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Cannabis, like any other plant, relies on light to grow and mature. To get the most out of your plants, keeping control of the light cycle is important. But do you know that when it comes to the health of your plants, the dark cycle is just as important, if not more so?

The light cycle of cannabis

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Growing cannabis can be easy. All it takes is dirt, water, and sunshine. When the plant is first growing, it is in the vegetative stage. This is the stage where it is simply getting bigger, and the bigger it is, the more it will eventually produce. To keep it in the vegetative stage, it has to have at least 13 hours of light. Most indoor growers want to mimic the conditions of long summer days, where it gets the most light.

To keep it in the vegetative stage, it has to have at least 13 hours of light. Most indoor growers want to mimic the conditions of long summer days, where it gets the most light.

Setting your lights to an 18-hour cycle will give them plenty of light to grow fast and strong. Some growers will even go for constant light, keeping them lit for 24-hours-a-day. During the flowering stage, where our beautiful buds are produced, the light cycle is reduced to a 12-on/12-off in order to mimic fall, where the days grow shorter, and the plants grow female flowers to attract pollen and produce seeds. But while light is vital for growth, it is the uninterrupted periods of darkness that really tell the plant what to do.

But while light is vital for growth, it is the uninterrupted periods of darkness that really tell the plant what to do.

Why the dark is just as vital

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It is natural in the great outdoors for clouds to cover the sky and rainy days to limit light at times. Similarly, when growing cannabis indoors, it is okay to turn your lights off for short periods of time to do maintenance on your grow space, replace bulbs, etc. But ask any veteran grower, and they will tell you that you never want light to leak into the grow space while the plants are sleeping. Why? A host of reasons.

But ask any veteran grower, and they will tell you that you never want light to leak into the grow space while the plants are sleeping. Why? A host of reasons.

First, it is the length of darkness that a plant is exposed to that signals its system on which stage of life it should be in. Longer nights mean it is time to grow buds. Short nights mean it is time to just grow mass. Like car headlights in your bedroom window, letting light in disturbs this cycle, and messes with their biological clock.

A strong enough exposure during their night cycle can trigger them to stop growing buds and revert back to vegetative growth.

The second reason is even more vital. Just like you or me, when our sleep gets interrupted, we get cranky. For plants, the stress can cause them to react far worse than we do. They can spontaneously become hermaphrodites, and even one hermie in your grow could ruin all your hard work by pollinating the females and making the whole crop go to seed.

The science behind the darkness

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Cannabis is a “short day/long night” plant, meaning it needs those conditions to bloom. The system cannabis uses to detect light changes is a group of receptors in the pigments of their leaves. There are 2 different receptors, each with a different task: Phytochrome Red and Phytochrome Far Red. They absorb light in different wavelengths and transmit that information to the plant.

Far-Red can be manipulated through the amount of light given to the plant. This receptor keeps the plant in the vegetative stage. Light in the far-red spectrum will signal this receptor to pass the chemical signal to veg. During times of light, both receptors are balanced in number, but in darkness, Far-Red receptors slowly change into Red receptors.

With longer dark periods, the number of Far-Red receptors reduces until there aren’t enough to counter the signal from the Red receptors, which tell the plant to flower.

Want to speed up your switch from veg to flower? Give plants an uninterrupted 24-36 hours of darkness before going to a 12-12 cycle. It will make more of the Far-Reds change into Reds, giving a more powerful signal to your plant that it is time to bloom. Normally, the transition can take a couple of weeks to be seen on your plants. By giving them a really long night, you can speed this up dramatically.

The power of the dark side

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The way in which plants can go “wonky” and revert to vegetative growth is that just because a Far-Red receptor begins to work as a Red one, doesn’t mean that it can’t revert back. It takes several hours of continual darkness to change Far-Reds to Reds, but only a brief moment of bright light to change them back.

Outdoors, normal moonlight and starlight won’t cause this to occur, but a grow in an area with overhead street lamps might experience this problem.

It can, and does also happen to growers who have night-time motion sensor flood lights aimed over their crops. They may scare off burglars or pesky critters, but they can also do just as much damage to your crop as the varmints you are trying to stop.

Bud growth can stop, leaves can begin to grow deformed when plants “re-veg”, and your plants may take a month or more to get back on schedule. Patio growers most often see this. Even the lights coming from inside the house can potentially cause it to happen.

Tips to prevent darkness interruption

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Commercial growers in greenhouses use light-proof covers for their grows to manipulate light cycles in order to make plants flower when they want them to, regardless of the season. For indoor growers, light control is as simple as a preset timer, and making sure there are no light leaks in your grow area.

Make sure doors have insulation around the edges, or that ventilation holes don’t let in light with the air. A vent cover, like what you see for dryer exhaust vents, is great for this. Often, ducting bends and the mass of the fan will take care of this for you. Ensure your grow will not be disturbed, and that your light times are written down, so you don’t accidentally go in at the wrong hour.

Also, make sure that any equipment, such as humidifiers or heaters, don’t have lights one them. If they do, cover them with black tape. Some growers will even use night vision goggles to tend their plants at night. Talk about love!

Have you ever had plants re-veg on you, or become hermaphrodites? How was your crop affected? How long did it take to recover? Tell us your experiences on social media or in the comments section below.

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