Light Sensitivity of Cannabis During Dark Cycles: How to Protect Your Plants
Cannabis plants are highly sensitive to light interruptions during their dark cycles in the flowering stage. During the flowering phase, cannabis typically requires 12 hours of continuous darkness daily. Even brief exposure to light during this period can disrupt the plant's hormonal balance and negatively affect growth.
How Light Affects the Dark Cycle
Cannabis responds to the dark cycle by producing a hormone called phytochrome, which triggers flowering. When the dark period is interrupted by light, the plant becomes confused, which can lead to:
- Delayed Flowering: Even short exposures to light during the dark cycle can delay the flowering process or cause the plant to revert to vegetative growth.
- Hermaphroditism: Plants may become hermaphroditic, developing both male and female reproductive organs, leading to unwanted seed production instead of potent buds.
How Much Light Causes Harm?
Even small amounts of light, such as a light leak through a zip or an LED indicator, can negatively impact cannabis plants during their dark period. Prolonged or frequent light leaks are more harmful, potentially leading to more severe problems.
How can you tell if a light source will cause problems in the grow room?
The light intensity levels that effect the cannabis dark cycle are very low.
The measure of light intensity used by plants is PAR (Photosynthetically active radiation) in units µmols/m²/second. As a reference point the PAR intensity most growers would use for flowering would be about 900 µmols/m²/second.
Recent studies of dark cycle interruption in cannabis measured the PAR threshold for light pollution at 0.01 µmols/m²/second. That is 90,000 times less light than lights on during flowering!
For outdoor growers the PAR intensity on a full moon in a clear sky is 0.0016 µmols/m²/second. Light intensity would have to be five times greater than moonlight to affect the plants light cycle.
So how can we measure or determine if a very low level of light is causing our plants problems during light cycle? You can use a specialised meter to detect very low PAR levels but they are very expensive.
Ways to assess if there is light pollution in your grow room
Enter the grow area with lights off and allow your eyes to adjust to the dark for a few minutes. If you can read a book with small print at a normal distance from your face the light levels are probably too high.
However, for humans low light visibility in red light is lower than relative visibility in white or green light. Therefore if the light source is a red LED indicator light this method may not be reliable.
If you have a single LED indicator light in the grow room it should not emit enough PAR to interrupt the light cycle but you can cover them in tape to remove the risk.
In power terms a light source of only 0.02 watts could cause light pollution in a small grow room is shining directly on the plants. A typical indicator LED is about this wattage so they must be covered up.
Can I use a green light in the grow room at night?
Humans have good sensitivity to green light in low light situations. This means we require less green light to see in low light compared to blue or red light. However plants are also sensitive to green light and too much for too long can interrupt the plant night cycle.
How to prevent light pollution in the grow room
- Seal the Grow Space: Ensure your grow room or tent is completely lightproof. Block any gaps where light might enter.
- Monitor Equipment: Cover LED lights on equipment with tape to prevent light leaks.
- Use reliable timers: Use high quality timers to ensure light cycles are maintained accurately and reliably.
- Check grow lights go fully off: Some grow lights can use a very small amount of power and the LEDs may glow, even with the timer switch off. To check for this enter the grow area with lights off and allow your eyes to adjust to the dark for a few minutes and inspect the LEDs
By eliminating light leaks, growers can ensure their plants thrive during flowering, leading to healthier, more productive harvests.
