Photoperiod and Flowering

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Weedguru Higher
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Photoperiod and Flowering

Post by Weedguru Higher » Tue Aug 03, 2010 1:18 pm

Photoperiod and Flowering


For the cannabis grower the most important plant/environment interaction to understand is the influence of the photoperiod. The photoperiod is the daily number of hours of day (light) vs. night (dark). In nature, long nights signal the plant that winter is coming and that it is time to flowers and produce seeds. As long as the day-length is long, the plants continue vegetative growth. If female flowers do appear, there will only be a few. These flowers will not form the characteristic large clusters or buds. If the days are too short, the plants flowers too soon, and remain small and underdeveloped.

The plant "senses" the longer nights by a direct interaction with light. A flowering hormone is present during all stages of growth. This hormone is sensitive to light and is rendered inactive by even low levels of light. When the dark periods are long enough, the hormones increase to a critical level that triggers the reproductive cycle. Vegetative growth ends and flowering begins.
The natural photoperiod changes with the passing of seasons. In the Northern Hemisphere, the length of daylight is longest on June 21. Day-length gradually decreases until it reaches its shortest duration on December 22. The duration of daylight then begins to increase until the cycle is completed the following June 21.

Because the Earth is tilted on its axis to the sun, day-length also depends on position (or latitude) on Earth. As one moves closer to the equator, changes in the photoperiod are less drastic over the course of a year. At the equator (0 degrees altitude) day length lasts about 12.5 hours on June 21 and 11.5 hours on December 22. In Maine (about 45 degrees north), day-length varies between about 16 and nine hours. Near the Arctic Circle on June 21 there is no night. On December 22 the whole day is dark. The longer day-length toward the north prevents cannabis from flowering until later in the season. Over most of the northern half of the country, flowering is often so late that development cannot be completed before the onset of cold weather and heavy frosts.

The actual length of day largely depends on local conditions, such as cloud cover, altitude, and terrain. On a flat Midwest plain, the effective length of day is about 30 minutes longer than sunrise to sunset. In practical terms, it is little help to calculate the photoperiod, but it is important to realize how it affects the plants and how you can use it to you advantage.
Cannabis generally needs about two weeks of successive long nights before the first flowers appear. The photoperiod necessary for flowering will vary slight with
(1) the variety, (2) the age of the plant, (3) its sex, and (4) growing conditions.

1. Cannabis varieties originating from more northerly climes (short growing seasons) react to as little as nine hours of night. Most of these are hemp and seed varieties that are acclimated to short growing seasons, such as the weedy hemp's of Minnesota or southern Canada. Varieties from more southerly latitudes need longer nights with 11 to 13 hours of darkness. Since most cannabis plants are acclimated to southerly latitudes, they need the longer nights to flower.
To be on the safe side, if you give Cannabis plant dark periods of 13 or more hours, each night for two weeks, this should be enough to trigger flowering.

2. The older a plant (the more physiologically developed), the quicker it responds to long nights. Plants five or six months old sometimes form visible flowers after only four long nights. Young cannabis plants (a month or so of age) can take up to four weeks to respond to long nights of 16 hours.

3. Both male and female Cannabis are long-night plants. Both will flower when given about two weeks of long nights. The male plant, however, will often flower fully under very long days (18 hours) and short nights (six hours). Males often flowers at about the same time they would if they were growing in their original environment. For most cannabis plants this occurs during the third to fifth month.

4. Growing conditions affect flowering in many ways. Cool temperatures (about 50F) slow down the flowering response. Cool temperatures or generally poor growing conditions affect flowering indirectly. Flower development is slower, and more time is needed to reach full bloom. Under adverse conditions, female buds will not develop to full size.

Applications of Photoperiod

The photoperiod is used to manipulate the plants in two basic ways:

1. By giving long dark periods, you can force plants to flower.
2. By preventing long nights, using artificial light to interrupt the dark period, you can force the plants to continue vegetative growth.


Most cannabis plants cultivated in the United States begin to flower by late August to early October and the plants are harvested from October to November. For farmers in the South, parts of the Midwest, and West Coast, this presents no problem and no special techniques and instructions are needed for normal flowering.
In much of the North and high-altitude areas, many varieties will not have time to complete flowering before fall frosts. To force the plants to flower earlier, give them longer night periods. If the plants are in containers, you can simply move them into a darkened area each evening.

Plants growing in the ground can be covered with an opaque tarpaulin, black sheet plastic, or double or triple-layers black plastic trash bags. Take advantage of any natural shading because direct sunlight is difficult to screen completely. For instance, if the plants are naturally shaded in the morning hours, cover the plants each evening or night. The next morning you uncover the plants at about eight to nine o'clock. Continue the treatment each day until all the plants are showing flowers. This usually takes two weeks at most, is the plants are well developed (about four months old). For this reason, where the season starts late, it is best to start the plants indoors or in cold frames and transplant outdoors when the weather is mild. This in effect lengthens the local growing season and gives the plants another month or two to develop.

