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When do Cannabis Plants Flower?

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Maybe you’re thinking about growing your very own weed at home, but you have about a thousand questions, such as when do cannabis plants flower? Well, in this article we’re going to try and give as clear an answer as possible to that delicate question. Cannabis plants have various different phases they must go through until their flowers can be harvested, dried, cured and smoked. During each phase your plants will have a certain set of requirements such as nutrients or the amount of light needed.

Cannabis plants have four main phases, which are germination, vegetative growth, flowering or bloom and the fattening period. Today we’re going to talk specifically about the flowering period; why it happens and when it happens.

What’s the flowering period?

The flowering period, or bloom as some people call it, is what happens when your plants begin to produce aromatic flowers which, with the right care, can later be harvested, dried, cured and smoked. Your plants will still grow a bit once they’ve begun the flowering stage, although they will eventually stop in order to focus all of their energy on producing large, resinous flowers. The flowering period tends to last around 7-9 weeks on average, although many sativa strains take quite a bit longer.

When do Cannabis Plants Flower?

Cannabis plants usually begin the flowering process when there’s a change in the amount of light and dark they receive. Many people believe that it’s due to them receiving less light when, in reality, it happens because they begin receiving more darkness.

In order to flower correctly, seasonal cannabis plants need at least 12 uninterrupted hours of darkness. During the day, your plants are photosynthesizing, turning light into energy and releasing oxygen – this is just general knowledge. When in total darkness, cannabis plants begin producing a hormone called florigen, which is in charge of starting the flowering period. If your cannabis plants begin receiving more hours of darkness, they will produce more florigen which forces them to flower.

Indoors and Outdoors

If you’ve ever growing cannabis indoors before, then you know that your plants will begin the flowering process once you switch the light period to 12h of light and 12h of darkness. The main difference between indoor and outdoor flowering is that outdoor plants will begin to flower according to the change in season, once the days get shorter and the nights get longer.

When the flowering period starts, your plants shouldn’t begin shooting off flowers straight away. They’ll keep growing for a couple of weeks, depending on the strain, so that they can make a smooth transition to the flowering period. This is technically called the pre-flowering period. Some plants can grow to double the size during this period if they haven’t grown much, which is something you’ll need to keep in mind when growing indoors.

Every strain has their own unique flowering process. Generally, seasonal strains follow the same pattern, although they can vary in as far as flowering times, pre-flowering times etc.

What happens if the dark cycle is interrupted?

Cannabis plants are living beings and they like their routines because any variation can easily stress them out. So, what would happen if you interrupted your plants’ dark cycle? Well, you’d stress your plant out quite a lot and it might end up revegging, which means that it reverts to the growth period and it’ll stop flowering because it’s stopped producing florigen.

In more extreme cases, your plant might actually end up becoming a hermaphrodite plant, which means that it’ll produce both male and female flowers. The male flowers will then pollinate the female flowers, leaving your buds full of seeds. If this happens and you manage to catch it fast enough, you can simply remove the male flowers and hope that it does its best.

The Exception: Autoflowering Plants

When it comes to autoflowering plants, they don’t flower when it comes to the quantity of light or Darkness that they get. Instead, autoflowering plants begin to bloom depending on the amount of time they’ve been growing for. Autos can flower while being given many more hours of light than dark, and indoors you can set them at 18h of light or more if you want. Autoflowering plants generally take 4-6 weeks to begin flowering. They can finish their full cycle in just 10 total weeks from germination to harvest, although some autoflowering strains may take a little longer.

When do cannabis plants flower? Why do cannabis plants flower? In this article we're explaining then how, when and why's of cannabis flowering.

Preflowering And Flowering


By now you have managed to set up the basic environment in which your indoor plant will grow. You have your plants in some pots under a grow light with some white surfaces nearby and some fresh air in the room. You have also been watering your plant properly and you have been adjusting the pH and Nutrients in the soil.

