Lighting is the most-researched part of indoor cannabis growing, and also the most heavily marketed. The research keeps pointing to three things: how much light the canopy gets, how efficiently that light is produced, and how long the lights stay on in flower.
Intensity: in a University of Guelph study, flower yield kept rising in a straight line up to 1,800 µmol·m⁻²·s⁻¹ at the canopy, while potency stayed the same. Spectrum: in CBD-type medical hemp at Utah State, raising the share of blue light from 4% to 20% lowered yield by about 12% without raising THC or CBD. An Italian comparison found HPS-grown plants produced more flower than two LED spectra, which mainly changed plant shape and the cannabinoid mix. Supplemental UV-B didn’t improve yield or potency.
Photoperiod: the 12/12 rule is being tested. Ten drug-type cultivars all started flowering under days as long as 14 hours, and in a full-cycle study two high-THC cultivars produced at least 35% more flower under 13-hour days than under 12. An Australian study found 50–100% more CBD in one variety when flowering began at 14 hours of light.
Caveats: most of these studies used one to three cultivars, and photoperiod responses differ between genetics. If you experiment with longer flowering days, try it on a single plant first.
Key terms
PPFD
Photosynthetic photon flux density: how much usable light reaches a surface each second, measured in µmol·m⁻²·s⁻¹. Usually measured at the top of the canopy.
DLI
Daily light integral: the total usable light a plant receives over a whole day, in mol·m⁻²·d⁻¹. It combines light intensity and hours of light.
Photoperiod
The number of hours of light per day. Photoperiod cannabis is usually switched to 12 hours of light to trigger flowering.
Studies
9 studies
Longer Photoperiod Substantially Increases Indoor-Grown Cannabis’ Yield and Quality: A Study of Two High-THC Cultivars Grown under 12 h vs. 13 h Days
Plants · 2024
Drug-type cannabis
Controlled experiment
Open access
13-hour days gave at least 35% more flower than 12-hour days in both cultivars.
Authors: Ashleigh Ahrens, David Llewellyn, Youbin Zheng
13(3):433
Study plant: Two high-THC cultivars grown indoors to commercial maturity
Building on their earlier screening work, the researchers grew two high-THC cultivars to commercial maturity under either 12-hour or 13-hour days. The 13-hour plants produced at least 35% more total flower, far more than the roughly 8% extra daily light would explain, with cannabinoid concentrations unchanged or higher. One cultivar started flowering about 1.5 days later under 13 hours.
Moving Away from 12:12; the Effect of Different Photoperiods on Biomass Yield and Cannabinoids in Medicinal Cannabis
Plants · 2023
Mixed / review
Controlled experiment
Open access
One variety produced 50–100% more CBD when flowering started at 14 hours of light instead of 12.
Authors: Tyson James Peterswald, Jos Cornelis Mieog, Razlin Azman Halimi, Nelson Joel Magner, Amy Trebilco, Tobias Kretzschmar, Sarah Jane Purdy
12(5):1061
Study plant: Three medicinal cannabis varieties, including the CBD variety Cannatonic
Three medicinal cannabis varieties were flowered under nine different photoperiod schedules, including longer days (such as 14 hours light, 10 dark) and gradually changing day lengths, instead of the standard 12/12. In the CBD variety Cannatonic, every schedule that started flowering at 14/10 raised CBD concentration and increased total CBD yield by 50–100%. The authors conclude that 12/12 is not optimal for every line.
Is Twelve Hours Really the Optimum Photoperiod for Promoting Flowering in Indoor-Grown Cultivars of Cannabis sativa?
Plants · 2023
Drug-type cannabis
Controlled experiment
Open access
All ten cultivars started flowering under days as long as 14 hours.
Authors: Ashleigh Ahrens, David Llewellyn, Youbin Zheng
12(14):2605
Study plant: Clones of ten drug-type cultivars in a growth chamber for 3–4 weeks
Clones of ten drug-type cultivars were grown for three to four weeks under six photoperiods to test whether days longer than 12 hours still trigger flowering. All ten cultivars began flowering under every photoperiod up to 14 hours. For some cultivars, early flowering measures peaked somewhere between 12 and 13 hours of light.
Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment
Frontiers in Plant Science · 2021
Drug-type cannabis
Controlled experiment
Open access
Yield rose steadily with light up to 1,800 µmol·m⁻²·s⁻¹, and potency didn’t change.
Authors: Victoria Rodriguez-Morrison, David Llewellyn, Youbin Zheng
12:646020
Study plant: Drug-type cannabis flowered indoors under sole-source LED lighting
Plants were flowered under a wide range of canopy light levels to see how intensity affects yield, potency and leaf photosynthesis. Dry flower yield kept rising in a straight line all the way up to 1,800 µmol·m⁻²·s⁻¹, even though individual leaves stopped gaining photosynthesis well below that level. Light level had no effect on cannabinoid potency and only minor effects on terpenes. The authors conclude that leaf-level measurements are a poor guide to whole-plant yield.
Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids
PLOS ONE · 2021
Hemp
Controlled experiment
Open access
More blue light cost about 12% of yield and didn’t raise potency.
Authors: F. Mitchell Westmoreland, Paul Kusuma, Bruce Bugbee
16(3):e0248988
Study plant: CBD-type medical hemp in growth chambers, three separate studies
Across three separate growth-chamber studies, researchers changed only the share of blue light (from 4% under HPS up to 20% under LEDs) and measured flower yield and cannabinoids. Yield dropped about 12% in a straight line as blue increased from 4% to 20%. Blue fraction had no significant effect on THC or CBD concentration. The authors argue that a fixture’s efficiency (light produced per watt) matters more than its color mix.
Cannabis Inflorescence Yield and Cannabinoid Concentration Are Not Increased With Exposure to Short-Wavelength Ultraviolet-B Radiation
Frontiers in Plant Science · 2021
Drug-type cannabis
Controlled experiment
Open access
Supplemental UV-B didn’t raise THC, other cannabinoids or yield.
Authors: Victoria Rodriguez-Morrison, David Llewellyn, Youbin Zheng
12:725078
Study plant: Two indoor drug-type cultivars (‘Low Tide’ and ‘Breaking Wave’)
Two indoor cultivars received daily supplemental UV from 287 nm LEDs at a range of doses throughout flowering. Total yield fell in one cultivar as the UV dose increased, and the share of yield coming from the top buds dropped in both. UV did not deliver any commercially meaningful increase in yield, cannabinoids or terpenes.
Study plant: 25-day-old hemp seedlings in an aeroponic system, 20 days of light treatments
Young hemp plants were grown aeroponically under ten LED spectra plus natural light for 20 days, and researchers tracked cannabinoids alongside markers of plant stress. A red-blue-green mix (R7:B2:G1) gave the highest CBD with relatively low stress. THCA behaved most like a stress marker, while CBD and THC showed little link to light-induced stress.
Study plant: Cannabis clones grown under one HPS and two LED spectra
Clones were grown under high-pressure sodium (HPS) and two LED spectra to compare plant shape and cannabinoid content. Plants under HPS grew taller and produced more flower dry weight than under either LED. The light sources shifted the mix of cannabinoids, with the LED richest in blue and UV-A giving the highest CBG, but total cannabinoid yield per plant was similar across all three.
Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions
Physiology and Molecular Biology of Plants · 2008
Drug-type cannabis
Lab study
Open access
Leaf photosynthesis was strongest at moderate temperatures and fell off above about 30 °C (86 °F).
Authors: Suman Chandra, Hemant Lata, Ikhlas A. Khan, Mahmoud A. ElSohly
14(4):299–306
Study plant: Leaves of a high-THC Mexican variety, measured in a leaf chamber
Using a leaf chamber, researchers measured photosynthesis and water-use efficiency in a high-THC variety across light levels up to 2,000 µmol·m⁻²·s⁻¹, temperatures from 20 °C to 40 °C, and CO₂ from 250 to 750 ppm. Photosynthesis and water-use efficiency rose with light at the cooler temperatures (20–25 °C), and temperatures above about 30 °C reduced photosynthesis. It remains a widely cited baseline for cannabis environment targets.