In nature, cannabis plants (excluding ruderalis) begin to flower in the late Summer and early Fall. When the days start to become shorter and the nights longer, flowering signals are triggered by photoperiodism, beginning the flowering process. The increasing hours of darkness signal to the plant that Fall is just around the corner. The female marijuana plant shifts its energy from vegetative growth into flowering, in order to attract pollen and produce seeds. By manipulating the hours of darkness, indoor and outdoor growers can delay flowering indefinitely or begin flowering whenever desired.
The traditional method used to sustain vegetative growth is to keep light on the plants between 18 and 24 hours each day. The key factor is not the hours of light, but the hours of darkness. With six hours of darkness or less, the plants will continue to put on vegetative growth as if they were experiencing an extended Summer.
When it is deemed time for the flowering process to begin, the light cycle is changed to a regimen of 12 hour on and 12 hours off. The increasing hours of uninterrupted darkness cause the plants to respond as if Autumn is approaching, and begin to flower. By using these two light cycles, you can control when your plants flower.
Over the years I have taught a few people how to grow cannabis. In doing so, I have learned a few lessons myself. Simply telling someone what to do is generally not enough. Unless I also explain the reasons behind the practice, it is usually only a matter of time before they forget or decide to change things up. It also seems to be a lot easier for growers to diagnose their own problems when they have a firm grip on the reasons plants grow how they do.
The Science in Layman’s Terms
Cannabis plants are referred to as long night or short day plants, because they require a long period of darkness to trigger the hormones that tell the plant to switch from vegetative growth to flowering. These light receptors are color pigments in the leaves called Phytochrome Red (PR) and Phytochrome Far Red (PFR). These pigments get their names from the types of light they absorb. PR absorbs red light between 660 and 760 nm and PFR absorbs far red light between 760 and 800 nm. These two pigments chemically react to the light, and trigger the plant to flower or not.
This is where it gets a little confusing.
In cannabis plants, the normal presence of PFR switches off the flowering signal. The level of PFR is what you can manipulate by adjusting the photoperiod. PFR is quickly produced when plants are exposed to light that contains far red wavelengths. When there is light, the PFR and PR maintain a balance. When the sun goes down or the lights go out, the darkness gradually switches the PFR to PR. Because of this, PR levels gradually increase and the PFR gradually decrease during the dark period. The presence of PR is a neutral condition to the plants and essentially tells them nothing. When the light returns, or if a small amount of far red light interrupts the dark period, the PR immediately switches back to PFR. If the plant is without light long enough, the PFR will decrease past a tipping point. This decreased level of PFR signals the plants that Fall is approaching; and the plant begins flowering.
In short, the presence of PFR due to long hours of light and short hours of darkness keep the plants in the vegetative phase. If the plant experiences enough hours of darkness, most of the PFR turns to PR ; and the low level of PFR signals the plant hormones to begin flowering.
Enough with all of this scientific mumbo jumbo. Let us look at how we can take advantage of photoperiodism.
How to Induce Flowering
Because of photoperiodism, you can easily induce flowering in cannabis plants simply by changing your grow room light cycles–reducing the light from 18 hours a day or more, to only 12 hours each day. Again, what is really important to the plants is changing from six hours of darkness or less to 12 hours of darkness. Within two weeks of switching the light cycles to 12 on and 12 off, you should see small buds starting to form.
I need to stress the point that your plants are very sensitive to any light during the dark period. If you have any light at all leaking into your grow room, during the 12 hours of dark– even momentarily–the PR can change back to PFR. This means that any light reaching your plants during the dark period may sabotage the flowering process.. For this reason, you should never–not even for a moment–enter your grow room when the lights are turned off in the 12/12 cycle.