Everyone makes mistakes on their first time growing. Very few get to this stage without problems, so don’t feel too bad if you did not get it right. Growing marijuana is like riding a bike. Once you get it, it is hard to forget.

You have been adjusting your lights and watching your plants grow. If you have not had any problems your plants should have a number of nodes and a small leaf mass at the top of your plant which you know is going to form the next set of leaves and branches.

Your leaves should be flat and stretched out. If they are, then your plant is enjoying its environment. If not then maybe you should consider turning to our problem solver chapter to see what has gone wrong.

You have been watching the height of your plant every week and now the plant is becoming more like the picture of the strain that you viewed in the seed-bank brochures. Then one day you notice that the plant is not growing much taller anymore. It seems to have stopped. You take a closer look and see that there appears to be small new growths at most of the nodes between the stem and the branch. This is new to you. These have not developed before so you ask yourself, “what are they?”

Your plant is now reaching the end of its vegetative stage. These small new growths are going to produce more leaf, branches and FLOWERS. The plant is now entering its Pre-flowering stage.


Now is the time when you should be hoping for as many females as possible and that all your work has not been in vain. You closely look at the new growths (Calyx) to see if you can identify your plants sex, but it is still too early to tell.

The early stages all look the same you say? What should you do now? You do not know? You will have to wait and see.

There are three things that can reveal your plant’s sex early on but these are not 100% accurate. They are more of an educated guess. So remember these methods can fail to be accurate but most of the time they are a good indicator.

First Early Sexing Method:

If you have been growing the same strain and all the seeds at the same time, then you may notice that some plants are taller than others. This is a sign that the smaller plants are female and the taller ones are male.

If you want too, you can separate your plants into two sections in order to see how good your guesswork was when you do finally identify sex. The other thing to know is that male plants in general start to pre-flower before females. If you have taller plants that are producing new growths before the smaller ones, then the taller plants are probably male.

Second Early Sexing Method.

A good way to identify plant sex at early date is to look at the calyx with the aid of a very fine magnifying glass. (See last page of this book for pre-flowering/calyx illustration)

If the calyx is raised on a stem then it is probably a male. If the calyx is not raised on a stem then it is probably a female.

Third Early Sexing Method.

Force-flowering a cutting is probably the best early sexing method out there. Simply take a cutting from the plant that you want to sex and place the cutting in a cup of water or into a cloning medium such as rockwool. Give the cutting 12 hours light and 12 hours total darkness. The cutting will flower and display its sex. Clones will carry the exact same genetic make-up as the plant it came from, including sex.

These methods are NOT 100% accurate but will help you to understand more about the visible differences between male and female plants. In a moment we will explain how to identify sex properly and completely.


Your plant will Pre-flower between 1 and 2 weeks and during this period the new growth regions begin to change shape depending on whether the plant is male or female. It is during this shape change that you can properly detect your plant’s sex.

Pre-flowering is a sign that your plant is mature enough to start flowering. As a grower you have a simple choice to make. Do you want to flower now? Or do you want to continue vegetative growth? Here are a few facts before you make that choice.

– Most cannabis plants can be kept alive for up to 10 years by simply keeping a light on the plant at all times. These plants will grow to a certain height and then form into a bush. It will then eventually stop producing branches and will spend the rest of its life growing new leaves to replace the old ones.

– Bud production is not equal to the height of your plant. Bud production is equal to your growing environment, your strain’s genetic make-up and the amount of nodes that the plant has. All nodes are potential bud areas, but every strain has a genetic threshold for bud production.

– It possible to get more bud with lots of plants which are flowered as soon as they are mature (which also keeps them shorter and smaller), than extending vegetative growth with less plants until they reach their maximum height and size. The time frame for the shorter option may also produce more bud turnover per annum.

Figure 7.1 – A Picture of a large indoor grow by

Figure 7.1 – A Picture of a large indoor grow by

Vic High.

