Flowering in Cannabis plants
Cannabis plants go through different stages of life before reaching harvest. These stages are: germination, pre-growth, growth and flowering. Depending on the phase of life, the care and feeding required can vary greatly, in this case we’re going to focus on the flowering period of cannabis and the changing nutritional demands upon our plants.
Phases of plant life
Pre-bloom phase of cannabis after the photoperiod change
At this point in their life cycle the plants are growing well, they’ve already occupied a little over half of the available space, and now is the moment to change the timer, so instead of using 18/6 we will switch to a 12/12 photoperiod and induce the plants to flower.
Depending on the type of plant and the genetics being cultivated, the time between changing the photoperiod to 12/12 displaying the first flowers can vary a great deal, and more if we are talking about differences between pure Indicas and Sativas.
Indica vs sativa
The stage between switching the photoperiod to 12/12 and seeing the first flowers appear is known as “the stretch”, as plants will tend to elongate and stretch in growth at this time. There’s a huge difference between the stretch of an indica plant and that of a sativa.
Indica plants stretch far less than sativas. The vast majority of Indicas originate in countries that have clearly defined seasons. The different seasons mean the plants can differentiate the period of growth from that of flowering by the hours of daylight and the time of year. More hours of sun mean vegetative growth and less hours mean it’s time for flowering, for this reason photo-dependent plants grow in spring and flower in summer.
Because Sativas usually come from tropical zones where there the photoperiod barely differs from 12 hours light and 12 of darkness, their stretch is greater than that of indicas since the plants have to be able to grow enough to ensure their offspring. The evolution of cannabis and its great potential to adapt to the environment has led to these different traits in indica and sativa plants.
Indica/sativa cannabis hybrid
So when you grow plants indoors you must consider their amount of stretch to enable you to give the correct vegetative growth period before switching to bloom and in this way control their final height, thus avoiding headroom problems.
In the case of pure indicas, during the stretch period they will usually only double their size. If you start with 30cm plants these will end up with a height of around 60cm at the end of the stretching period.
In the case of sativas, it’s important to remember that these plants can as much as quadruple their initial size, meaning a 30cm plant can finish at 1.20m in height. For this reason, when it comes to very long flowering sativas with a significant stretch as mentioned, the plants can be started at 12/12 from germination, avoiding excessive vegetative growth. They can also be pruned and trained if you want to grow in SCROG with fewer plants per m2.
2nd week of flowering, after 13 days
Sativa varieties with significant stretch:
Indica plants with reduced stretching:
Start of flowering in Cannabis plants
The start of flowering in cannabis plants occurs when the first pistils emerge from the female pre-flowers. At this moment the plant is indicating that its nutritional needs have changes and that from this time will need more phosphorus and potassium than nitrogen.
2nd week flowering – stretching but still no pistils
At this time the plants have already switched hormone production to focus on creating buds. The nutrient levels required will vary depending on the variety being grown, whether more indica or more sativa, it will require higher or lower levels of nutrients. This is also ruled by the duration of flowering in the genetics cultivated. If it is a more sativa-leaning plant with a long flowering period, nutrient levels are usually lower with a slower and more gradual increase week by week. If more indica-leaning plants are grown, having a shorter flowering, these usually consume a larger amount of nutrients in fewer weeks, having less time to finish their flowering.
We’ve talked about flowering indica plants and sativa plants, but in the case of growing cannabis hybrids you’ll need to take into account the genetic composition of the cross to be able to apply a nutrient schedule, depending if the hybrid leans towards the indica or sativa side, or if it falls in the middle between the two.
End of 2nd week flowering, the first buds emerge
When liquid fertilisers are used, their nutritional composition in terms of NPK and the ratio of these 3 macro nutrients should be taken into account. With commercial bottled nutrients, the same flowering fertiliser is used for both indica and sativa, but with a dose that varies according to the week. So we won’t give the same amount of nutrients to the indica plants as to the sativa plants over the same time period, but we’ll vary them as needed.
We can also find certain fertilisers like Powder Feeding that are formulated specifically for long or short flowering varieties:
The flowering period of indica plants is always shorter than that of sativas. This means that an Indica needs a higher concentration of nutrients to provide the maximum flower production. With sativas the opposite is true, with more weeks of flowering, the lower the amount of nutrients required weekly although if we check the total at the end of the crop both plants will have used almost the same amount of liquid fertiliser.
EC levels during flowering*
* This grow chart is indicative, the nutrient levels of each week may differ and should be adapted according to the genetics being cultivated. It’s possible to modify the table as needed, using this chart as a point of reference to raise or lower the levels as required.
