how dark does flowering room need to be

How Dark Does Flowering Room Need to Be? Explained!

Light and Darkness are two important factors in cannabis cultivation. The plants need darkness as much as they need light.

This is applicable regardless of the growing environment and location.

If you are growing in a flowering room, you might think, “How dark does flowering room need to be?”

For starters, 12 hours of light and 12 hours of darkness are best. The room should be as dark as it gets for the dark cycle. The light should not surpass 0.3 lumens anyhow. In particular, the photoperiod strains are prone to light burns. However, in auto-flowering strains, this will not be a problem. If you are growing indoors, you can adapt the 18/6 or 6/2 plan.

That’s the summary. We will dig deep into the matter and provide you with useful insights.

Stick with us till the end to find them out.


  • It’s better for the room to be as dark as it gets.
  • Usage of grow lights is paramount if you want to maximize your yield. 

How Do Cannabis Buds Grow in the Dark?

Cannabis buds grow in the dark by secreting cannabinoids. Because of the cannabinoids, the buds stay protected and healthy. That’s why 12 hours of darkness in the flowering stage is quite crucial for cannabis.

Plants don’t let the extra energy collected throughout the day go to waste. It is stored as starch to be used during the night. Hence, plants grow at day and night alike.

The process of photosynthesis in plants in response to light is well known. But like animals, they undergo certain distinct nighttime procedures. In order to maximize yield, your cannabis plants need a sufficient amount of time in the dark.

Dark Cycle In Cannabis Cultivation
Source: royalqueenseeds

However, it is well-established that continuous lighting does not cause cannabis plants to automatically provide higher-quality harvests.

In addition, photoperiod cannabis plants adjust their development accordingly in response to the available light. They tend to emerge in the wild between spring and summer. They spend the rest of the year growing in cooler climates.

Less than about 14 hours of light each day induces the flowering stage. During this, females make flowers, and males generate pollen sacs.

How Much Dark Does Cannabis Flowering Room Need to Be?

How dark your cannabis plant needs depends on whether you’re cultivating photoperiod or auto-flowering strains. 

When it comes to blooming time, auto-flowering strains are not affected by seasons. They have ruderalis DNA and bloom at their own discretion. 

So what are the effects of ruderalis?

The effects of ruderalis are minimized largely by its low concentration of THC. What this implies is that they are not tied to any certain schedule. They are not bound to when it is light and when it is dark. 

I’ll elaborate on that later part. However, you can take a peek at the optimal level of darkness for cannabis plants-

Before going on to autoflowers, I’ll first discuss the darkness needs of photoperiod weed plants. I will focus on various stages of their development.

Photoperiod Strains:

The darkness in a flowering room needs to be adjusted dynamically for photoperiod plants. Unlike auto-flowering strains, they are susceptible to light and dark. 

Hence, you must follow a different schedule for every cultivation phase. Worry not, I have compiled them for your ease.

Here is a quick info table to sum things up for you, here is a quick info table-

Strain Type Vegetative StateFlowering StateLumen Rating
Photo Period18/6 ( indoors), 6/2 (Optional) 12/120.2-0.3 lux
Auto Flowering18/6 (Standard) 12/123.5 lux (max)

Vegetative State: 

This development occurs during the transition between the seedling and flowering phases. The seedling begins the vegetative stage shortly after it emerges from the soil. 

Right now, its primary goals are upward expansion and the formation of leaves for photosynthesis. This vegetative phase is when the buds grow the most.

During this time, you can use the 18/6 light cycle if you are an indoor grower. For example, this could be in a grow room for a 10 lb plant. I have 18 hours of daylight and 6 hours of night. This signals to the plant that it should remain in its vegetative state.

Here it may take advantage of abundant light for photosynthesis and hence continue to generate growth fuel. The darkness level will be no more than 0.3 lux.

During the vegetative period, a minority of indoor growers will adopt a 6/2 light schedule. This means that every day, plants experience three distinct phases of light and dark. 

Claimed advantages include mitigating the effects of light stress and allowing your grow space to chill down. In addition, there is the disputed idea that it provides plants with needed downtime.

Flowering State:

The cannabis plant’s photoperiod must be changed to 12 hours of light and 12 hours of darkness (12/12). This has to be done in order to trigger the blooming phase. 

This is a reflection of the shorter days of autumn. This is also a signal to the plants to reproduce before the onset of winter.

