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.

When do Cannabis Plants Flower?

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.

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Erik Collado

With more than 10 years of background in the cannabis sector, his experience and knowledge are the base of GB The Green Brand’s success

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22 replies on “When do Cannabis Plants Flower?”

I have a plant that is less than 3 ft tall I’ve been growing it since early April and I’m ready to go to my flowering stage being that I do not have much room in my apartment should I wait or start my 12 12 cycle

Hey Jim!
Do you know what strain/type it is? Sativa plants can sometimes triple in size when flipped to flower, which can be an issue for indoor growers. Many indicas tend to double in size, also, so I’d say flipping to flower asap is probably a good idea if you don’t want a possibly 6ft tall plant growing indoors. If you’re worried about growth and size, you can always top it and train its branches out sideways using a net; if done right, when you flip to flower you should be able to continue training it out, although this is assuming that you have the space in as far as width, which leads to larger yields.
Kind regards

Hi there, sorry with dumb questions,
I’m new to all this but certainly not new to cannabis world after enjoying it for over 40 years…!
Will be picking up a PH pen tomorrow to test my PH which should have done a long time ago obviously… (Unfortunately Didn’t know any better) only have 2 cannabis plants growing in 5 gallon bods each in nice organic compost soil in my yard with pretty good sun usually.

These plants did grow very nicely up to about 7’ tall now but slowly started showing many yellow fan leave since about 3 weeks ago, I take these leaves off but week later same thing again, needless to say there are no flowers or even sign of it at all anywhere.

Plants are about 4-5 months old now and in vancouver BC we’ll start seeing cold temp coming up soon. Wondering if I have light issues (not enough dark hrs?) or nutrition problem? Or just have to wait longer for flowers to show up one morning?

If there are serious issues, will I still have time to save these plants and hope for some flowering if I provide them with proper nutrients if anyone with experience can advise What kind of nutrients I should use at this stage of growth etc… Of course I’ll have the PH pen soon and would have some idea what my soil would look like with PH etc… I would really appreciate your advise!

Hey Oliver!
First off, there’s no such thing as a dumb question! We all started somewhere!
The yellowing in the leaves is most likely a pH-related issue, certain minerals may be being blocked in your roots. You’re already halfway there to figuring it out; make sure to measure the pH in your runoff water too in order to know your soil and root pH. You shouldn’t remove the yellow leaves, this is usually only done when they yellow naturally towards the end of the bloom cycle.
Some plants can take until late August/beginning of September to flower, especially more sativa strains. Do you happen to know what strain you’re growing?
In as far as nutrients, until it begins to flower you should be using growth nutrients, although I wouldn’t recommend using nutes again until you manage to sort out your possible pH issue. Also, switching to flower nutes won’t cause your plants to flower earlier, you’ll simply be giving them nutes they don’t quite need yet.
I’ve done some digging and late flowering seems to be a common issue with many growers in Canada, I’m not sure if this is to do with the light quality or amount of light, but unfortunately the best advice I can give you is to measure your pH runoff, adjust it accordingly, and wait for it to flower. Let us know how you get on!

Hey Ciara,

Tnx a lot for your quick respond and all the valuable info.
I’m told what I’ve planted was Sativa, as per your suggestion maybe this could be one reason for late flowering! I’ll know the PH levels by tomorrow and will report back.

Again, thanks for your help, I/we really appreciate it.

Hey Ciara,
Just measured my PH last night with this cheap PH pen. It shows something around 6.75. Thought I’m not 100% sure of this pen being really precise cause depends on which side of Pod I push pen down in the soil it reads between 6.5 to just under 7!
What do you think? Tnx again

Hi Oliver!

pH Values for cannabis plants in soil/peat mixes are usually as follows;

Growth: 5.5 – 6.0 pH
Pre-bloom: 6.0 – 6.2 pH
Bloom: 6.2 – 6.5 pH

When given too high or low a pH, plants struggle to absorb certain nutrients, so it could be the reason for the leaf yellowing, as your plants are not in bloom yet. What kind of feeding schedule are they getting, and what kind of soil/substrate are they in? You can also shoot us some clear pics alongside any relevant information at and my co-workers will be able to get a clearer idea of what it might be.
Kind regards

I planted OUTSIDE in mid April. It’s about 2′ from the soil. I never topped it but it grew as if it did. Leaves are growing in between the leave and the stalks. Very green and very healthy. I just started putting it in the dark around 5:30 pm and putting it back outside 6:30 am.

