For most indoor grows, the top pick is a steady 12 hours on and 12 hours off, run on the same clock every day, with LED output set to a daily light integral (DLI) target rather than a fixed dimmer percentage. It wins on five criteria: reliable flowering trigger, enough total light for dense flowers, fit with the spectrum your fixture already produces, protection of the dark period from leaks and heat, and electricity use that stays reasonable. Every option below is judged against those five.
outdoor cannabis flowering month
Option 1: steady 12/12 on a fixed clock
Cannabis is a short-day plant, so flowering starts when the uninterrupted dark period passes a critical length. A 12 hour dark block sits past that threshold for the cultivars grown indoors, and holding it steady keeps the flowering signal on one track instead of resetting it with small changes.
how long is flower stage for cannabis
Pros
- Predictable trigger and finish timing for photoperiod cultivars.
- Simple math: 12 hours at a given PPFD gives a DLI of PPFD x 0.0432, so 600 µmol/m²/s lands near 26 mol/m²/day.
- The long dark window lets the room shed heat, which keeps canopy temperature even under high-output fixtures.
- Easy to automate with a timer and a contactor, with no weekly adjustments to track.
Cons
- Roughly 12 hours of fixture runtime per day, which shows up on the power bill.
- Timer drift or a light leak during the dark block hits flowering directly.
- Long-running cultivars still need 10 to 11 weeks no matter what the timer says.
Use it when you grow photoperiod plants in a tent, closet, or sealed room and want one schedule you set once and leave alone.
cannabis flowering stage lights on off hours
Option 2: 11/13 or 11.5/12.5 (longer dark)
Cutting 30 to 60 minutes off the light period lengthens the night, which some growers use to strengthen the short-day signal and shave a few days off the finish.
outdoor cannabis flowering month
Pros
- Stronger dark signal for cultivars that stall at 12/12.
- Lower runtime and less heat dumped into the room during the light block.
- Useful in hot climates where the dark window coincides with the coolest hours.
Cons
- About 8 percent less DLI at the same PPFD: 11 hours gives a 0.0396 multiplier, so 600 µmol/m²/s is about 24 mol/m²/day.
- You need to raise PPFD, add CO2, or accept smaller yields to keep total light where it was.
- Any gain in finish time is cultivar dependent and often small.
Use it when a specific cultivar runs long at 12/12 and you have already maxed out intensity.
Option 3: 13/11 (longer light)
This schedule trades dark time for extra photons. At the same PPFD it raises DLI by about 8 percent, since 13 hours gives a 0.0468 multiplier and 600 µmol/m²/s reaches about 28 mol/m²/day.
Pros
- More total light without buying a larger fixture.
- Helps when your LED array is modest and you cannot push PPFD higher.
- Keeps the room warmer during lights-on in cold basements or garages.
Cons
- An 11 hour dark block sits close to the critical night length for some cultivars, and a short night can delay flowering or push plants back toward vegetative growth.
- It is a minority practice, not a standard, so fewer growers can help you troubleshoot it.
- Longer runtime per day cuts into the efficiency advantage LEDs are chosen for.
Use it only as a trial with a cultivar you already know, and change nothing else for the full cycle.
Option 4: diminishing schedule (12/12 down to 10/14)
Start flowering at 12/12, then add 15 minutes of dark every week or two until you reach 10/14 near harvest.
Pros
- Mimics the shortening days of late outdoor season.
- Some growers report tighter flowers and faster ripening in the final weeks.
- Small weekly steps are easy to program on a smart timer.
Cons
- Controlled-trial support is thin, so results lean on grower reports.
- Each change adds a variable if flowers stall or stress appears.
- DLI drops as hours fall: at 10/14 the multiplier is 0.036, so 600 µmol/m²/s yields about 21.6 mol/m²/day unless you raise intensity.
Use it when you run one cultivar repeatedly and want a slow, low-risk taper into harvest.
Criterion: total light (DLI) beats hours
Hours set the schedule, but total photons set the yield. Track DLI and adjust PPFD when you change the photoperiod.
Pros
- Lets you compare a 12/12 room and an 11/13 room on equal terms.
- Explains why dimming a fixture without shortening hours can cost weight.
Cons
- Requires a PAR meter or a fixture with credible PPFD mapping.
- Very high DLI without matching CO2, water, and nutrients causes light stress, not more flowers.
Criterion: spectrum, including far-red
Pros
- Broad white and warm spectrums cover the red and blue bands cannabis uses for photosynthesis.
- Small amounts of far-red can influence stem elongation and canopy light penetration.
Cons
- Heavy far-red can stretch internodes under an already tall canopy.
- Extreme single-color fixtures limit your ability to correct problems mid-cycle.
Criterion: dark period integrity and temperature
Pros
- A fully dark, sealed room protects the flowering trigger.
- Running lights overnight in summer shifts heat load to cooler hours.
Cons
- Timer lights, power strips, and controller displays can leak enough light to interrupt the night.
- LED fixtures radiate little infrared heat, so leaf surface runs close to air temperature and rooms often need to be a degree or two warmer than HPS setups.
Use-case recommendations
- First photoperiod run in a tent: steady 12/12, no experiments.
- Long-flowering sativa leaning cultivars in a hot room: 11.5/12.5 with PPFD raised to hold DLI.
- Underpowered fixture in a cold space: 13/11 as a single trial, with dark period checks.
- One cultivar, dialed in, chasing quality: diminishing schedule from 12/12 to 10/14.
- Autoflowering plants: light hours do not trigger flowering, so run 18/6 or 20/4 for the whole cycle and skip this debate.