The dark cycle is crucial at this point. So, make sure no light is leaking into the growing area. You may use lightproof vents for this purpose.

You can use covers such as Magnetic Air Vent Covers or Magnetic Vent Covers 4 PCS to prevent light leaking. Or, you can try Polycarbonate TWINWALL Bronze Sheets to let your room be completely dark.

The darkness should be around 0.2-0.3 lux. Some plants may revert to a vegetative state if they detect longer days. This can have devastating effects on the quality of the harvest.

Even though it’s not required, many growers will let their plants go into total darkness for about two days before harvest. It’s a very common technique used by many veteran growers-

Darkness triggers the plant’s final burst of cannabinoid and terpene production

Whatever is going on, by this point your plant should be finished growing, so you shouldn’t worry too much.

Autoflowering Strains:

The situation is quite different with auto-flowering plants. The timing of their development is independent of external factors. Therefore, exposure to light will not force them to progress through a specific stage. 

Still, they require darkness for sleep, and each cycle’s period is arbitrary.

Image of autoflower strains
Source: royalqueenseeds

The 18/6 light cycle is the most popular one for auto-flowering plants. They get plenty of light for development and some rest, too. It is common practice for some cultivators to use a photoperiod light schedule. They may shift to 12/12 for the flowering schedule. 

However, in this scenario, it is the light that is adapting to the plant. Growing operations sometimes use 24 hours of continuous illumination instead of the standard 12 hours (12/0). But never opt for 24 hours of darkness during the flowering stage.

We advise against it because it is too costly and too risky in the form of light burns. The lumen rating will be up to 3.5 lux for the auto-flowering strains.


Do make sure which strain you are growing before jumping onto the light schedule. It will help you in the long run.

Effect of Darkness On Cannabis Plant Parts 

It’s not only the leaves that are affected by light and darkness. The other parts of the plant are also affected by it. I’ve discussed the effects for your convenience only.

Effects of darkness on cannabis plant

Cannabis Seeds: 

There is a direct correlation between the lack of light and the failure of cannabis seeds to germinate. If seeds are kept in the dark for long enough, they will think they are buried. They will begin to germinate. 

Incredibly, cannabis seeds can actually sense light. They use a pigment called phytochrome for this purpose. It transforms in response to light. 

In order for cannabis seeds to germinate, there must first be a significant amount of darkness.

Cannabis Roots:  

When tending to your roots, remember that keeping them in the dark is crucial to their continued health. Light poses a significant risk to roots because numerous organisms, including fungus, flourish in it. 

There are plenty of creatures that can cause problems for them even when it’s dark. But turning on the lights can attract even more unwelcome guests.

Setting Up the Proper Dark Flowering Room

For photoperiod plants, a truly dark cycle is crucial. The term “dark-ish” is inadequate. When they are around, they require pitch-black conditions.

A newly setup flowering room
Source: 2fast4buds

There are many sources of artificial light that can trick them into thinking it’s daytime. This includes headlights on cars, porch lights, and even the moon.

As was previously noted, light leaks can prevent blossoming and cause flowering to quit. It can have other negative consequences on your plant. 

Assuming you’re using a grow tent, you won’t be able to see the sun. It should be possible to patch any holes with duct tape. Maybe you should get a new one if it isn’t.

Put a black hydroponic sheet over the top of the greenhouse to prevent light from reaching the plants.


Try not to use low-quality duct tape to avoid light leaking.

Frequently Asked Questions (FAQs): 

How Far Should LED Lights Be During the Flowering Stage?

There is no universal rule for this duration. However, it is recommended to keep the LEDs 12-18 inches away from the plants. Light is one of the most vital factors for plant growth. Light triggers the photosynthesis process in the plant.

In What Week Of Flowering Do Buds Smell?

Odors begin to appear at a young age of 3-6 weeks (depending on strain). However, once plants begin to blossom or form buds, the odor becomes unbearable. There is only a slight odor to the cannabis plant’s leaves. But the buds have a strong aroma.

How Many Nodes Should I Have Before Flowering?

Growing experts advise waiting until plants develop four or more nodes before topping them. Also, many insist that the sixth node be the cutoff point. The nodes of a plant are the junctions between the main stem and its newer branches.


That will be all about how dark does flower room need to be. Getting the correct dark circle is as important as the light cycle.

Your plants will be happy, healthy, and productive if you pay attention to both of these factors.

Have a great day.

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