Are there benefits to changing my grow room's lighting schedule?

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Q:

I’ve always thought using an 18/6 (18 hours on, six hours off) light schedule while plants were in veg stage was the way to go, but recently I’ve been hearing a 6/2 (six hours on, two hours off) schedule is better because pot plants only flower when exposed to long periods of darkness. Is this true? Are there benefits or drawbacks to changing my light schedule?

A:

I abandoned using the traditional 18/6 schedule long ago due to the following facts: First, you can veg a plant under 24 hours of light, growing more biomass in less time and getting you to the point of flowering quicker. However, after a few weeks, this benefit lessens due to plants needing a break.

When the plants are photosynthesizing, all the movements of water, nutrients, and pressure go upward. At night, the motor—so to speak—is turned off. This allows for the pressure to reverse, root exudates to be fully discharged, and plants to rest.

Secondly, most of today’s strains are so hybridized that they sometimes begin flowering under an 18/6 photoperiod. They require less darkness to begin to flower. So, I adopted a 20/4 (20 hours on, 4 hours off) vegging schedule after two to three weeks of continuous light. I find this to be the best combination between nature and production.

As with almost any subject regarding cannabis cultivation, there are many different schools of thought when suggesting photoperiods. Are you most concerned with maximum production and yield? Is finding the most efficient input of energy in return for yield what you’re after? Does closely mimicking nature make your heart sing? All the above are valid intentions and widely practiced throughout the home and commercial agricultural communities.

I tend to fall into the category of those trying to closely imitate nature whenever possible. The second lighting schedule you suggested above reminds me of a cloudy day. The lights or sun come out for six hours, then go behind the clouds for two hours. I don’t see any gain by denying your plants full sunny days. I’m sure my vegetable garden grows bigger when there are more sunny days.

However, there are noticeable gains to be made regarding light intensity during flowering. That’s the science behind the new lights coming onto the market that sequence short periods of lower wattage, thus mimicking cloudier days and giving plants a chance to recover from direct, intense light.

Outdoors, in addition to clouds, the sun moves across the sky, lighting different parts of the plant. Indoors, the most intense projection of your light hits the exact same spot for the entire lit portion of the photoperiod. This is one reason why it is extremely important to have lots of circulation, most importantly across the space between your lights and plant canopy. Plants should gently sway from an intermittent breeze.

Late season sun is far less intense. Having the same light intensity on your buds at the end of the growing cycle, as in the beginning, is less than optimal. I always leave room for raising the lights to double their distance from the plant, or have the ability to lower wattage on digital lamps just in case I don’t have the vertical space, near the end. Growers may want to consider 10 K finishing lamps, which give off higher UV levels, mimicking the late fall sun.

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Written by Kyle Kushman
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Kyle Kushman is an internationally renowned marijuana cultivator whose collaborations have earned 13 Medical Cannabis Cups awards, including three US Cannabis Cups for Best Flowers. As the creator of Vegamatrix, the only line of vegan and organic nutrients designed for growing cannabis, Kyle continues to make advances for people who want to cultivate the purest, cleanest medicine possible. Full Bio

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