Keeping these things in mind, you can either choose to flower now or choose to keep your plant growing until it reaches its size threshold before you start flowering. If you take the longer route then prepare to have the space for it, because when you flower most cannabis strains they can sometimes more than double in height and width.

If you have pre-flowers and want to flower you only have to do one thing. You must put your plants under a 12/12 schedule.

Figure 7.2 – A Picture of an indoor grow by

Figure 7.2 – A Picture of an indoor grow by


Most people never get good results or bud quantities from cannabis plants because they have never heard of 12/12.

12/12 is easy to explain. Cannabis plants grow outdoors naturally between the months of April to October/November. This means that towards the Sept/Oct/Nov dates the plants will be in flowering. During this time the days get shorter and the nights get longer. When this occurs the plant is under 12 hours of light and 12 hours of darkness. When this 12/12 PHOTOPERIOD occurs the plant is naturally stimulated to flower. If the 12/12 continues then the plant will produce its flowers. As long as 12/12 continues the flowers will grow larger and more plentiful. This is part of the cannabis plants evolution and how it has taught itself when to flower. Naturally as a grower we want lots of flowers so we need to put the plant through a light cycle of 12/12. This means that we must turn our lights on and off at these given ratios.

During pre-flowering you will either manually turn on your lights for 12 hours and turn off your light for 12 hours everyday or you will use an automatic timer. If your light comes with a timer set the timer to do this. Throughout the 12 hours of darkness it is best to keep your grow area as dark a possible. A small desk light at the other side of your room will cause your plant not to react properly to 12/12 which will result in continued vegetative growth. In fact any light that penetrates the darkness will stop your plants from flowering properly. That means that your grow room must be sealed to the point where it is completely light proof. If you want to learn how to do this perfectly then I suggest that you read up on ‘Photography Dark Rooms’, either on the Internet or in your local library.

Photographers use common items that can be bought in most hardware shops to make their film-processing rooms light tight. If you can replicate their ideas (basically a thick black screening around the doorframes or any open light points) then you will have a great system for flowering plants in. I think most of you would have done this by now anyway if you listened and took the advice on covering your grow with either Mylar or white walls/boards. If you have prevented any light from leaking out, then you have prevented light from leaking in!

Problems with 12/12

If you switch to 12/12 before pre-flowers have shown then you may encounter the following problems.

– Stress related sex problems (Hermaphrodites).

– Abnormal bud growth.

Stress related sex problems MIGHT produce hermaphrodite plants. The stress of what is sometimes called “early flowering” (it is not really early flowering, there is no such thing as early flowering. We will explain this in a moment) triggers the plant into a situation where it thinks that its chances of reproduction are slim to none. That situation is a condition called – ‘self-pollination’. It does this by producing both male and female flowers on the same plant. The male flowers then pollinate the female flowers which will eventually produce seeds. The reason for this is that the plant notices that the photoperiod is irregular and that it should no longer be in the vegetative cycle but in flowering. This shocks the plant into a last ditch effort to receive pollen because it feels that it has missed its chance to receive pollen already (in the wild males release their pollen just around the time that females begin to flower and sometimes even before that).

Figure 7.3 – This is what hermies look like. Notice that both male pollen pods and female pistils are present on the plant. Picture by Rasta Linus.

Figure 7.4 – Here is another shot of the condition. The male pods are clearly visible.

Figure 7.4 – Here is another shot of the condition. The male pods are clearly visible.

Figure 7.5 – These series of shots also show the hermie condition clearly. Picture by Tick.

Hermies cause problems because they may carry the hermie trait with their offspring. In fact, genetically the hermie will only produce female seeds and hermaphrodite seeds. It will never produce a male seed. If you have ever seen all female seeds been advertised by seed-banks then you should have the right to know that these seeds come from female plants which are stressed into producing male flowers. The plants then self-pollinate themselves and the results are female and hermaphrodite seeds. In a special case a female known as an XX female will produce more female seeds than hermaphrodite seeds. That is how female seeds are created. In general growers try to keep away from any hermie plants because they will spoil a Sinsemilla crop. Also having pollen floating around in your grow room from a hermie plant will spoil everything else including breeding projects.