Before you make the switch to a 12/12 light cycle, you need to make sure your grow room has no light leaks. Enter the grow room while the lights are out and close the door behind you. Wait about 15 minute minutes for your eyes to adjust fully, then look around and make sure no light is entering the room. No light under the door, through a curtain, or shining from a CO2 generator or dehumidifier. I have made this last mistake myself. If any of your equipment produces light from the display, cover it with duct tape, making sure no photocells have been accidentally covered. If you can see your hand in front of your face, you have a light leak that needs to be fixed.
While simply changing to a 12/12 light cycle will induce flowering, there is a trick to jump start the process. Between the switch from an 18/6 to a 12/12 light cycle, let your plants sit in total, uninterrupted darkness for 36 hours. This will cause the PFR to drop substantially, giving the plants a strong signal to flower. After the 36 hours of darkness, begin the 12/12 light cycle. In a side by side experiment, I saw significant results from this method. Be sure to flush out high nitrogen “grow” fertilizers from your growing medium and change to a high-phosphorus “bloom” formula. Also, adding high potassium supplements for the first two weeks can help increase rapid bud development
Light Deprivation For Outdoor
Through manipulation of photoperiodism, growers can induce early flowering in outdoor cannabis plants. It is much more difficult to make the great outdoors artificially dark than it is to turn off the lights–but not impossible. Many growers have perfected the art of light deprivation, and used it to harvest their outdoor crop in midsummer, or even multiple times each year. This can be achieved by building a garden that can be covered after the sun goes down, and then uncovered part way through the morning. If timed correctly, this can lengthen the natural night to a full 12 hours of darkness. The garden must be covered for several hours each day without exception, through the entire flowering period; but the effort can bring impressive early harvests.
I can’t count how many times I have received urgent phone calls from alarmed acquaintances who planted clones outside too early. Despite what anyone says, clones can do amazing things if properly grown outside. Unlike seeds, however, you need to be aware of the hours of natural light when planting outside. Because clones are likely accustomed to 18 or more hours of light, they often begin to flower once placed outside in early spring. It will depend on your latitude and the strain; but at least in California, the nights are usually too long to plant clones outside before mid-May.
By using supplemental light to decrease the hours of darkness, you can plant your clones outside as early as you like. Simply clip a florescent light with an aluminum reflector onto a stake or cage around the plants. If the light shines on the plant for even a few hours after the sun goes down, it will usually be enough to prevent early flowering. If you are afraid of late frost, you can use an incandescent bulb instead, and also take advantage of the heat it generates.
Power Outages & Light Interruption
Even with light timers and a fully automated grow room, sometimes things go wrong. If the power goes out, or you need to change lights around, keep your light cycles in mind.
When your grow room is running on a 12/12 flowering cycle, a short power outage is not a major problem. A few extra hours of darkness will not really mess things up. Of course, the plants will not grow much without light; but they should be fine for at least two days. Any longer than two days, and they will start to suffer. This will not kill them, but may cause them to become stressed.
When running on a 18/6 vegetative light cycle, a power outage resulting in long hours of darkness can trigger the plants to flower. You need to find an alternative source of light for the grow room. An electric or gas camp lantern will be enough to prevent the PFR from dropping. If you need to change things around and interrupt your normal dark period, just leave the lights on until the following dark period. A few hours of extra light will not hurt anything. Remember, you do not need enough light to keep them growing–just enough to make it not dark.
- To maintain vegetative growth, use 18-24 hours of light; six hours of darkness or less.
- To promote flowering, alternate 12 hours of light with 12 hours of darkness.
- When switching light cycles from vegetative to flowering, first give plants 36 hours of darkness.
- To harvest outdoor plants early, cut the time light reaches the plants to 12 hours each day.
- To plant clones outside before mid-May, supplement with artificial light at night to prevent early flowering.
- During 12/12 flowering, additional dark hours are acceptable if necessary.
- During 18/6 vegetative growth, additional hours of light will not cause problems.
Successful marijuana growers know how to manipulate the environment of their garden. Once you become comfortable with light manipulation and photoperiods, the sky is the limit.
Light Cycles and Flowering Cannabis,