Grow basics · 8 minute read · Updated October 15, 2026

Light leaks and light schedules: protect the dark period

Short answer. Photoperiod cannabis flowers because of a long, unbroken dark period, so the standard flower schedule is 12 hours on and 12 hours of total darkness. Hemp researchers reported that stray light as low as about 2 µmol/m²/s, roughly twilight level, disrupted flowering of one cultivar in their greenhouse; that is less than 1 percent of the light a typical flowering canopy gets. Find leaks by sitting inside the closed tent with its lights off and the room lit, then seal every pinhole, zipper gap and glowing LED you see.

Your light is on a timer, so the schedule is handled. Or is it? The plant does not count the hours of light. It counts the dark, and it notices things you stopped noticing months ago.

Why the dark period, not the light, starts flowering

Photoperiod cannabis is a short-day plant. In plain words: it starts to flower when nights get long enough. Inside the leaf, a light-sensing pigment called phytochrome keeps track of how long it has been dark. A pulse of light in the middle of the night resets that clock, and the plant reads the night as short.

That is why growers flip to 12 hours on, 12 hours off to start flower. It is also why a few minutes of light at 2 a.m. can matter more than an extra hour of light at noon.

Growers have even used this on purpose. In a 2023 trial in Colombia, posted as a preprint and not yet peer reviewed, short night-time light pulses kept clones from showing most of the flowering signs that plants on a plain short day showed. A night break is a tool for keeping mothers in veg. In a flower tent it is an accident.

So ask yourself: when did you last look at your tent at night, with your own eyes, after the lights went out?

How much stray light is too much?

Far less than most people guess. Here is what the research measured.

Light during the dark period: what studies found
Study	Plants	Finding
Zhang et al., 2021	One hemp cultivar (a greenhouse observation, not a controlled test)	Stray light as low as about 2 µmol/m²/s, roughly twilight level, disrupted flowering
Ahrens, Llewellyn and Zheng, 2023 (citing earlier work)	Cannabis (citing Zhang 2021 on hemp, and a web resource)	Delayed or blocked flowering reported at stray light of 2 µmol/m²/s or more; one cultivar delayed at 0.1 µmol/m²/s or less at leaf level (reported in a web resource, not a peer-reviewed paper)
Kusuma et al., 2021	Cannabis and soybean	Near-infrared LEDs, which people can barely see, delayed cannabis flowering by about 12 days at the highest dose

For scale, a flowering canopy under a good LED usually gets several hundred µmol/m²/s. Two µmol is well under 1 percent of that. In their own growth chamber, the 2023 researchers kept stray light below 0.05 µmol/m²/s to be safe.

The near-infrared result matters for one more reason: "I can barely see it" does not mean "the plant cannot see it".

Do light leaks cause hermaphrodites?

You will read this in almost every grow book, and the books we use say it too. It is a reasonable worry, but be clear about what is proven.

Proven: stray light during the dark period can delay or disrupt flowering, as shown above.
Reported, not proven: that light leaks cause intersex flowers. A 2020 study of hermaphroditism lists long darkness, short photoperiods and cool temperatures among reported triggers, and says the causes have not been studied much.

Either way, the fix is the same, and it is cheap. If you are already seeing male parts on a female, read hermaphrodite cannabis and nanners next.

The 10 minute light leak test

Do this at the start of every flower and any time you add new gear.

Wait until the tent lights are off and the room around the tent is bright. Daytime with the blinds open is ideal. If your lights are off at night, turn the room lights on.
Get inside the tent, or put your head in and zip it closed around your shoulders. Close your eyes for a minute or two and let them adjust.
Look slowly at every seam, corner and zipper. Then the duct ports, cord ports and vent flaps. Then the floor tray and the roof.
Now look down: power strips, timers, fan controllers, humidifiers and sensor screens all glow.
Mark every point of light with a piece of tape so you can find it again with the lights on.

Then flip the test. With the tent lights on and the room dark, walk around the outside. Any light that gets out is a hole that also lets light in.

