Nico’s Nuggets: Understanding Light and Dark Cycles During Veg and Flower Stages

Hello Nico,
During the vegetative stage, is it best to have 24-hours light or better to use a shorter light cycle such as 20 hours?
Also, I have not been able to find much info on 13-hour dark and 11-hour light during flower, but I have heard it mentioned on a few podcasts by the breeder DJ Short. I have tried to research this a bit more to see what advantages it may have other than the obvious saving of electricity. Thanks in advance for your thoughts.
– Michael via

Greetings, Michael. Thanks for your questions and for reading High Times!

Your questions are often asked by beginner growers, and this topic can be a bit confusing if this is your first time growing any type of fruiting plants. The first thing to understand is that it is the light cycle changes that signal to the cannabis plant it is time to begin flowering. Anything equal to or less than 12 hours of light will trigger flowering in cannabis plants.

The vegetative stage is perhaps the most misleading for new growers as cannabis plants do not require 24 hours of light to stay in veg. In fact, plants need some down time, just like humans. Keeping them under 24 hours of light essentially keeps them awake and driving hard without any rest. Most advanced growers will utilize a 20-hour or even an 18-hour light cycle during the vegetation stage, giving the plants 4-6 hours of dark time to rest.

Another little known fact is that the roots grow and develop during the dark hours, which is critical for the development of young plants. Roots also breathe in oxygen (as opposed to the CO2 that the rest of the plant breathes in), and this occurs much more prominently during the nighttime hours. It is for these reasons that giving your plants a few hours of darkness during their vegetative stage is recommended.

During flower, the standard light cycles are 12-hours light followed by 12-hours dark. However, some advanced growers (such as DJ Short) will tinker with this schedule to maximize growth and development of certain strains. Adding extra down (dark) time during flower can have beneficial effects for some varieties as discussed above, however, the opposite will not.

You cannot add more light beyond the 12 hours during flowering otherwise your plants will become confused and stressed. This will lead to plants going back into veg or worse—your plants could turn hermaphrodite (“herm”) whereby your female flowers develop male sex organs (small pollen sacs) and self-pollinate causing seeded bud.

Thanks for reading everyone and remember: Grow… And help the world grow, too!

Got questions? Email ‘em over to Nico at and be sure to put “Nico’s Nuggets” in the subject line!
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