Here is the 04 post from the new Light chapter. The photoperiod is the hours of light and darkness plants receive each 24 hours. The photoperiod affects different varieties differently. It is not so simple as turning the light to 12 hours on and 12 hours off for all varieties of cannabis to induce flowering. This is a key section in the Light chapter!
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The photoperiod is the relationship between the duration of the light period and dark period. In nature, cannabis normally flowers in the fall, when nights grow long and days short.
Foto cannabis growing outdoors
Cannabis is a short-day plant (C. ruderalis is a long-day plant) that will flower when it receives short 12-hour days and 12-hour nights. Most varieties of cannabis will stay in the vegetative growth stage as long as an 18 to 24-hour light and a 6 to 0-hour dark photoperiod are maintained. However, there are exceptions. Eighteen hours of light per day will give cannabis all the light it needs to sustain vegetative growth. Cannabis can efficiently process 16 to 18 hours of light per day, after which it reaches a point of diminishing returns, and the electricity is wasted. (See Chapter ???? Breeding).
Foto of cannabis flowering indoors
Flowering is most efficiently induced in most varieties of cannabis with 12 hours of uninterrupted darkness in a 24-hour photoperiod. When plants are at least two months old–after they have developed male and female sexual characteristics– altering the photoperiod to an even 12 hours, day and night, will induce visible signs of flowering in one to three weeks. Older plants tend to show signs of flowering sooner. Varieties originating in the tropics generally mature later and more hours of darkness shorten flowering time. The 12-hour photoperiod represents the classic equinox and is the standard daylight-to-dark relationship for flowering in cannabis.
In indica-dominant varieties less than 12 hours of light will not induce flowering any faster and less light reduces flower formation and yield. More than 12 hours of light often prolongs flowering in most varieties.
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A relationship exists between photoperiod response and genetics. Little scientific information is available about which specific varieties of cannabis are affected by photoperiod.
Foto sativa Haze type plant
Sativa-dominant varieties that originated in the tropics respond to long days better than indica-dominant varieties. On the equator, days and nights are almost the same length year-round. Plants tend to bloom when they are chronologically ready, after completing the vegetative growth stage. For example, the pure sativa variety, 'Haze’ flowers slowly for three months or longer, even when given a 12-hour photoperiod. Haze will continue to grow and flower for months.
Give Haze varieties more darkness and less light hours to speed harvest time and make flower buds fill in faster. Start with the 12/12 photoperiod and change to a 14 dark/10 light photoperiod after the first month. Play around a little with the photoperiod on pure sativas to dial in it in for specific varieties.
You can start 'Haze' on a 12/12 day/night schedule, but it still must go through the seedling and vegetative stages before spending three months or longer flowering. Plants grow more slowly in 12-hour days than when given 18 hours of light, and inducing flowering takes longer.
Foto indica plant
Indica-dominant varieties that originated in northern latitudes tend to flower sooner and respond more quickly to a 12-hour photoperiod. Many indica varieties will flower under a 14/10 or 13/11 day/night photoperiod. Again, the hours of light necessary to induce flowering in an indica-dominant plant is contingent upon the genetics in the variety. More hours of light during flowering can cause some varieties to produce bigger plants but flowering time is usually longer and some gardeners have reported looser, leafier flower buds as a result.
Some gardeners have achieved higher yields by inducing flowering via the 12-hour photoperiod, then changing to 13-14 hours of light after two to four weeks. This practice works best with early-flowering indica-dominant varieties, but flowering might be prolonged. I spoke with gardeners who increase light by one hour two to three weeks after flowering is induced. They say the yield increases about 10 percent. Flowering takes about a week longer, and different varieties respond differently.
Foto ruderalis plant
Ruderalis-dominant varieties are auto-flowering.
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Some gardeners experiment with gradually decreasing daylight hours while increasing hours of darkness. They do this to simulate the natural photoperiod outdoors. This practice prolongs flowering and does not increase yields.
Genetically unstable varieties could express intersex (hermaphroditic) tendencies if the photoperiod bounces up and down several times. If you plan to give plants a photoperiod of 13/11 day/night, stick to it. Do not decide you want to change the photoperiod to 15/9. Such variation will stress plants and could cause intersexuality.
Tropical gardeners that get 12-13 hours of light and at least 11-12 hours of darkness all year round can grow plants with artificial light the first month or two of life and set them outdoors to induce flowering with the long nights. Such gardens can flower for two or three months, harvested and replanted all year round. Other gardens in latitudes further north with good weather could grow auto-flowering feminized plants during the long summer days to avoid covering greenhouses to induce flowering.
The photoperiod signals plants to start flowering; it can also signal them to remain in (or revert to) vegetative growth. Cannabis must have 12 hours of uninterrupted, total darkness to flower properly. Dim light during the dark period in the pre-flowering and flowering stages prevents cannabis from blooming. When the 12-hour dark period is interrupted by light, plants get confused. The light signals plants, "It's daytime; start vegetative growth." Given this signal of light, plants start vegetative growth, and flowering is retarded or stopped.
Foto of cannabis flowering
Cannabis will not stop flowering if the lights are turned on for a few minutes once or twice during the two-month-long flowering cycle. If a light is turned on for a 5-30 minutes–long enough to disrupt the dark period–on three to five consecutive nights, plants will start to revert to vegetative growth.
Less than one half of one foot-candle of light will prevent cannabis from flowering. That is a little more light than reflected by a full moon on a clear night. Well-bred indica-dominant plants will revert within three days. Sativa-dominant plants take four to five days to revert to vegetative growth. Once they start to re-vegetate, it can take from four to six additional weeks to induce flowering again!
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Some gardeners give plants 36 hours of total darkness just before inducing flowering with the 12/12 photoperiod. This heavy dose of darkness sends plants an unmistakable signal to finish flowering. Gardeners using this technique report that plants normally show signs of flowering, such as pistil formation, within two weeks and develop pistils after a week of flowering.
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The smart way to visit a garden room during the dark period is to illuminate it with a green light. Cannabis does not respond to low levels of green light. Low levels of green light will not affect the photoperiod of flowering plants.
Half of this 'Haze' plant received light from a streetlamp, causing it to remain in the vegetative growth stage. The other half of the plant received total darkness at night and flowered!
There are other photoperiods possible. For example you can give plants 12 hours of HID light and the remaining 6 hours of incandescent light for a total of 18 hours to save on electric bills. But other light regimens that do not allow for 11-12 hours of darkness in 24 hours are going against Mother Nature. If they promise higher yields watch out for disproportionate use of electricity. There are also some screwy photoperiod regimens that should not be followed! Check out www.marijuanagrowing.com for more information.