By the end of August the plants are physiologically ready to flower; they sometimes do with no manipulation of the photoperiod. More often female plants show a few flowers, but the day-length prevents rapid development to large clusters. The plants seem in limbo - caught between vegetative growth and flowering. The natural day-length at this time of year will not be long enough to reverse the process, so you can discontinue the treatment when you see that the new growth is predominantly flowers.
In areas where frosts are likely to occur by early October, long-night treatments may be the only way you can harvest good-sized flower clusters. These clusters, or buds, are the most potent plant parts and make up the desired harvest. Forcing the plants to flowers early also means development while the weather is warm and the sun is shining strongly.

The flower buds will form much faster, larger and reach their peak potency. A good time to start the treatments is early to middle August. This allows the plants at least four weeks of flowering while the weather is mild.
Another reason you may want to do this is to synchronize the life cycle of the plants with the indigenous vegetation. In the northeast and central states, the growing season ends quite early and much of the local vegetation dies back and changes color. Any cannabis plants stick out like green thumbs, and the crop may get ripped off or busted. Plants treated with long nights during late July will be ready to harvest in September.

Outdoor growers should always plant several varieties, because some may naturally flower early, even in the northern-most parts of the country. These early-maturing varieties usually come from Mexican, Central Asian, and homegrown sources. By planting several varieties, many of you will be able to find or develop an early-maturing variety after a season or two. This, of course, is an important point, because it eliminates the need for long-night treatments.

Preventing Flowers

Manipulation of the photoperiod can also prevent the plants from flowering until a desired time. For example, in Hawaii the weather is mild enough to grow winter crops. The normal summer crop is harvested anytime from September to mid-November. The winter crop is generally planted from October to December. Because the winter days are so short, the plants flower almost immediately, usually within two month. The plants are harvested in their third or fourth month and yield about 1/4 the yield of summer plants.

A large Hawaiian female can yield a pound of buds. Most of the plant's overall size is reached while it is vegetatively growing. By interrupting the night period with light, you can keep these plants vegetatively growing for another month, yielding plants of about twice the size.
The amount of light needed to prevent flowering is quite small (about .03 foot candles95 - on a clear night the full moon is about .01 foot candles). However, each plant mist is illuminated fully, with the light shining over the whole plant.

This might be accomplished with either electric light or a strong flashlight. The easiest way is to string incandescent bulbs, keeping them on a timer. The lights need be turned on for only a flash at any time during the night period, from about 9:00 PM to about 3:00 am. The interrupts the long night period to less then nine hours. Start these night treatments each night or two, until you want the plants to flower.

Indoors Instructions

Natural Light.
The growing season lasts all year. The night period is much easier to control. Sometimes people grow plants in their windows for more than a year without any female flowers ever forming. This is because household lamps are turned on sometime at night, illuminating the plants. Under natural light exclusively, indoor plants flower at about the same time they would outdoors (sometime a bit sooner because it is warmer indoors or the plants may be shaded).

When plants are well developed and you want them to flower, make sure that no household lamps or nearby street lamps are shining on them. During late fall and winter, the natural day-length is short enough for the plants to flower naturally, if you simply keep off any lights at night that are in the same room as the plants. If you must use light, use the lowest wattage possible, such as a six-watt bulb. (The hormone is also least sensitive to blue light.) Shield the light away from the plants. Or shield the plants from any household light with aluminum foil curtains.

Once the flowers are forming clusters, you can discontinue the dark treatments, especially if it is more convenient. However, if it is too soon (when you see only a few random flowers), household lights can reverse the process.
By using natural light, you can grow indoor crops all year. The winter light is weak and the days are short, so it is best to use artificial lights to supplement daylight, as well as to extend the photoperiod. The extra light will increase the growth rate of the plants and hence size and yield. You should allow winter crops to flower during late January or February, using the natural photoperiod to trigger flowering. If you wait until spring, the natural light period will be too long and may prevent flowering.

Artificial Light Instructions

Under artificial light the photoperiod is, of course, any length you wish. The most popular way to grow with artificial lights is the harvest system. Start the plants under long light periods of from 16 to 18 hours daily. After the plants have reached a good size, usually between three and six months, shorten the light cycle to about 12 hours to force flowering.

To decide exactly when to force the plants to flower, let their growth be the determinant. If male plants are showing their flowers, then the females are physiologically ready to flower. Most of the plant's overall height is achieved during vegetative growth. Some varieties, of course, are smaller and grow more slowly than others. Wait until the plants are nearing the limits of the height of the garden or are at least five feet tall. This is large enough to support good flower development and return a good yield. If you turn down the light cycle when the plants are young and small, you'll harvest much less grass because the plants simply can't sustain a large number of flowers.

Some leaf growers prefer a continuous growth system, emphasizing leaf growth and a continuous supply of grass. The light cycle is set for 18 to 24 hours a day. This prevents flowering and the plants continue their rapid vegetative growth. Growing shoots and leaves are harvested as used, and plants are removed whenever they lose their vigor and growth has noticeably slowed. New plants are started in their place. In this way, there will be plants at different growth stages, some of which will be in their rapid vegetative growth stage and will be quite potent. Male plants and some females eventually will form flowers, but the females will not form large clusters. People often use this system when the lights are permanently fixed. Small plants are raised up to the lights on tables or boxes. This garden never shuts down and yields a continuous supply of grass. ... _guide.php

Ganja God
Ganja God
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Re: Photoperiod and Flowering

Post by (weedguru)scarf » Fri Sep 17, 2010 8:14 am

thanks higher.. this was really helpful

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