ask whether it still gives us the best results.

It also discussed eleven-hour nights.

The idea is to give photoperiod cannabis a long, uninterrupted night. Twelve/twelve gives a strong flowering signal across a wide range of cultivars, making one schedule practical for a mixed grow.

Twelve/twelve is a dependable standard. But a schedule that starts flowering isn't necessarily the schedule that gives you the most flower.

Plants don't read the grow manual.

In a twenty twenty-four study at the University of Guelph, researchers compared twelve/twelve with thirteen hours of light and eleven hours of darkness throughout flowering.

Incredible Milk produced thirty-five percent more dry flower weight per plant. Gorilla Glue produced fifty percent more.

That came with about eight percent more light each day at the same intensity. In the sampled top flowers, total potential THC concentration increased in Incredible Milk and showed no significant difference in Gorilla Glue.

Wait, more yield without a significant drop in THC?

So why wouldn't you just switch to thirteen/eleven?

Well, other research hasn't always found the same payoff but there's a caveat.

An earlier greenhouse trial compared twelve- and thirteen-hour days across ten clone lines. It found no significant overall increase in the combined leaf-and-flower harvest. Two vigorous clone lines produced more leaf-and-flower biomass, but had lower cannabinoid concentration and yield.

However, the newer study used full-spectrum LEDs indoors and measured dry flower yield, making it a closer match to how many of us grow today. That gives us a good reason to test thirteen/eleven, but it doesn't erase the older trial's warning: more plant growth doesn't always mean a better harvest.

And the newer study gives us more to consider than the extra weight.

The thirteen-hour plants appeared larger. More flower per plant doesn't establish the same increase from a crowded tent, where plants compete for space and light.

They harvested the twelve/twelve and thirteen/eleven plants together, using the twelve/twelve plants to set the harvest date.

The thirteen/eleven plants were slower to show signs of maturity, and Incredible Milk's top flower was less dense at harvest.

There's still a solid case for testing thirteen/eleven.

Compare clones of the same plant under twelve/twelve and thirteen/eleven in separate light-tight spaces, starting when you switch to flowering. Keep light intensity, growing conditions, area and plant count similar.

Compare dry flower yield, density, harvest maturity, crop time and electricity use. Don't force the usual harvest date if the flowers aren't ready.

Keep that eleven-hour night uninterrupted. A shorter planned night doesn't make light leaks harmless.

So here's my recommendation.

If predictable harvest timing is your priority, stick with twelve/twelve.

If your priority is more flower per harvest and you have flexibility on the harvest date, give thirteen/eleven a trial.

Then judge the finished crop. Did you get more usable dry flower from the same space? Did the quality hold up? How many days did it take?

If you're trying to maximize production over time, compare dry flower weight per flowering day, too.

Keep thirteen/eleven if the finished harvest delivers a worthwhile improvement. Otherwise, go back to twelve/twelve.

But I think you'll find thirteen/eleven is worth testing.

And yes, I'm running 13/11 in flower and I'll keep you posted.

Related Grow Answers: When Should You Flip Cannabis to Flower? https://www.growpops.com/grow-answers/when-should-you-flip-cannabis-to-flower Can a Tiny Light Leak Ruin Your Cannabis Grow? https://www.growpops.com/grow-answers/can-a-tiny-light-leak-ruin-your-cannabis-grow

Research discussed: Ahrens, Llewellyn and Zheng (2024), Plants 13:433. https://www.mdpi.com/2223-7747/13/3/433 Potter (2009), The Propagation, Characterisation and Optimisation of Cannabis sativa L. as a Phytopharmaceutical, Chapter 5. https://ia801501.us.archive.org/8/items/CANNABISSATIVAASAPHYTOPHARMACEUTICAL/THE%20PROPAGATION%2C%20CHARACTERISATION%20AND%20OPTIMISATION%20OF%20CANNABIS%20SATIVA%20AS%20A%20PHYTOPHARMACEUTICAL.pdf