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Cannabis Light Schedules: When should I change my light schedule?

by Nebula Haze

There are a million opinions about the best time to change your marijuana plant over to the flowering stage.

Vegetative Stage - Indoor and outdoor growers must make sure plants get at least 13 hours of light each day - most indoor growers provide 18-24 hours of light a day (known as 18-6 or 24-0 light schedules) and outdoor growers generally plant in mid to late spring.

Young cannabis plants in the vegetative stage

Long days keep cannabis in the vegetative stage. For those who don't know, your plant will stay in the vegetative or growing stage for as long as its daily 'dark period' is shorter than 12 hours a day. Or in other words, when the 'days' experienced by the plant are longer than 12 hours.

Happy cannabis plants

Most indoor growers give their plants 18-24 hours of light a day to encourage the fastest growth. When a grower provides 18 hours of light a day and 6 hours of darkness, this is known as the 18/6 light schedule. For 24 hours a day, it's referred to as the 24-0 light schedule.

Outdoor growers simply need to make sure they plant cannabis outdoors after the days are already lasting at least 12-13 hours and days are getting longer, usually in mid-spring (varies depending where you live).

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 2-3 months no matter what light schedule is provided. For some growers, an auto-flowering strain may be more simple than a traditional (photoperiod) strain.

During the entire vegetative stage the plant does not produce buds at all.

In nature, as the days get shorter, cannabis plants will start flowering (producing buds) because they think winter is approaching.

Vegetative vs Flowering stage of cannabis

Flowering Stage - cannabis plants start budding when they start getting at least 12 hours of uninterrupted darkness each night (short days). 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!

When to start flowering a cannabis plant?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.

Outdoors, growers just need to wait until 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. 

Learn more about cannabis growing timelines

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 time to start flowering your cannabis indoors?

The real answer is that i'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:

LST - low stress training involves bending over plants so they stay shorter

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. 



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