Am I on the right track or not?

Being on the right track really depends on what you’re looking for in as far as results. What’s your climate like? Mid-April in the UK is vastly different from mid-April in Spain, for example.
If you’re trying to mimic the bloom period by putting it in the dark and back outside, you can indeed do it this way was long as they’re getting 12h of absolute, constant darkness, but one little mistake (a light shining when it shouldn’t, forgetting to take it in one day) can cause it to reveg and stress out enough to end up turning hermaphorite and filling its flowers with seeds. Also, you need to be sure that the sun is strong enough where you live for it to actually successfully flower, as it’s not just the hours of daylight that influence the results, the strength of the sun at that time of year is also incredibly relevant.
Kind regards!

Wassup nor cal novice grower here . I receive about 12 hours direct sunlight on my plants and the rest is indirect light or ambient light and I know that affects the plant so if I only have 12 hours direct sunlight and 4 to 5 more hours good indirect ambient light my plant will still grow vegetatively until the fall when the sun is up for less hours, right? So I guess my question is it’s July, I started early April my plants are less than 3 ft but are in the ground now and growing fast and I am wondering if they are growing so fast because they are in the preflower stage when they double (maybe triple I have some NYC Sour D Sativa dominant hybrid that are growing really nice ) in size because of only having 12 hours direct sunlight if it affects them and throws them to flower too soon. I hope they get bigger and don’t bloom till August at least.
Sorry for the long reply super stoned
Later thanks

Hi JV!
Contrary to popular belief, cannabis plants don’t flower when they only get 12 hours of direct sunlight, they flower when they receive 12 uninterrupted hours of darkness. This means that those hours of indirect sunlight are still being absorbed by the plant. However, the dip in hours can instigate the pre-flower period, and depending on the strain they can start properly flowering towards the middle/end of July/beginning of August. I’m not sure what your set up is like, but try and ensure that they don’t get light contamination at night (streelights etc) or this could cause them to reveg or stress out and become hermies.
Kind regards!

Hi Ciara,
We planted outside in Chicago on a balcony with unobstructed daylight all day. We have very hot and humid sun that starts in july and runs usually through mid October (it can be 70F-90F for that period of time with lots of consistent humidity). We planted seeds from a medical grade cannabis that was packaged between may and June and then planted those seeds in 6/20/20. They are all high-grade (from hybrid strains testing over 30%- wedding cake, G6, Super Glu, Gelato), and have all grown to 1’+ from the pots they are in- look green, lots of healthy leaves growing from every angle. They are all in organic soil. Looking for tips on following: do we move them on a 12/12 schedule or just let them grow naturally? Any plant food tips (we mix in food 1x a week)? Watering schedule (currently every 2-3 days and we test soil for moistness)

How long should we expect until we see buds (generally- 8 weeks? 12 weeks?) what do we do once we see buds? Let grow naturally or start a 12/12 schedule?

Thanks! Any advice is better than what we have now!

Hi Jake!

I have a few questions; did you get the seeds from cannabis flowers you were meant to smoke? If so, I wouldn’t buy from that vendor again, and I’d take care with the seeds as they almost certainly come from a plant that got stressed out, turned hermaphoridite and pollinated itself; deliberately pollinated cannabis doesn’t produce just a few seeds and it’s hard to accidentally pollinate without a bigger turnout than that. Simply keep an eye on them when they flower, as they might also be hermaphrodites.