Common leaks and how to fix them

Where light gets in, and the fix
Leak	Why it happens	Fix
Zipper ends and corners	The zipper never seals fully where it turns a corner	Fold a strip of black cloth over the corner, or tape a small flap on the outside
Pinholes in the fabric	Wear, stitching, old tents	Black duct tape or foil tape on the outside
Duct and cord ports	Drawstring sleeves never close tight around a cord	Cinch tight, then wrap with tape or a sleeve of black cloth
Passive intake vents	Mesh windows let air and light in	Use a light trap or a duct with a bend, or switch to active intake
Indicator LEDs on gear	Power strips, controllers and timers glow all night	Cover with black electrical tape, or move the gear outside the tent
Phone and flashlight checks	You open the tent at night to look at something	Plan checks during lights on; see the work light rule below

If you are weighing vent setups, our post on passive vs active intake covers how each one handles light.

Light schedules that work

Common indoor light schedules
Stage	Common schedule	Notes
Photoperiod veg	18 on / 6 off	Long days keep plants vegetative; some growers run longer days
Photoperiod flower	12 on / 12 off	The industry standard; the dark must be unbroken
Autoflower, whole grow	18 on / 6 off is common	Autos flower by age, not night length, so leaks are a smaller trigger
Mother plants	18 on / 6 off	Keep them out of the flower tent's schedule

Is 12 hours magic? Not exactly. A 2023 University of Guelph study grew ten drug-type cultivars at 12 to 15 hours of light. All ten started flowering at up to 14 hours, with delays of zero to about four days compared with 12 hours, depending on the cultivar. At 15 hours, three cultivars started flowers that did not go on to develop. The authors advise growers to test their own cultivars before moving away from 12 hours.

The lesson for a home tent: the length of the day has some slack. A broken night has much less. Spend your effort on the dark, not on fine-tuning the day.

Want to understand how the two schedules shape a whole grow? Read autoflower vs photoperiod and the stage-by-stage plan in cannabis grow week by week.

Timers, power cuts and the work light rule

A leaky tent is not the only way to break a night. The timer can do it too, and it fails quietly.

Check the timer after any power cut. Some digital timers keep time on a battery; many cheap ones lose their place. A mechanical timer stops while the power is off and runs late after.
Check clock changes. If your timer adjusts for daylight saving time, your schedule shifts an hour twice a year. That alone will not ruin a grow, but you should know it happened.
Keep the schedule fixed in flower. Pick a lights-on time and leave it. If you must move it, move it once, by making one night longer, never shorter.
Write down the on and off times. A note in your journal turns "I think it was fine" into a fact you can check. A temperature sensor that logs through the night also shows when lights came on, because the tent warms up.

The work light rule: avoid opening the tent in the dark at all. Many growers use a green headlamp, based on the idea that plants ignore green. We found no cannabis study showing a green work light is harmless, and a 2026 Utah State dissertation on light during the dark period reports that intensity, color, timing and total amount of light all affect the flowering response. If you must go in, keep it dim, brief and pointed away from the buds.

How to tell if a leak has already cost you

Plants that read their nights as short tend to show it within the first few weeks of flower.

Flowers start late, or start and then stall.
New growth at the tops looks leafy again, with single-blade or rounded leaves, which is a sign of a plant going back toward veg.
Plants nearest the leak are behind plants on the far side of the tent.
Male parts appear on females more often than in past grows.

Fix the leak, keep the schedule steady, and give the plants time. Note the date you fixed it so you can see whether things change after. If you log your grow in Canna GLAP, a dated note and a photo per plant make that before-and-after easy to compare.

Questions growers ask

Can a small light leak ruin a flowering cannabis grow?

It can delay or disrupt flowering. Hemp researchers reported stray light as low as about 2 µmol/m²/s, roughly twilight, disrupting flowering of one cultivar in their greenhouse. A pinhole aimed at a leaf can matter; a glow on the floor of the room outside the tent matters less.

Is green light safe during the dark period?

It is not proven safe. We found no cannabis study that shows a green work light is harmless, and research reports that the color, intensity and timing of night light all affect flowering. Keep any light brief, dim and away from the plants.

Do autoflowers care about light leaks?

Much less. Autoflowers start flowering with age, not night length. Many growers run them on 18 hours on and 6 off for the whole grow.

What if my timer failed and the lights stayed on one night?

Reset the timer, go back to your normal schedule, and watch the plants closely for the next couple of weeks. One event is a stress, not usually a disaster. Write down the date so you can connect it to anything you see later.

Should I give plants 36 or 48 hours of darkness before harvest?

Some growers do, but we found no controlled study showing it raises potency, so we do not recommend it as a rule. Harvest when the trichomes are ready.

Sources

Education only, for adults 21+ where home cultivation is legal. Updated October 15, 2026.