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Cannabis Light Periods – What do I need to know about marijuana light cycles? (length of sunlight hours each day)

If you’re growing a cannabis plant grown from a random seed (“bagseed”), unless you somehow have an auto-flowering seed, you will need to understand about cannabis life stages and how they are affected by light periods.

If you don’t understand light periods, your plant may never start making buds! The light schedule experienced by your plant will actually change its life stage. Learn more…

Cannbis plants have two life stages:

1.) Vegetative – Seedling or clone leads to Vegetative Stage

  • Give 18-24 hours of light a day (indoors)
  • Cannabis grows only stems and leaves

2.) Flowering – Flowering (Budding) Stage leads to Harvest

  • Give at least 12 hours of uninterrupted dark each day (short days)
  • Cannabis starts growing flowers/buds

The first stage, “Vegetative” begins when marijuana plants first sprout, at the beginning of their life.

Most indoor growers give their cannabis plants 18-24 hours of light a day during the vegetative stage. The exact number of hours needed to keep a plant in the vegetative stage is dependent on the strain, but 18+ hours/day will keep basically all cannabis plants in the vegetative stage.

Outdoor growers plant their seeds in Spring when the days are naturally longer. In the wild, cannabis seeds naturally germinate in the Spring.

For an indoor grower, when a plant is about half the final size you want it to be, you should change it over to the “Flowering” stage. This is the stage when your plant starts growing buds.

You do this by changing your light so that it only shines for 12 hours a day, and the other 12 hours a day your marijuana plants are kept in TOTAL darkness.

After 2-3 weeks of the 12-12 light schedule, most cannabis plants will show the first signs of their gender (they either are a female plant which starts growing buds, YAY! or they are a male plant which start growing balls/pollen sacs, NO!).

Boy cannabis plants don’t give you any usable amounts of THC, so most growers toss them on sight. These male plants can also impregnate (pollinate) your female plants, which causes your female plants to produce seeds and less buds.

So unless you’re planning on breeding, it’s important that most growers destroy male plants as soon as you notice them growing grape-like balls where their buds would normally be.

Unfortunately, about 50% of all regular (unfeminized) cannabis seeds are male (though this varies from strain to strain, and from environment to environment). Fortunately for small growers, you can purchase feminized (all-female) seeds so you don’t have to worry about male plants if you don’t want to. Learn more about buying seeds.

When does a cannabis plant start budding?

Marijuana plants have an internal process that allows them to detect how long they receive darkness each night. This is because they are a “photo-period” plant, specifically a “short-day” plant which means these plants start making flowers/buds when days start getting short.

In the wild, as the days get shorter and nights grow longer, a marijuana plant “realizes” that winter is coming and will start budding/flowering. It “knows” it’s approaching the end of its life cycle so it frantically starts making buds in time before winter.

When growing marijuana outdoors, a grower doesn’t need to do anything to induce flowering because the sun will take care of things on its own. It’s just important to make sure that there are no lights shining on your plants during their night period (which will disrupt their dark cyle).

However, when growing weed indoors, a marijuana gardener will have to fool their plants into “thinking” winter is coming to induce flowering and kickstart the creation of buds.

This is done by changing the plant’s light schedule to 12-12, where the weed plants gets 12 hours of light a day and 12 hours of total darkness.

You’ll get the best results if the start and end time for the light is exactly the same each day, which is why most growers end up getting a timer to flip their lights on and off, like an automatice light switch.

I tend to set my timer to shine line from 8pm-8am. This gives me time to check on my plants at night when the lights first come on, and I can also check them quickly in the morning before I go to work. It also keeps things cooler since the lights go on at night.

But ANY 12 hour period will work, as long as you remain consistent.

Check out my cannabis grow light guide for more info about picking out suitable lights!

Photoperiod dependent strains vs. auto-flowering strains

So all strains of cannabis that respond to light in this way (where the light period effects what stage they’re in) are called “Photoperiod dependent” strains.

Auto-flowering” marijuana strains pretty much ignore how much light they get each day. Generally you don’t run into these unless you buy them particularly from a cannabis seed bank.

Cannabis Light Periods – What do I need to know about marijuana light cycles? (length of sunlight hours each day) If you’re growing a cannabis plant grown from a random seed (“bagseed”), unless

Stages of the marijuana plant growth cycle

Cannabis plants, like all living things, go through a series of stages as they grow and mature. If you’re interested in cultivating cannabis, it’s especially important to understand the changes a plant undergoes during its life cycle, as each stage of growth requires different care.

