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.
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.
Benefits of a 6/2 Light Schedule
There is a lot of research that suggests cannabis plants can only process a certain amount of light per day. After that level has been reached, the plant can no longer absorb light and any additional light is essentially wasted.
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.
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.
While under light, cannabis tries to prevent CO2 from leaving its cells by cutting off transpiration. However, this prevents new CO2 from entering the cell, blocking new growth. When the lights are turned off and no photosynthesis is occurring, the plant can absorb new CO2 into its cells.
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.
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 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.
While there are many approaches to veg light cycles for cannabis, a 6/2 schedule allows for maximum plant growth. A 6/2 schedule also allows plants to process more intense light, prevents plants from becoming stressed, and puts less stress on your cooling system. It’s a win all around. Give your plants a break every two hours and you might be amazed at the results.