Light Leak
Definition: A light leak is any unwanted light that reaches cannabis plants during their dark period, such as gaps in a tent zipper, glowing equipment indicators, or a hallway lamp. For photoperiod plants in flower, repeated interruptions of the 12-hour night can delay blooming, cause regrowth, or contribute to hermaphrodite traits.
How Light Leak works
Photoperiod cannabis uses light-sensitive pigments to measure how long each night lasts. A long, unbroken night tells the plant to keep flowering. Light during that window can interrupt the signal. One brief accident rarely ruins a crop; the real danger is a leak that hits the plants every single night for weeks.
Common sources
- Zipper gaps and corners on tent doors.
- Duct and cord ports that aren’t cinched tight.
- Pinholes in the fabric of older tents.
- Status LEDs on power strips, timers, controllers, and humidifiers.
- Room lights, street lights, or porch lights for greenhouse and window grows.
How to find leaks
Two checks work well. First, with the grow lights off, sit inside the closed tent in a lit room and let your eyes adjust for 5 to 10 minutes; any pinpoint of light is a leak. Second, at night with the room dark, turn the grow light on and walk around the outside looking for glow.
Fixes
Cover pinholes and indicator lights with black electrical tape or foil tape, fold a towel over the zipper bottom, and use light traps on passive intake vents. If you must check plants during the dark period, use a dim green work light briefly rather than room lights.
Autoflowers don’t rely on night length to bloom, so leaks matter less for them, though steady darkness still benefits plant rhythm. With photoperiods, chronic leaks are a known stress that can push some genetics toward becoming a hermaphrodite.