As far as we know, the cultivation of cannabis requires a lot of light, but did you know that? Darkness is also very important to cannabis. Enough darkness will make them happier, and harnessing the power of the dark cycle is the key to successful enrichment and healthy cannabis growth.
First let's learn what happens to plants in the dark.
The biggest change in plant metabolism in the dark is that they stop photosynthesis, but they still need to respire like animals do. They take in oxygen, grow using their nutrients, and release carbon dioxide.
So plants never go dormant when it gets dark, in fact, this is usually when they grow more as they store up their energy throughout the day and get their metabolism ready. Therefore, if the environment is not completely dark at this time or the brightness of the light is enough to affect the growth of cannabis, the cannabis will not grow normally and healthily, and may wither or even "degenerate" and destroy the entire plantation area.
The biggest influence of light and darkness on plants is actually the combination between them. Plants cannot do without light and cannot do without darkness.
Many activities in plant metabolism (growth, flowering, fruiting, sowing) are regulated by day length, and these phases are triggered by longer or shorter light. Not the absolute number of hours, but the varying amount of light over time that the plant is looking for.
It's called the photoperiod, and it's the primary mechanism plants use to align with the seasons. It's not temperature or rainfall, but how the amount of light changes over time.
Most importantly, during the photoperiod, cannabis plants decide how they should grow based on the amount of light they detect. In the wild, they tend to sprout in spring and early summer, spending long summers in strong sunlight.
When the days start to get shorter and there are less than 14 hours of light, the flowering phase is triggered—the females flower and the males produce pollen sacs. If these photoperiod rhythms are disrupted, it is possible to keep the cannabis plant in a permanent vegetative stage, reversing it from the flowering stage back to a vegetable, inducing early flowering, or killing it. In general, irregular photoperiods are ineffective or even harmful.
Therefore, the light time required in different growth stages of cannabis is also different.
Below I will list the most commonly used light and dark time adjustments for cannabis in various growth stages.
Plant Growth Stage
Most indoor growers use an 18/6 light cycle during this period. That's 18 hours of light and 6 hours of darkness. This tells the plants to stay in the vegetative stage and provides them with enough light to photosynthesize and generate the energy they need to grow.
A few indoor growers will use a 6/2 light schedule during the vegetative phase. This means that plants go through three periods of light and darkness each day. The stated benefit is that it prevents light stress and allows your grow room/tent to cool down. More controversially, some claim that this affects plant rest, and it's unclear if there's a benefit to be gained from it.
I would prefer to use methods that imitate the laws of nature as much as possible.
To force cannabis plants into flowering, the light schedule must be shifted to 12 hours of light and 12 hours of dark (12/12). This reflects the shortening of days in the fall and tells plants that winter is coming and it's time to stop breeding before the cold arrives.
At this stage, it is very important to prevent light leakage into the growth chamber during the dark cycle. If plants feel longer days, they may revert to the vegetative stage, which can seriously affect the quality of the final crop.
Dark Period Before Harvest
Many growers will keep their plants completely dark for two days or so before harvesting. While it's not entirely clear if this is true, it appears that this darkness causes the final burst of cannabinoids and terpenes in the plant. The Stichting Institute for Medical cannabis (SIMM) once found through experiments that placing mature plants in the dark before harvesting improved their harvest quality. Growers at SIMM split the crop of mature plants into 2 different environments, harvesting half of the cannabis and keeping the other half in absolute darkness for 72 hours before cutting and drying. A final analysis of the resulting cannabis flower showed that the flower kept in the dark for 72 hours increased THC by up to 30%, while CBD and CBN remained unchanged.
Notice! Be Sure To Avoid Light Leaks
For photoperiod plants, it is very important to ensure that their dark cycle is really dark. And "darkness" that is not dark enough is not acceptable. For them, it needs to be as dark as night. Light pollution from street lights, lights in your home, or any other light source is enough to make them think it's still daytime.
According to a research course by Dr. Bruce Bugbee, director of the Crop Physiology Laboratory in the Department of Plant, Soil, and Climate at Utah State University,
This area with light pollution below 10nmol/㎡/s is the area we consider safe for cannabis. But if you see something strange happening to a plant in a corner, that doesn't necessarily mean it's light pollution. It could be the temperature, or it could be the effect of something else. Also, when nights are cooler, plants seem to be more sensitive to light pollution.
If you're growing in a grow tent, it should block all outside light anyway. If it has a little crack, a little simple tape will be enough to seal the crack. If the crack is very large, it may be time to invest in a new tent.
If growing in a greenhouse, cover the top with a tarp to keep the plants in the dark.
If grown outdoors, cannabis is primarily affected by the season. However, you can use some light deprivation techniques such as placing portable structures over/around plants to cast them into the dark. But these costs are very high, which is a difficult problem for ordinary growers. A small amount of outdoor potted planting is fine and can be brought directly indoors for dark periods, but if there is a large amount of planting, this cannot be done.
Can Cannabis Survive Artificial Light?
You need to trust that artificial light can also provide very good results for the light levels nature gives you.
Any light bulb labeled "Full Spectrum" is a good addition to keeping your plants bright, and it's up to you to decide if you want to use incandescent, fluorescent, LED, or halogen bulbs. Some are on the pricier side, but they last longer and may be cheaper in the long run.
If you're concerned about electricity consumption, consider the SK series of modern LED lights from Baylabs. They use the least amount of electricity to get very good results. In the plant light market, many LED plant light manufacturers have not studied and understood the dark demand of cannabis, so many plant lights will still emit residual light after they are turned off, the light pollution is far greater than 10nmol/㎡/s. The growth of cannabis has a very bad effect, and it may lead to genetic mutations of cannabis! The Baylabs plant light has been completely improved for this demand. The Baylabs driver can completely control the light amount of the LED, so that no residual light is left after the plant light is turned off, so that cannabis can grow and develop in a completely dark environment. .
All in all, cannabis plants need darkness just as much as they need light. Darkness can tell plants when to bloom, when seeds will open, and protects roots from pests.
It's important to get the right lighting controls in your cannabis cultivation so you can have more healthy, productive harvests from it.
1. Bruce Bugbee. (2022). How dark is dark enough?.
2. Max Sargent. (2021). Why Is Darkness Important for Cannabis Plants?.
3. Lisa. (2022).Do Plants Need Darkness to Grow?.