The photoperiod is the relationship between the duration of the light period and dark period. Most strains of marijuana 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 marijuana all the light it needs to sustain vegetative growth.

Flowering is most efficiently induced with 12 hours of uninterrupted darkness in a 24-hour photoperiod. When plants are at least two months old-after they have developed 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. The 12-hour photoperiod represents the classic equinox and is the optimum daylight-to-dark relationship for flowering in cannabis.

Research has proven that less than 12 hours of light will not induce flowering any faster and reduces flower formation and yield. More than 12 hours of light often prolongs flowering. Some growers 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. However, flowering is often prolonged. 1 spoke with growers who increase light by one

This rude ml is cross is blooming in the middle of summer. The photoperiod does not induce flowering in this plant.

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.

A relationship exists between photoperiod response and genetics. We can make generalizations about this relationship, because little scientific evidence documents the extent to which specific strains of cannabis are affected by photoperiod. For example, so//vo-dominant plants that originated in the tropics respond to long days better than indica-dominant plants. 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. However, most growers are familiar with the pure saliva strain, 'Haze', which flowers slowly for three months or longer, even when given a 12-hour photoperiod. 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

Give plants 36 hours of total darkness just before inducing flowering with the 12/12 photoperi-od. This heavy dose of darkness sends plants an unmistakable signal to flower sooner. Growers 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.

Growers have experimented with giving plants up to 40 hours of total darkness to jump-start flowering and have found that 36 hours-three contiguous 12-hour nights-is most effective.

Half ol this 'Haze' plan! received light from a street-¡amp, causing it to remain in the vegetative growth stage. The other half of the plant received total darkness at night and ¡lowered!

Turn on a green light halb to work in the indoor garden at night. Green light will not affect the photopcri-od of flowering plants.

or longer (lowering. Planls grow more slowly in 12-lionr days than when given 18 hours of light, and inducing flowering takes longer.

/nrf/co-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 rndicci-dominant plant is contingent upon the genetics in the strain. More hours of light during flowering can cause some strains to produce bigger plants with reduced flowering time, but some growers have reported looser, leafier buds as a result.

Giving any cannabis variely less than 12 hours of uninterrupted darkness will not make it flower faster. Instead, the plant will take longer to mature, its buds will be smaller, and the overall harvest will be lessened.

Genetically unstable strains could express 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 produce hermaphrodites.

Some growers 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 yield.

The photoperiod signals plants to start flowering; it can also signal them to remain in (or revert to) vegetative growth. Marijuana 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 marijuana 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 slopped.

Marijuana will not stop flowering if the lights are turned on for a few minutes once or twice during the flowering cycle. If a light is turned on for a few minutes-long enough to disrupt the dark period-on two or three 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 /nc//cij-dominant plants will revert within three days. SoftVff-dominant plants take four to five days to revert to vegetative growth. Once Ihey start to re-vegetate, it takes four to six additional weeks to induce flowering!

When light shines on a green object, green pigment in the object absorbs all spectrum colors but green, and the green light is reflected. Tin is is why we see the color green.

The smart way to visit a grow room during the dark period is to illuminate it with a green light. Marijuana does not respond to the green portion of the light spectrum, thus a green bulb is usable in the grow room at night with no ill effects.

Some growers leave the HID on 24 hours a day. Marijuana 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 Sixteen Breeding.)

I talked with Dutch and Canadian growers who

You can put outdoor plants in a closet daily to induct' dowering with 12 hours of uninterrupted darkness.

claim their plants flower under a 6-hour dark and 12 hour light photoperiod. This expedited, 18-hour photoperiod regimen is supposed to work, but I'm not sold on it. Growers say that their harvest is undiminished, and that they are getting 25 percent more marijuana in the same time. I have not visited their grow rooms to verify these claims. No electricity is saved by adopting this regimen.

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