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
Cannabis Light Schedules: Vegetative Stage vs Flowering Stage
Cannabis plants keep getting bigger and bigger with long days, and start making buds when you give them long nights.
Cannabis is a “photoperiod” plant, which means the amount of light received each day decides when the plant starts flowering or making buds. This article explains how much light a day your photoperiod cannabis plants need to grow and start budding, so you get to a happy harvest day. What about auto-flowering strains?
Vegetative – Seedling or clone leads to Vegetative Stage –
Give 18-24 hours of light a day
Flowering – Flowering (Budding) Stage leads to Harvest –
Give 12 hours light & 12 hours dark each day
Seedling or Clone
While not technically a “stage,” all grows start with cannabis seeds or clones.
Plant your seeds or clones when you’re ready to start growing! What are clones? https://www.growweedeasy.com/cloning
Some outdoor growers start their plants indoors to give them a headstart before putting plants outside.
If you’re growing cannabis outdoors with seeds, you should wait until a few weeks after the spring equinox to put your seeds outside. In the northern hemisphere this means seeds go outside in-or-after April, In the southern hemisphere seeds go outside in-or-after October.
For growers starting with cannabis clones, generally you should wait a few weeks longer than with seeds. Cannabis clones are more prone to flowering early outdoors than seeds, so you might want to put your clones out in late Spring or early Summer. (What are clones?)
If you live in a cold climate, you must also wait until after the last frost before putting your plants outside. Freezing temps will kill cannabis plants. Strain choice is very important. Some strains flower earlier than others. For outdoor growers in cold climates, it’s important to make sure you grow a strain that is matched up with your local weather, so that plants are ready for harvest before temperatures drop.
The vegetative stage is one of the most important parts of the life of your cannabis plant.
The vegetative stage is the growing stage of the plant. When in veg, cannabis plants grow bigger and taller, growing only stems and leaves. As a grower, you are able to control the size and shape of your plants in the vegetative stage using simple training methods.
During the entire vegetative stage the plant does not produce buds at all. It only grows stems and leaves. During the vegetative stage plants tend to grow very fast, especially when conditions are right.
What keeps cannabis in the vegetative stage?
Short nights keep cannabis plants in the vegetative stage. You can keep a cannabis plant in the vegetative stage for basically forever as long as the plant continues to get short nights (shorter than 1s-12 hours, depending on the strain).
Cannabis will stay in the vegetative stage as long as the plant gets short nights (less than 11-12 hours of darkness each day)
Whether you’re growing indoors or outdoors, you must make sure your cannabis plants get at least 13 hours of light each day to stay in the vegetative stage. If your plant gets a few long nights, it may start budding before you want.
The plant can receive as much as 24 hours of light a day while in the vegetative stage. Many indoor growers provide 18-24 hours of light a day (known as 18-6 or 24-0 light schedules) during the vegetative stage to encourage faster vegetative growth.
Don’t want to worry about light schedules? For growers that don’t want to pay attention to light schedules, there are auto-flowering strains of cannabis, which will automatically go through their whole life in about 3 months no matter what light schedule is provided. For some growers, an auto-flowering strain may be more simple than a traditional (photoperiod) strain.
Most indoor growers provide 18-24 hours of light a day (known as 18-6 or 24-0 light schedules). Giving your cannabis plants more hours of light each day in the flowering stage will encourage faster growth.
Lingo: When a grower provides 18 hours of light a day and 6 hours of darkness, this is commonly known as the 18/6 light schedule. For 24 hours a day, this is referred to as the 24-0 light schedule.
As long as your plant is getting plenty of light a day, your plant will automatically stay in the vegetative stage from late spring until late summer. Every strain is a bit different.
Cannabis starts budding when plants get at least 12 hours of uninterrupted darkness each night. After plants start budding, they must continue to get long dark nights until harvest or they may revert back to the vegetative stage.
Indoors most growers put their plants on a 12-12 schedule to initiate flowering. Outdoors the plant will naturally start budding in late summer when nights are growing longer and longer as winter approaches. Just make sure plants aren’t exposed to light during their dark period!
What is 12-12 Lighting?
The indoor grower will need to artificially induce flowering/budding in plants by changing the light schedule so the plant receives only 12 hours of light a day, and 12 hours of uninterrupted darkness.
Once the plant is changed over to the flowering (12/12) light schedule, there is generally another 6 weeks-5 months (average 2.5 months) before the plant’s buds are ready for harvest.
Outdoor growers wait until their cannabis plants start naturally flowering on their own, usually after mid-summer when days start getting shorter than 12 hours.
It’s important to make sure plants aren’t exposed to light at night during their dark period, even street lights or spotlights, as this can prevent cannabis plants from flowering properly.
Growing Indoors? Not Sure When To Switch To Flowering?
So indoor growers have a choice to flower their plants whenever they want… When is the best t ime to start flowering your cannabis indoors?
The real answer is that it’s a matter of personal preference and also depends on what end result you’re looking for. There are two major considerations when choosing the right time to switch to 12/12, the age of the plant and the height of the plant:
Age: Some growers feel that a marijuana plant which has been grown from seed will not produce as many buds or have enough resin production if the plant is not given at least 60 days in the vegetative stage to mature before it’s changed over to the flowering stage. This is not true. many growers initiate flowering soon after germinating a seed in order to keep plants small and short. This is often called “12-12 from seed.” Just remember, no matter what you do, a young cannabis plant will not start flowering until it is 2-3 weeks old. Even if you put a seed on a 12-12 schedule from the beginning, it will not start properly budding for about 3 weeks. When growing with cannabis clones, age is not an issue and growers can switch directly to flowering once your clone has established roots. This is because even though a clone may be small, it’s still a ‘mature’ plant since it is made of a piece from a mature plant. Rooted clones tend to grow much faster for the first few weeks than plants grown from seed. In any case, age is not much of an issue, and you should switch your light schedule at the time that best fits your needs.
Height: A general rule is that your marijuana plant will double or triple in size during the flowering stage from the point where you first change over the light schedule to 12/12. Some plants will grow more, some will grow less, but a good rule of thumb is to change your light schedule over to flowering when your plants have reached half of their final desired height. Bending, known as “LST” or “low stress training” can be used to control colas that get too tall. Simply bend too-tall colas down and away from the center of the plant. Some growers will even slightly break or “supercrop” branches to get them to bend at a 90 degree angle. For those growing in a small space, height may be the primary concern. However, there are many techniques available to grow a short, bushy weed plant or basically train your cannabis plant to grow into any shape you want.
Here’s an example of LST to keep a plant short:
In optimal conditions if height and space is not an issue, you would probably want to vegetate your cannabis plant for 60 days or more before switching it over to flowering. This gives your plant plenty of time to grow big (so you get bigger yields), and allows new growers to dial in their grow before plants enter the sensitive flowering stage. In the vegetative stage, it is easy to recover from problems, but problems are a lot more serious in the flowering stage, where mistakes can dramatically hurt your final yields.
Giving cannabis plants more time in the vegetative stage, and taking time to train them to fit your space, will give you the best final yields. However, if space is tight, then it’s better to switch when the plant is half the final desired height, or even to just attempt to flower your cannabis plant straight from seed.
After the vegetative and flowering stage are over, it is time to harvest your plants!
What do I need to know about light cycles and flowering my marijuana plants? Plants keep getting bigger and bigger with long days, and start making buds when you give them long nights.