Many growers who are new to medial marijuana do not realize that marijuana is photosensitive. In other words, the amount of light hours and dark hours in a 24 hour period trigger particular responses within the marijuana plant. So what are the best light cycles for growing cannabis?
In nature, marijuana’s blooming (flowering) cycle is triggered by the lengthening dark periods which occur in the late summer and early fall. Indoor growers trigger flower production by changing the light cycle so that the plant receives a longer duration of darkness. In fact, the reason why many indoor gardens have two separate grow rooms is to keep the vegetative light cycle and the blooming light cycle separate. In order to have a successful harvest, new growers must grasp the concept of and the differences between a vegetative light cycle and a flowering light cycle.
Vegetative Cycle: 24 Hour vs 18 Hour
There are many gardeners who will argue that a 24 hour light cycle for vegetative growth is the way to go. In other words, leaving the light on all the time and forcing the plant to photosynthesize continuously. Personally, I prefer an 18 hour light duration for vegetative growth, which is closer to nature in the summer. I argue that plants are biological creatures and all biological creatures benefit from rest. Although some plants may grow a little faster when under a 24 hour light duration, many of these plants will develop nutrient deficiencies or have stunted growth in the flowering stage.
Many gardeners don’t realize that the problems they are experiencing in the blooming stage are occurring because the plants didn’t receive a proper dark cycle in the vegetative stage. During the vegetative stage, the time spent in darkness enables marijuana plants to translocate nutrients, translocate energy stored in the form of sugars, and prepare itself for another day of vigorous growth. This is why I always recommend an 18 hour lights on and a 6 hour lights off cycle for the vegetative stage.
Flowering Cycle: 12 Hours On and 12 Hours Off
As stated earlier, a medical marijuana plant’s flowering hormones are triggered by the amount of darkness it receives. Most marijuana strains require at least 12 hours of darkness to trigger the hormones responsible for the flowering process. I have experimented with numerous lighting techniques for flowering and, as long as the plants receive 12 hours of darkness, they all work (the plants create flowers).
Some growers believe that alternative light cycle (12 hours off 6 hours on or 12 hours off 10 hours on) are beneficial because they allow the grower to harvest more times throughout the year. However, I have never been able to get the results I desired using these methods. A gardener may be able to harvest more throughout the year but that doesn’t make up for the reduced yield in each harvest. In other words, alternative light cycles may cut down on the overall flowering time, but they end up diminishing yields, making them a wash (at best) and definitely not worth the extra effort.
I would never stop a gardener from experimenting with the light cycles in his or her garden. After all, many of the great techniques we use to grow medical marijuana came from growers who were willing to try new things. However, there is something to be said about sticking with the tried and true lighting cycles used by the most successful indoor horticulturists. As a new grower, the most important thing to remember is that marijuana needs at least 12 hours of darkness to produce bountiful yields. Also, there is no shame in giving your vegetative plants a short dark period to “rest”. They will repay your kindness at harvest time.