Abnormal bud growth is a side effect of this. Because the plant produces male pollen sacks in with female flowers you may notice that the bud looks different. Also the quantity of female bud produced is decreased because of pollination.

Early induced flowering is not technically forcing your plant to flower. If you force flower on one strain that has not pre-flowered it will flower at roughly the same time as an exact copy of the same strain which has been flowered only when the pre-flowers appear naturally. Force flowering simply acts by stressing the plant into a crisis condition.

Get the best out of your plant and wait until your plant starts pre-flowering before switching to 12/12.

Keep feeding and watering your plant as normal. Pay attention to the flowering areas as they begin to grow. At this stage you may want to switch to your flowering feeds. Soon you will be able to see your plant’s sex.


You now have spent time and money on your plants. You have grown a small selection in the hope that you will get some females out of them in the end. If you end up with no female plants out of 15 seeds then send the seed-bank a letter explaining how out of the 15 seeds 15 where male. If you are lucky and sincere in your writing then the seed-bank may send you some free seeds or give you a discount on your next order. Seed-Banks or breeders are not responsible for male/female ratios. It simply is not under their control. People get 100% females and others get 100% males, but it is rare that such a thing will happen. To get 5 or more females in a pack of 15 is a good thing.

Here are some pictures of male and female plants.

Figure 7.6 – This is a picture of a male plant. You can see the male pod clusters clearly. The picture is by the ‘Chronic Couple’.

Figure 7.6 – This is a picture of a male plant. You can see the male pod clusters clearly. The picture is by the ‘Chronic Couple’.

Figure 7.7 – This is a female plant by BigIslandBud.

Notice how the female produces little white hairs and the male produces little ball like shapes. This is the definitive male/female telltale factor. Do not kill away any plants before you see these flowers. It is the only time that you can really say for sure if the plant is male or female.


If all things have gone well and you have taken good care of your plants, you will now enter the flowering stage of the plant’s life cycle. You have removed and maybe killed off the males. You now have a number of females to work with. This is going to be the most important time you will spend taking care of your plant.

The male plant produces pollen sacks which, when ripe, burst and scatter pollen to the female plants. The female plant produces white hairs at the internodes and top cola (head) of the plant during flowering. These hairs (pistils) begin to curl slightly and grow longer and thicker. The top cola should carry the most pistils. These pistils are sticky too touch (do not touch them too much! they also contain your much wanted THC) and begin to cover in resin over the flowering period. The reason for the stickiness is that it is used by the female to catch falling pollen. If the female plant is not Pollinated she will try to grow more sticky areas. Hence the results of a sinsemilla crop Bigger and Better Buds!

Figure 7.8 – Picture by BushyOlderGrower.

During the strict cycle of 12/12 she will eventually reach a peak period of flowering. Along with the flowering cycle the plant will also fill out more. More leaves, more branches and more flowers. Your plant will start to almost take the shape of a Christmas tree. The lower fan leaves will be stretched out to the max in order to receive the most light. Running upward in a cone shape she will get tighter with floral and leaf development.

During the peak period of flowering the female pistils on the flowers tips will swell up. When the swelling takes place the pistils will begin to change in colour. They will generally change from a white to an orange tint to a red tint to a brown tint. All strains are different but in general it is a white to red or a white to brown colour change (It is best to use the breeders recommended flowering times for harvest guidelines though). When she does this you are ready to harvest her and sample your favourite herb.

Each strain does have their own flowering times and each strain also may have a different colour tint when they reach a flowering peak.

We will talk more about harvesting in another chapter.

By now you have managed to set up the basic environment in which your indoor plant will grow. You have your plants in some pots under a grow light with some