Photoperiodism in Cannabis


Cannabis is a plant which exhibits photoperiodism. This means that changes in the length of day and night can cause a physiological reaction.

Cannabis exhibits both obligate and facultative photoperiodism. Cannabis is a “short day” plant.

What does this Mean?

As the seasons change, the lengths of day and night change. This is more pronounced in locations further away from the equator.
In Summer days are long and nights are short, in Winter days are short and nights are long.
Cannabis has evolved to germinate in spring (seeds will only germinate when temperature rises- normally in spring), vegetate (grow big) in Summer when the climate is benign and there are many hours of good light for photosynthesis, then flower in Autumn so as to complete the life cycle before temperatures fall and light levels drop off too much.
Thus it is a short day plant- flowering is triggered as the days become shorter.

How does cannabis know when to vegetate and when to flower?

The photoperiod. When days are long and nights short, the plant will grow (vegetate). When days grow shorter and nights longer, the plant will flower.
To be specific a longer dark period of night is required to trigger flowering. This must be uninterrupted.
So if a cannabis plant is being grown indoors under lights (or indeed in any situation where the light/dark cycle can be controlled) the photoperiod can be used to control when the plant flowers.

Typical light schedules for seedlings and vegetative growth would be-
-Lights on all the time, 24 hrs.
-Lights on 18 hrs, lights off 6 hrs.
-Or anywhere in between.

Cannabis is unusual in that it does not need a dark period to grow. However returns diminish if the lights are left on for more than 18 hrs per day- essentially the plant cannot really use the last 6 hrs of light so electricity is wasted and lamp life expended.
In the vegetative phase of growth, the dark period can be occasionally interrupted without bad consequences.

Typical light schedules for flowering
-Lights on for 12 hrs, lights off for 12 hrs. This is the most commonly used flowering schedule.
-Lights on for 10 hrs, lights off for 14 hrs.
-Diminishing lights on, increasing lights off. So say the plant was vegged 18 on/6 off. Every day the period the lights are on is reduced by 30 mins. After 12 days the schedule is 12 on/12 off. For the rest of flowering the lights are left at 12 on/12 off per day.
-Variations of the above.
The reason schedules such as 10 on/ 14 off are used is to encourage reluctant strains to finish flowering.

Generally speaking flowering will be triggered when there are 10 or more uninterrupted dark hours. If the dark time is interrupted (say the lights are turned on) then this can interrupt flowering and cause the plant to revert to vegetating. Even if the lights are only turned on for a second, this could set a plant back for a week or longer. This may well stress the plant resulting in hermaphrodism. So don’t turn the lights on randomly when plants are flowering.

You could flower some cannabis with lights on for 14 hrs and lights off for 10 hrs per day. However flowering will take longer and the plant may continue to grow while flowering.

For some reason, plants grown indoors are very sensitive to any light at all during the lights off period while flowering. Even the slightest amount of light may delay or prevent flowering.
Outdoor plants are a little less sensitive (they will have no problem with a full moon for instance), however street lighting in urban environments can cause issues.

Do all strains of cannabis behave similarly? What is obligate and facultative photoperiodism?

Different strains of cannabis from different places around the world have adapted to the different conditions in those places.

Obligate photoperiodism depends upon photoperiodism pretty much absolutely.
Facultative photoperiodism means the growth of the plant is affected by the photoperiod but not absolutely, the plant will eventually flower regardless of the photoperiod.
Most cannabis exhibits photoperiodism somewhere between these two kinds.

Cannabis Indica is adapted to areas away from the equator.
This means that indicas must complete their life cycle before Winter, or they will perish before they can reproduce. Indicas tend towards obligate photoperiodism. So their vegetative stage and flowering stage are pretty much entirely controlled by the length of day/night.
One stage of growth not so heavily controlled by photoperiod is that immediately after germination. Indicas (and indeed most cannabis) will not flower immediately after germination but after about a month the plant will show small immature flowers called “preflowers”. Once these start to form, if the nights are long (10 hrs or more) then the plant will start to flower.
Generally indicas can be vegetated indefinitely, they will only flower when the dark hours are increased.

Cannabis Sativa is adapted to areas close to the equator.
At the equator days and nights (and temperatures) do not vary very much with the seasons. So being in synch with the seasons is not quite so important.
In general sativas will tend vegetate for longer than indicas regardless of the light schedule. Even if a sativa is germinated and grown for it’s whole life under 12 on/12 off, it will still vegetate and grow for quite a while. This can cause problems for indoor growers as plants tend to grow too tall.. Also sativas carry on growing while they are flowering.
Like indicas, sativas can usually be vegetated indefinitely, if the days are long they will not flower.

Ruderalis is another variety adapted to places away from the equator.
Ruderalis is autoflowering. This means it will flower regardless of the photoperiod. Thus ruderalis can be flowered outdoors during Summer, or flowered indoors under an 18 on/6 off schedule. This can be useful for the plant to reproduce in areas with harsh weather conditions.

Note that the above descriptions of photoperiodism for indicas, sativas and ruderalis are ideal. Most cannabis will exhibit photoperiodism somewhere between all three.
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