by Professor Lee – Author of Marijuana 101: Professor Lee’s Guide To Growing Grade A Bud

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Indoor growers can speed up the flowering cycle of traditional and feminized strains of cannabis by simply changing the amount of light the plants are exposed to between dark periods.  The following technique won’t be needed for the newer auto flowering strains that finish from germination to harvest in a few weeks regardless of the light cycle.  The two methods I’m talking about are commonly called the short light flowering cycle and the long light flowering cycle. Both have the effect of forcing plants to finish flowering sooner than the traditional 12 hours on, 12 hours off light cycle, but the long light cycle has one distinct advantage.  The short light cycle involves reducing the amount of light by up to four hours. Common short light cycles are 11 on 13 off, 10 on 14 off, 9 on 15 off, 8 on 16 off.  Each hour that the light cycle is reduced directly translates to a reduction in the length of the floral cycle. Basically the plants think that winter is bearing down on them and they need to finish up as quickly as possible.  The disadvantage is a proportionate reduction in harvested bud weight.


The alternative long light cycle technique shortens the flowering cycle of plants without sacrificing bud weight.  It involves maintaining consistent 12 hour periods of darkness while increasing the length of the light period by up to 12 hours. Examples of long light cycles are 16 on 12 off, 18 on 12 off, 20 on 12 off, and the one I always recommend 24 on 12 off.  This method can reduce the flowering cycle of cannabis plants by up to a week for every month of growth. In other words a plant whose floral cycle normally takes 60 days can finish up to two weeks early, and/or a plant that normally takes 90 days can finish up to three weeks early. The equipment needed is just a timer that allows the grower to program the on off cycles over the course of a week.  I found lots of models by simply searching for “7 day timers” on the internet.

It might get a little confusing while explaining exactly why this system works, so I’ll try not to ramble too much.  First one must remember that there are 168 hours in a week. A traditional 12 on 12 off light cycle gives the plants 84 hours of light to grow with during the course of the week. An extended light cycle of 24 on 12 off provides the plants with 120 hours of light per week.  Over the course of a 30 day month the plants get exposed to 516 hour of light instead of only 360 hours. The 156 hour advantage translates into 6.5 days worth of extra growing time per month. Funny enough a 30 hour on 12 hour off cycle also provides the plants with 120 hours of light a week. This technique has its limits and the 24 on 12 off cycle is about it.  Using a crazy cycle like 36 hours on 12 off or 48 hours on 12 hours off looks good on paper, but most plants simply can’t finish any faster than the more accommodating 24 hour on 12 off cycle.  And, using any other long light cycle has the tendency to leave the grower with a mild form of schizophrenia when they try to remember if the lights will be off and on during any given day.




  1. Excellent article ! While I am not a cannabis grower I find this information nonetheless very useful, I am curios to see its application on tomatoes and strawberries.