The importance of darkness for cannabis plants

The importance of darkness for cannabis plants

Give your plant plenty of long days in the sun and you’ll see her grow big and strong in no time. But did you know that dark periods are just as important to a plant’s health and development? Read on to find out why.

Like any plant, cannabis needs light to flourish. Give your plant plenty of long days in the sun and you’ll see her grow big and strong in no time. But did you know that dark periods are just as important to a plant’s health and development? Read on to find out why.


Cannabis plants require a very strict light-dark cycle to flourish. In nature, cannabis plants will usually sprout in spring or early summer. They’ll enter their vegetative phase and eventually flower at the end of summer/first weeks of autumn.

What triggers flowering at the end of summer is a change to the plant’s light cycle. This reaction is called photoperiodism. As the days get shorter, the plant produces flowers in order to reproduce before the winter season.

In order to keep from flowering, a cannabis plant will need at least 14 hours of light per day. When growing outdoors, it’s virtually impossible to control the amount of light your plants get, unless you use specific light deprivation techniques. Indoors, however, you can manipulate a plant’s light cycle using grow lights.

During the vegetative phase, most indoors growers will give their plants around 18 hours of uninterrupted light per day. Some even choose to keep their plants on a 24 hour light cycle.

But it’s important to remember that a plant’s dark period is crucial to its development and growth. Here’s why:


In order to understand the importance of a plant’s dark period, we need to first understand the processes of photosynthesis and respiration.

Photosynthesis involves three main elements: light energy, water, and carbon dioxide. Plants use sunlight to metabolize carbon dioxide in the environment and the water they take up from the soil. In doing so, they essentially produce carbon and oxygen compounds. They then use the carbon as energy to grow and release oxygen back into the atmosphere.

Whereas photosynthesis breaks down the compounds in water and carbon dioxide, respiration essentially rebuilds them. Rather than separating carbon molecules from water and carbon dioxide, plants will respire to recreate these compounds from the carbon they have stored from photosynthesis and the oxygen in the air.

Plants can only photosynthesize actively in the presence of light, meaning they only do so during the day. However, plants can respire without light and do so both during the day and at night.

Most plants will also automatically store the energy they produce during the day in order to get through the night. Most plants store this energy as starch. They then gradually metabolize this energy during the night, before producing more of it once the next day begins.

Hence, the night is actually a really important time for plants, as it is when they naturally metabolize the energy they’ve worked so hard to create during the day. However, in order for a plant to properly metabolize that energy, it needs to follow a strict light cycle.



You’ve likely heard many growers talk about “light leakage” and how important it is to ensure no light leaks into your grow room during a plant’s dark period. There’s a simple reason behind this.

Much like humans, plants have an internal clock. After all, you’ll notice that your body automatically wakes up in the morning and gets tired during the day, encouraging you to rest. Maintaining this rhythm (known as the circadian rhythm) is key to ensuring our bodies function properly.

For plants, following a strict rhythm is just as important. First and foremost, plants use this internal clock in order to regulate their energy levels and metabolism to ensure they can make it through the night. However, plants also use their internal clock to detect what time of year it is and when to flower.

For cannabis plants, this rhythm becomes extremely important during the flowering cycle. Disruptions to a plant’s dark period can stress the plant and can actually reverse it back into the vegetative state or worse.

Alternatively, light stress can also lead to hermaphroditism. Female cannabis plants can develop male traits in order to pollinate their own flowers if they see that the conditions might be too difficult to flower in.

Essentially, hermaphroditism is a kind of defense mechanism to ensure the plant can still reproduce, even when the conditions aren’t right, leaving behind one more generations of seeds.


As you can see, darkness is just as important to growing healthy and strong cannabis plants as light. Not only does it help plants metabolize energy and know when to flower, but interruptions to a plant’s light cycle can cause them to reverse into the veg phase or potentially cause hermaphroditism.