How Many Hours of Darkness Does Weed Need? Veg, Flower and Autos

In cannabis growing, light gets all the attention, but darkness matters just as much. Photoperiod plants need about 6 hours of uninterrupted darkness a day in veg (an 18/6 schedule) and a full 12 hours in flower (12/12) to start and keep blooming. Autoflowers don't depend on the dark to flower, but they still do better with a nightly rest.

During the dark period, cannabis plants run processes that are different from what they do in daylight. The length of that nightly stretch tells the plant what time of year it is, and that's what kicks off each stage of its development.

Right from germination, cannabis seeds rely on darkness to signal that they're underground. Darkness also shields the roots from pests and diseases, creating the ideal environment for them to grow.

Skipping or messing with the dark hours can hurt your plants' development. The good news: with a little care, it's easy to give them the darkness they need to flower properly. Come harvest time, you'll be glad you did.

Why darkness is crucial for photoperiod cannabis

If you're growing photoperiod cannabis (strains that flower based on day length), you need to understand why darkness plays such a big part in the process.

Everybody knows plants photosynthesize when they're exposed to light. But, much like animals, they also run other processes at night. That's why it's essential to let them spend enough time in the dark.

During the day, cannabis plants photosynthesize and generate energy to grow. At night, they burn through those reserves. The extra energy captured during the day is stored as starch, a backup fuel the plant uses during the dark hours.

Scientists still don't fully understand how plants metabolize that starch, but it's key to healthy development. Growers have also observed that cannabis plants kept under constant light don't produce better or more potent harvests.

On top of that, photoperiod cannabis plants regulate their growth according to how much light they get. In nature, they sprout in spring and early summer and grow under the strong sun of the long summer days. When the days start getting shorter, that naturally triggers the flowering stage.

But if those photoperiod cycles get thrown off, for example by an unreliable light schedule, it can wreck the plants' growth and development, anywhere from locking them in endless veg to killing them outright.

Bottom line: understanding and respecting the light and dark cycle is the foundation of a successful photoperiod grow and a high-quality harvest.

How many hours of darkness does a weed plant need?

Once you get into growing weed, it becomes clear that the light-dark cycle is everything. So how much darkness does your cannabis plant actually need, and why does sticking to a strict light schedule matter so much?

Let's break those questions down so you can get the most out of your harvest.

Does a cannabis plant need different amounts of darkness at each stage?

How much darkness your plant needs depends on whether you're growing a photoperiod strain or an autoflower. Autoflowering strains run on their own internal clock, so they don't need a specific light/dark cycle.

Photoperiod strains, on the other hand, follow the rhythm of the seasons and have specific light and darkness requirements.

Vegetative stage: giving the plant the energy it needs

During the vegetative stage, between the seedling phase and flowering, the plant is focused on growing and putting out leaves for photosynthesis. Indoor growers usually run an 18/6 light cycle: 18 hours of light and 6 hours of darkness. That schedule drives vegetative growth by giving the plants the energy they need.

Some growers experiment with alternative schedules, like a 6/2 cycle, where plants get three rounds of light and dark per day, but the jury's still out on whether these less conventional methods actually work.

Flowering stage: getting ready for harvest

To flower, the light schedule needs to switch to 12 hours of light and 12 hours of darkness (12/12). This change mimics the shorter days of fall, telling the plants it's time to reproduce before winter hits.

During this stage, it's critical to avoid any light leaks during the dark period, since even a little stray light can disrupt flowering.

Do cannabis plants need darkness before harvest?

Some growers give their plants roughly two days of total darkness right before harvest. The idea is that it boosts cannabinoid and terpene production and improves the quality of the final product. The science behind it isn't fully understood, but plenty of growers swear by it.

In short, understanding and respecting light and dark cycles is essential to maximize both the quality and the size of your harvest. Give your plants the right care at every stage of their life cycle and you'll see the results.

Do autoflowers need darkness?

Autoflowering plants are a special case when it comes to how light and darkness affect their development.

Unlike photoperiod strains, which respond to the light-dark cycle, autoflowers run on their own biological clock, so their relationship with light and darkness works differently.

Autoflower stages: light and darkness

During flowering, autoflowers don't follow the strict light-dark rules of photoperiod plants. They still need light to grow and darkness to rest, but the length of each period can vary a lot.

Common light schedules for autoflowers

The most popular light schedule for autoflowers is 18 hours of light and 6 hours of darkness (18/6) for their entire life cycle. That setup gives the plants the energy they need to grow, followed by a rest period.

Some growers switch autoflowers to a 12/12 schedule (12 hours of light, 12 of darkness) during flowering. In that case, though, the light is adapting to the plant and not the other way around, since autoflowers aren't driven by photoperiod the way photoperiod strains are.

What about 24/0 constant light?

Some growers give their autoflowers constant light: 24 hours on, 0 hours off (24/0). It can work, but there are downsides to weigh, like a higher power bill, the risk of light burn and heat building up in the grow space.

In the end, even though autoflowers have different light and darkness needs than photoperiod strains, striking the right balance between the two still matters for a successful grow. Experimenting with different light schedules can help you find what works best for your plants.