Why 12:12 Probably Costs You Money: What New Photoperiod Research Says

Lead: 12 hours of light, 12 hours of darkness. For decades this rule has been the standard for bringing cannabis into flower indoors. Three studies from the last two years suggest that the standard protocol is suboptimal in many cases, and that the resulting yield loss can be measured in double-digit percentages.

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What this is about

In the previous post we covered the broad lighting toolkit for cannabis cultivation (PPFD, spectrum, UV). One variable we deliberately only touched on: the photoperiod. It deserves its own post because the data here has become considerably stronger over the last two years than for many other cultivation levers.

We looked at four studies: three direct photoperiod experiments on indoor cannabis (Peterswald et al. 2023, Ahrens et al. 2023, Ahrens et al. 2024), plus one nutrient study under a long photoperiod (Saloner and Bernstein 2020) as context for the vegetative phase. Three core findings that growers can act on right away.

Finding 1: 14 hours of light means more flower, almost always

In 2023, Peterswald and colleagues tested three cannabis cultivars against nine different photoperiods in a facility in Australia licensed by the Office of Drug Control (ODC). One CBD line (“Cannatonic”) and two THC lines (“Northern Lights”, “Hindu Kush”). They compared 10L:14D, 12L:12D and 14L:10D, plus switch treatments in mid flower (28 days after onset).

The result was consistent across all three cultivars: flower yield was highest in treatments that started with 14L:10D. The plants received more light for photosynthesis in the early flowering phase, grew more vigorously and set more flower.

A year later, Ahrens and colleagues from Guelph (Canada) replicated this with two THC-dominant cultivars (“Incredible Milk”, “Gorilla Glue”), this time with only 13 instead of 14 hours of light, but running the plants all the way to commercial maturity. Yield in the 13-hour treatment: 1.35 times the 12-hour yield in IM, 1.50 times in GG. That is 35 to 50% more flower.

Worth noting: the DLI (daily light integral, the total amount of light per day) rose by only about 8% from 12 to 13 hours. The yield jump was therefore four to six times larger than the light quantity alone would explain. So the effect does not seem to be just “more light = more flower” but also a different physiological response of the plant.

Finding 2: But THC can suffer, a cultivar question

This is where it gets practical. In the two THC lines, Peterswald did measure more flower under 14L, but at the same time saw a significant drop in THC concentration. The trichomes were less dense, the leaves longer and narrower: the reproductive phase apparently had not run its full course.

Cannatonic, the CBD line, showed the exact opposite: CBD concentration rose under the long photoperiod, and combined with the higher yield that added up to +50 to +100% total CBD per plant. For CBD programs, that is a substantial win.

The THC lines, in contrast: higher flower mass but lower THC%. Whether you end up with more total THC per plant depends on how much the drop in concentration offsets the yield gain. In Northern Lights, the treatment that started at 14L and switched back to 12L delivered +50% yield without the THC dip.

Here the Ahrens 2024 study partly disagrees: at 13 h (instead of 14 h), THC concentrations showed no significant difference or were even higher than under 12 h. Total THC per plant rose by at least 38%. In other words: 13 hours seems to be a kind of sweet spot that captures the yield advantage without stalling THC ripening. 14 hours goes one step too far in some cultivars.

The takeaway: test cultivar by cultivar, in 0.5-hour steps between 12 and 14 h. The data does not point to one universal recommended value; it suggests that every cultivar has its own optimum.

Finding 3: Some cultivars show no flowering delay at 14 h at all

In a broader 2023 study, Ahrens et al. tested ten cultivars against six photoperiods between 12 and 15 hours of light. The study only tracked the early flowering phase (three to four weeks, roughly one third of the commercial ripening time).

The picture is heterogeneous, as expected:

• Four cultivars (study codes BD, BT, IM, GJ) had their optimum between 12.4 and 12.7 hours, close to the 12-hour standard but slightly longer.
• Four cultivars (GT, LL, GG, OG) showed no delay at all in flowering onset up to 14 hours of light.
• Two cultivars (CC, PD) showed a linear delay, but only 1.3 and 2.3 days respectively at 13 h.
• Three cultivars even initiated flowering under 15 h, but the flower tissue did not develop further, so no practical yield advantage there.

The bottom line for practice: photoperiods between 12 and 14 hours are worth testing. Beyond 14 hours, results get unstable. The optima apparently sit between 12.5 and 13 h for the early phase and can go up to 14 h if you wait for commercial maturity.

What the studies do not settle yet

Four points that remain open and should be considered before recommending “13 instead of 12”:

1. Maturity drift at 13 h. In the Ahrens 2024 study, the 13-hour group was harvested when the 12-hour group was ripe. Clues such as slightly elevated CBGA concentrations suggest the 13-hour plants may not have reached their ripeness peak yet. If you run 13 h, decide harvest time by maturity indicators (stigma browning, trichome ambering), not by day count.

2. Interaction with PPFD. All studies worked with moderate PPFD values (360 to 700 µmol m⁻² s⁻¹). At very high values (>900 µmol) the effects are unclear. Plausible: under very bright light, 12 h of DLI could already be saturating, so extra hours would add less.

3. Cultivar library. Across the three studies, around 15 cultivars have been tested in total. That is far more than two years ago, but the pool of cannabis genetics is orders of magnitude broader. Generalizations to all cultivars should be treated with caution.

4. Energy balance. More hours of light = more electricity. The 35% yield increase at 13 h has to be weighed against roughly 8% higher lighting costs. At current electricity prices that usually pays off, but hard ROI data on photoperiod changes is still missing.

If you take away only one thing as a grower

Try 13 instead of 12 hours, on one batch, on one cultivar. It is the cheapest, fastest and best-supported test you can run in the lighting department. Hardware cost: zero. Operational change: extend the timer by one hour. In the studies, the extra yield for THC cultivars was in the range of 30 to 50%, with no measured drop in THC concentration, in some cases even a slight increase; only your own test will show whether your cultivar follows suit.

Is this a silver bullet? No. Some cultivars gain little, some lose potency under a longer photoperiod, some benefit above average. But the risk of a single test run is minimal, and the knowledge of what your genetics need stays with you.

Practical takeaways

• Default test: 13L:11D on one THC cultivar; measure yield, judge ripeness visually, not by day count.
• Keep a cultivar map: which cultivar gains at which photoperiod? That is an asset that grows with every batch.
• Check CBD cultivars specifically: Cannatonic-like lines can benefit dramatically from a long photoperiod (+50 to +100% total CBD).
• Decouple harvest timing: if you change photoperiods, anchor the harvest to trichome ambering and stigma browning, not to “day 60”.
• What stays open: interaction with high PPFD, energy costs over cycles, generalization across a much larger pool of genetics.

Sources

1. Peterswald et al. (2023), Plants 12, 1061. doi:10.3390/plants12051061
2. Ahrens, Llewellyn, Zheng (2023), Plants 12, 2605. doi:10.3390/plants12142605
3. Ahrens, Llewellyn, Zheng (2024), Plants 13, 433. doi:10.3390/plants13030433
4. Saloner & Bernstein (2020), Front. Plant Sci. 11, 572293. doi:10.3389/fpls.2020.572293