If I’m understanding you wrong and they are store-bought or reputable seeds, then go ahead, everything sounds great! Whether or not to flip them to 12/12h yet (assuming that you’re certain they aren’t autoflowering) depends on how much space you have and how tall you’re willing to let them grow. When flowering, some strains can grow up to three times the size (most shoot up to almost double in size). Nitrogen is good for the veg period, and phosphorus and potassium are also good for veg and bloom especially. You could try a growth base with a decent NPK and a bloom base with high PK levels – here’s a suggestion: Crecimiento and Floración are what you’re looking for.

Your watering schedule sounds just fine – it really depends on your climate, the substrate and how much water it retains etc. If it’s working for you, perfect.
I would also recommend getting a pH meter if you plan on increasing the amount of food given to the plants, as the wrong pH can prevent them from absorbing any nutrients.
I can’t in good faith give you an estimate of when they’re going to finish flowering, as I don’t know what strain they are, so it could be anywhere between 8 and 14 weeks. However, I can tell you that the pre-flower starts once the days start getting shorter. If you’re growing on a balcony you also need to make sure that there is absolutely NO light contamination – cannabis plants begin to flower when they start receiving a cetrain amount of uninterrupted darkness, fully flowering at 12/12h light and dark, so if there’s a lamp post or your neighbor has they’re lights on until 2am, your plant might not ever flower.

I hope this could help, let us know how you get on!

Of all the articles I have read in the past and over the internet this (plant flowering stage) is by far the most informative, useful, and easy to read article I’ve ever seen. Thank you very much as I learned so much!! , Mike pann.

My plant growing since march but the flowering stage didn’t occurred when the flowering stage will occur

Hi Roman,
Flowering occurs in photo-dependant plants once they start receiving a certain type and amount of light. I need a bit more information here to help you out; where are you growing your plant? I assume it’s an outdoor set up. What strain is it? Sativas can take much longer to flower. Late-august is the cutoff period for many plants to begin flowering, so don’t start worrying for another couple of weeks.
Kind regards,

My plant (I believe Sativa) just started growing flowers on Friday (2 days ago). I germinated July 15 n now it is October 4. I wanted to start adding vitamins to the water, like Fe, B, Mn, Cu, Zn, N, P, and K. Should I add this to the water and if so, how often…..every time, once a week, once a month. I added to another plant and I think I killed it with too much.

Hi Abram!
You should use products that are specifically designed for cannabis flowering, most of them have a specific schedule to follow (a certain dosage per week/liter of water). Are you using any sort of base fertilizer, or a pre-prepared soil?

I am new to this growing thing 1st plant a random bag seed that has been on 24 hour light cycle for the last 2 months and I am super confused it has started what I assume is pre-flowering or maybe even flowering not sure. Could this be a autoflower strain and should I change my light cycle or keep it on the 24 hours of light all help is much appriciated.

Hi Mike
It sounds like they’re autoflowering, most likely. You can keep the light cycle, although some would argue that cannabis plants also need a dark cycle to develop correctly, say 18/6 or 20/4.

Hi ciara, just looking for a bit of advise! I have two plants which are outdoors and currently are about 3ft. It looks like there are buds! I clearly planted these to late in the year as they haven’t flowered yet! Is there anything I can do to save them before the cold gets them? I was aiming to start putting them in the darkness or should I move them inside? As you can tell, I am a novice grower!!

Hi Paul
This time of year is actually when plants should be flowering due to the natural light schedule; you’re going to want to protect them from the cold as good as you can. If you can bring them in at night and make sure they get 12h of uninterrupted darkness that’s great, (the dark is actually what causes flowering) but any minor variations in this can cause revegetation. To save them from the cold outside you can cover them at night to stop any frost or dew from settling, which can help avoid rot. If you want, you can shoot us a picture and we might be able to give you an idea of how long you’ll need to hold out. Also, if you can give us as much information as possible – strain, climate, temperatures, when you germinated etc. Contact Form

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