Professional Marijuana Grower


The Light Cycle and How it Affects Flower Initiation

Posted February 27th, 2019 by Eric Hopper in ,

TThe photoperiod is an extremely important aspect of the flowering cycle in a cannabis garden. The photoperiod is the duration of the light period versus the dark period within a 24 hour cycle. Generally speaking, cannabis is considered a short-day plant, which means it will begin to flower as the amount of light hours per day lessens (12 hours of light or less). In nature, cannabis plants will begin to produce flowers in the fall when the daylight hours become shorter. For indoor gardeners or greenhouse growers using light deprivation, a cannabis plant’s bloom cycle is initiated by shortening the duration of the light cycle (and, thus, increasing the duration of darkness).

When discussing horticultural lighting and photoperiods, the focus is typically on how much light is being provided to the garden. However, in order to encourage cannabis plants to produce flowers, it is the duration of the dark cycle that is of the utmost importance. It is the extended dark period that actually triggers and maintains a cannabis plant’s hormones to produce flowers. Interruptions in the dark cycle of an indoor garden (or greenhouse with light deprivation) can cause a series of problems, including reduced yields and hermaphroditic tendencies (female plants that create male pollen, resulting in seeded buds). To avoid issues that originate from an interrupted dark cycle, indoor horticulturists should strive to make the dark cycle as “light-tight” or as dark as possible.


Generally speaking, 12 hours of darkness is enough to trigger flowering in photosensitive plants, such as cannabis, and also to maintain healthy flower development. However, many growers often underestimate the importance of how dark the dark cycle should be. In an ideal situation, the garden’s dark cycle would be pitch-black. Any light entering the flowering stage during the dark cycle could potentially cause the plants to continue vegetative growth or, for dioecious plants, cause hermaphrodites to develop. When a cannabis plant is provided with a dark period of 12 hours or more, it produces specific hormones for the development of flowers. Interruptions in the dark cycle can hinder the development of these flower-inducing hormones and, in some cases, stop the production of these hormones altogether. When this happens, a cannabis plant that has begun creating its flowers will actually revert back to vegetative growth. Although reverting back to a vegetative stage can be a useful tool for cannabis breeders, the vast majority of indoor growers do not want their cannabis plants to revert back to vegetative growth after the flowering stage has been initiated.

A light leak does not always cause a noticeable problem, like the flowers becoming seeded by a hermaphrodite’s pollen. However, this does not mean there are no negative effects to the cannabis yield. It is quite common for cannabis plants whose dark cycles have been regularly interrupted to grow more spindly as the interruptions cause the plants to attempt to convert from vegetative growth to flowering growth time and time again. One of the most common questions asked by indoor horticulturists in regard to photoperiods and the dark cycle is how the moonlight affects cannabis plants in nature. Yes, it is true that in nature the moonlight interrupts the dark cycle and is seemingly bright. However, cannabis plants flowering outdoors do not seem to be negatively affected by the moonlight’s interruption of the dark cycle.

In order to understand why this is, we need to take a look at the inverse square law. Essentially, the inverse square law states that light diminishes exponentially from its source. Simply put, the farther the distance from the light source, the less light energy is present. In nature, moonlight, which is reflected sunlight, has a great distance to travel and has minimal intensity when compared with the actual sun. In other words, it is the great distance the light has to travel (past the Earth and then reflected back) that makes it relatively weak when compared with direct sunlight. Artificial light sources act much differently than sunlight. The close proximity of any light source in an indoor grow space to the cannabis plants will make the light significant enough to interrupt the dark cycle. The relative intensity of the artificial light sources is why having a pitch-black, light-tight grow room is so important.

Look for Light Leaks

The first step to ensuring a pitch-black dark cycle is to physically check the grow room’s darkness. This involves more than just flipping off the lights for a few seconds, seeing that the room appears dark, and moving on. Cannabis growers should spend at least five to ten minutes in the grow space during the dark cycle to see how dark it really is. It can take up to five minutes for the human eye to adjust to the darkness and become sensitive enough to detect any protruding light. After waiting five minutes, a grower should turn around very slowly to get a full 360 degree view of the grow room. A grower should look for any signs of light leaks. This includes lights on electric devices in the grow room, such as atmospheric controllers, power strips, or light timers. The tiny lights on these devices may not seem like they would affect the growth of plants, but any light in the dark cycle should be considered bad light. Light leaks most commonly occur around and through ventilation ports. Closely examine intakes, exhausts, and air-cooled ducting systems to ensure no light is entering the cannabis garden. Once a grower’s eyes have adjusted to the dark, it should be very easy to see any light leaks. Basically, if a grower sees anything other than complete darkness, he or she should address the areas of concern.

Repairing Light Leaks

Once a grower has determined where the light leaks, if any, are occurring, he or she should immediately take action to repair it. Electrical tape is a great way to cover operational lights on any electronic devices that must remain in the grow room. It may take multiple layers of tape to completely smother the light. For light leaks coming through ventilation ports or air-cooling systems, a manipulation of the ducting itself is usually the best solution. By bending the ducting upward to 90 degrees, a gardener can stop the light from entering the grow room. Light reflects, but it doesn’t bend. Black interior ducting should be the go-to ducting for indoor horticulturists as it negates light reflection within the ducting. Any other cracks or holes within the grow room can be repaired with caulk or expanding foam insulation. Black and white poly-plastic sheeting (if thicker than six mils) can be used as a black-out material and hung on walls or ceilings where light leaks are present.

Check and Recheck

After making the necessary repairs, a cannabis grower should go through the entire process again. Once all of the light leaks in the flowering room have been corrected, a horticulturist (again waiting approximately five minutes) should not be able to see their own hand during the grow room’s dark cycle. Although this may seem extreme, an indoor grower will not be able to achieve a cannabis crop’s full potential without this level of darkness during the dark cycle. When all else is equal, a pitch-black dark cycle will make the difference in cannabis flower density and quality. Checking and maintaining the grow room’s light-tight dark cycle should be included on every indoor grower’s regular maintenance checklist.

Light Tight with Air-cooled Reflectors

When using air-cooled reflectors, getting the dark cycle to be completely pitch-black can be more difficult than in an indoor garden without air-cooled reflectors. Air-cooled reflectors generally draw air in from outside the garden space. When light from that outside source travels through the ducting, it reaches the reflectors and gets reflected into the garden space. There are a few different ways a grower can tackle this problem. One option is to draw the fresh air from another room, not directly outside. This way, the other room can be kept dark and will not allow light to enter the ducting. Another solution is to bend the ducting at the outside source so that it creates an “s” shape. This is effective because light can reflect, but it cannot bend. It is also helpful to use ducting with a black or dark interior. When dark interior ducting is bent in an “s” shape, the light is not able to reach the garden space.

Indoor cannabis growers spend a lot of time and money to produce a high-quality, prolific harvest. Maintaining a light-tight, pitch-black dark cycle in the cannabis flowering room is a relatively easy and inexpensive way to make a significant impact over the yield and quality of the crop. Many indoor cannabis growers, even experienced ones, are operating a flowering room that, due to light leaks, is not providing the optimal environment for cannabis. When designing a cannabis flowering room, the photoperiod, especially a light-tight dark cycle, should be a top priority. An indoor horticulturist who takes the time to address any light leaks and includes a “light-tight” test on his or her regular garden maintenance checklist will be rewarded with larger yields, tighter buds, and higher quality cannabis.

Eric Hopper is a Professional Marijuana Grower Senior Editor. He can be contacted at

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