Light Leaks

A flowering cannabis plant measures the length of its dark period to decide what to do. Light during that period tells it the season has reversed, and it responds by reverting toward vegetative growth or by producing pollen to save itself.

What you are seeing#

The classic sign is revegetation. Out of a bud that was forming normally, you get smooth-edged single-blade or three-blade leaves emerging on thin stems, growing in odd directions. Growers call these whiskers. They look wrong because they are vegetative growth coming out of a flowering structure.

Buds stop filling. They stay loose and airy, new pistils keep appearing without the bud gaining weight, and growth goes slow and confused. Some plants foxtail, pushing new spires of calyxes out of the top of an existing bud. Stretching returns too, with a plant that had settled into flowering extending internodes again.

The other outcome is worse and has its own page: yellow banana-shaped anthers appearing in the buds, a stress hermaphrodite response that will seed your entire tent.

Why it happens#

Something is on during the dark period. The usual suspects, roughly in order of how often they cause it: tent zippers and corner seams that no longer close fully, the green LED indicator on an inline fan, timer, power strip or humidifier inside the tent, a room light on its own timer, a streetlight through a window, someone opening the tent during lights off, and a failed timer briefly powering the light.

Growers also do this to themselves by running 12/12 in a closet with a door gap, or by putting the tent in a room they walk through at night.

How to fix it#

1. Sit in the tent during the dark period. Zip yourself in, wait 5 full minutes for your eyes to adjust, then look around. Anything you can see is a leak. This costs nothing and finds the problem almost every time.
2. Tape the seams and zippers with gaffer or aluminum tape. Do not rely on the tent's velcro flaps, and put a square of electrical tape over every indicator LED.
3. Fix the room, not just the tent. Blackout curtains on any window, and no room light on a timer during your dark period.
4. Do not open the tent during lights off. Not to check, not for a second. If you must work in there, do it during lights on, or use a dim green headlamp and keep it brief.
5. Check the timer. Plug a lamp into it for a day and confirm it switches cleanly. Cheap mechanical timers fail.
6. Inspect every plant for banana-shaped anthers over the next 7 days.
7. Leave the reveg growth alone, or remove just the worst whiskers. Cutting into a bud to get them out does more harm than they do.

How to tell it apart#

Versus hermaphrodite plants. These overlap, because a light leak is one of the main causes of stress hermaphroditism. The distinction is what you find. Odd single-blade leaves and airy buds with no pollen structures means revegetation only, which is a yield problem. Yellow banana anthers or green pollen sacs in the calyxes means the plant has gone hermaphroditic, which is more urgent.

Versus light burn. Light burn bleaches the topmost buds white or pale yellow on the side facing the lamp, and it comes from too much light during the light period, not light during the dark. Light burn is strictly on the tops. Leak damage shows as growth behavior across the whole plant.

Versus heat stress. Heat curls leaf edges upward on the tops and comes with a canopy reading above 85F. It does not produce single-blade leaves out of buds. Check the thermometer before you tear the tent apart.

How long until it looks better#

Once the dark period is genuinely dark, the plant settles back into flowering within 7 to 14 days and starts putting weight on buds again.

Reveg leaves and whiskers do not go away. They stay in the bud through harvest and you trim around them. Foxtailed tops stay foxtailed. Judge the fix by whether new whiskers stop appearing and whether buds start swelling again over two weeks.

Be realistic about the cost. A leak caught in week 3 costs you a little bud density and extra trimming. A leak running through week 7 can add two or three weeks to flowering time and leave you with loose, airy flower. It does not ruin a crop on its own, but it is entirely preventable and there is no reason to have it twice.