Different stages call for different amounts of light, nutrients, and water. They also help us decide when to prune and train the plants. Determining a plant’s sex and overall health rely on stages of growth as well.

How long does it take to grow a marijuana plant?

Generally speaking, it takes anywhere from 14-32 weeks, or about 4-8 months, to grow a weed plant.

The biggest variability in how long a marijuana plant takes to grow will happen in the vegetative cycle—if you’re growing indoors, you can force it to flower after only a few weeks when it is small, or after several weeks when it is big. If you’re growing outdoors, you’re at the whim of the seasons and will have to wait until fall to harvest. The plant will develop buds in the last 8-11 weeks.

The life cycle of cannabis can be broken down into four primary stages from seed to harvest:

  • Germination (5-10 days)
  • Seedling (2-3 weeks)
  • Vegetative (3-16 weeks)
  • Flowering (8-11 weeks)

Seed germination (5-10 days)

Light cycle: 18 hours of light

The first stage of life for a cannabis plant begins with the seed. At this point, your cannabis plant is dormant, patiently waiting for water to bring it to life.

You can observe the quality of the seed by its color and texture. The seed should feel hard and dry, and be light- to dark-brown in color. An undeveloped seed is generally squishy and green or white in color and likely won’t germinate.

To begin growing from a seed, learn more about germination here. This stage can take anywhere between 5-10 days.

Once your seed has popped, it’s ready to be placed in its growing medium. The tap root will drive down while the stem of the seedling will grow upward. Two rounded cotyledon leaves will grow out from the stem as the plant unfolds from the protective casing of the seed. These initial leaves are responsible for taking in sunlight needed for the plant to become healthy and stable.

As the roots develop, you will begin to see the first iconic fan leaves grow, at which point your cannabis plant can be considered a seedling.

Seedling stage (2-3 weeks)

Light cycle: 18 hours of light

When your plant becomes a seedling, you’ll notice it developing more of the traditional cannabis leaves. As a sprout, the seed will initially produce leaves with only one ridged blade. Once new growth develops, the leaves will develop more blades (1, 3, 5, 7, etc.). A mature cannabis plant will have between 5-7 blades per leaf, but some plants may have more.

Cannabis plants are considered seedlings until they begin to develop leaves with the full number of blades on new fan leaves. A healthy seedling should be a vibrant green color. Be very careful to not overwater the plant in its seedling stage—its roots are so small, it doesn’t need much water to thrive.

At this stage, the plant is vulnerable to disease and mold. Keep its environment clean and monitor excess moisture.

Vegetative stage (3-16 weeks)

Light cycle: 18 hours of light

The vegetative stage of cannabis is where the plant’s growth truly takes off. At this point, you’ve transplanted your plant into a larger pot, and the roots and foliage are developing rapidly. This is also the time to begin topping or training your plants.

Spacing between the nodes should represent the type of cannabis you are growing. Indica plants tend to be short and dense, while sativas grow lanky and more open in foliage.

Be mindful to increase your watering as the plant develops. When it’s young, your plant will need water close to the stalk, but as it grows the roots will also grow outward, so start watering further away from the stalk so the roots can stretch out and absorb water more efficiently.

Vegetative plants appreciate healthy soil with nutrients. Feed them with a higher level of nitrogen at this stage.

Flowering stage (8-11 weeks)

Light cycle: 12 hours of light

The flowering stage is the final stage of growth for a cannabis plant. Flowering occurs naturally when the plant receives less than 12 hours of light a day as the summer days shorten, or as the indoor light cycle is shortened. It is in this stage that resinous buds develop and your hard work will be realized.

If you need to determine the sex of your plants (to discard the males), they will start showing their sex organs a couple weeks into the flowering stage. It’s imperative to separate the males so they don’t pollenate the flowering females.

There are a number of changes to consider once your plant goes from its vegetative stage to flowering:

  • Your plants shouldn’t be pruned after three weeks into the flowering stage, as it can upset the hormones of the plant.
  • Plants should be trellised so that buds will be supported as they develop.
  • Consider feeding plants with blooming nutrients.

What week of flowering do buds grow the most?

Buds typically grow the most toward the end of the flowering cycle, around week 6-7. You probably won’t notice much budding out at the beginning of flower, and it will slow down toward the end of the cycle, when buds become fully formed.

Once the buds have reached full maturation, it’s time to harvest.

This post was originally published on July 18, 2017. It was most recently updated on January 17, 2020.

It’s important to understand the changes a growing cannabis plant undergoes during its life cycle, as each stage of growth requires different care. ]]>