Takeaway: The key factor in growing cannabis is not the light periods; it’s the dark periods. Surna’s Stephen Keen weighs in with his preferred light schedule for growing big, productive plants.
Cannabis is a short-day plant, meaning it will only flower when exposed to long periods of darkness and short periods of light. For cannabis to flower, there must be at least 12 hours of continuous darkness. This allows for the use of a series of shorter light schedules while the plant is in veg—as long as the plant receives less than 12 hours of continuous darkness, it will stay in veg.
Giving plants six hours of intense light at a time not only puts less stress on the plants, it also spreads out the load on your cooling system over a longer period. The cooling system works the hardest when lights are on.
By breaking the light cycle into multiple six-hour periods, the plant can rest and process the light it has received. When the lights come back on two hours later, the plant will be ready to process additional light, allowing you to get the most plant growth out of every minute your lights are on.
Additionally, when plants are exposed to 18 straight hours of intense light, they become stressed. Signs of stress, including droopy or curled leaves, will usually appear toward the end of the light cycle. While some stress can be beneficial to plant growth, too much stress can cause harm to your plants and prevent them from reaching maximum growth potential.
By turning off the lights for two hours at a time throughout the day, your cooling system will get a break between light cycles, allowing the room to be cooled to desired temperatures before the lights come back on. With a properly sized cooling system, this benefit will be minimized as the system will be designed to handle the heat load throughout the entire light cycle.
At a biological level, cannabis’s inability to grow more once it has received a certain amount of light can be attributed to the way the plant processes carbon dioxide (CO2). A majority of the mass accumulated in cannabis is associated with the amount of CO2 found inside plant cells.
Many growers advocate the use of an 18/6 light schedule (18 hours on, six hours off) while plants are in veg. However, this may not be the most beneficial light schedule. Switching to a series of 6/2 light pattern (six hours on, two hours off) may increase plant growth while also potentially creating a more stable controlled environment.
While this new schedule may sound risky, it actually comes with a number of benefits.
Many growers advocate the use of an 18/6 light schedule (18 hours on, six hours off) while plants are in veg. But this may not be the most beneficial
The first stage, “Vegetative” begins when marijuana plants first sprout, at the beginning of their life.
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!).
When does a cannabis plant start budding?
Cannbis plants have two life stages:
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.
Photoperiod dependent strains vs. auto-flowering strains
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.
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).
Check out my cannabis grow light guide for more info about picking out suitable lights!
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