Not all indica or sativa plants are the same, so their diet shouldn’t be either. For example, if two Indica strains are grown in the same crop, we may have to make small variations in the feeding between the different plants. However, often these nutrient variations are not very large, so we can find a midpoint that works well for both types of genetics.
The nutrient charts are very useful as a rough guide to the nutrient concentrations week to week but it’s only through looking at the leaves, stems, roots, etc. that we can truly determine if the plants are properly fed, or if they’re suffering a deficiency or excess of nutrients.
Here’s a graph from Aptus that explains the plants nutrient requirements according to the week of life.
The graphic shows the different nutrients that the plant requires and the difference between one type of nutrient or another (nitrogen, phosphorus, potassium or calcium) depending on the stage of the plant.
As we can see, in the first weeks of flowering during the stretching period, (table for indica of 8 weeks) the requirement for nitrogen is very high. During this stage the plant requires high doses of nitrogen to be able to stretch, grow and start to form the plant structure where the bud will appear.
The first pistils appear
During these two weeks, when nitrogen is necessary in large doses for indica plants, it is not so vital for sativas, as they can continue growing free of deficiencies with slightly lower levels, because they absorb less nutrients than indicas (as a general rule).
If the plant is deprived of nitrogen at this stage of life, the leaves of the lower part begin to yellow, typical of a nitrogen deficiency. If we enter flowering without enough nitrogen, (or any other type of nutrient) the plant won’t develop fully, neither in height nor in number of internodes per branch, giving fewer, smaller buds and consequently a lower yield.
Flowering period of Cannabis plants
Once in the flowering stage, we see that in the 3rd and 4th week of bloom plants require nitrogen levels more or less similar to the previous week. The peak addition of nitrogen has already been given and now it is decreasing in relation to the contribution of phosphorus and potassium.
From the 2nd week of flowering we see how the phosphorus is increasing, in the 3rd week twice as much is required as in the previous week, much like the N during the stretching phase.
3 weeks flowering – we start adding extra phosphorus and potassium
It is at this time when we can provide an extra boost of potassium, either through the use of PK13/14 or other, more concentrated products such as Monster Bloom, although ideally we would be to be able to supply each element separately, as is the case with Aptus fertilisers which provide each nutrient separately, allowing growers to adjust the level of each nutrient according to the week of cultivation. In fairness they’re not the best option for the novice growers but they are perfect for experts seeking perfection in the feeding of their plants.
From the 3rd week of flowering we see that calcium requirements are much higher. After N, P & K, this is the fourth most needed nutrient by the plants. Here at Alchimia we always recommend its use throughout the crop, from growth to flowering, so that the plant does not have deficiencies. Fertilisers are becoming increasingly concentrated, but without extra calcium added in the same bottle, so for this reason it must be provided separately and from the start to avoid imbalances.
If calcium is not used at this stage of life, Calcium deficiencies can easily be spotted. These are first seen in the oldest leaves, both in the lower and middle part of the plant.
33 days, the structure is created, now the buds begin to fatten
These deficiencies are seen in early stages as white spots on the leaves, similar to insect damage from spider mites or thrips. Later these marks degenerate, growing larger until a hole appears in the middle of the spot. When this deficiency has reached this advanced state, the only thing that can be done is to add extra Calcium with products such as Calgreen, and take preventive measures for next time.
From the 4th week of flowering we see how magnesium also comes into play. Calcium and magnesium should always go hand in hand. If you raise the magnesium you must raise the calcium and vice versa. Keep in mind that you must add 1 part of magnesium for 2 of calcium. Fortunately calcium supplements are usually accompanied by magnesium in the same bottle to avoid nutritional imbalances. If this is the case, you just have to worry about raising the EC levels with Ca and Mg to the necessary point according to the week of cultivation.
38 days, still increasing EC and fattening bud
Between the 4th and 5th week of flowering we see how potassium comes into play. Now the plant has already formed the structure on which the buds will grow until the end of flowering.
The weekly increase of potassium is essential so that the plant can continue to create more flowers and finish flowering with fully developed buds, covered with resin and with some remarkable aromas and flavours.
According to the graph we refer to, from the beginning of the 4th week of flowering until the middle of the 6th week, potassium is increased progressively. This is what we call the fattening phase, where the plant focuses on the massive creation of flowers and the secretion of resin. This is the point of the grow when plants require the most nutrients, in particular potassium, although it must be accompanied by phosphorus to ensure a balanced metabolism of the plant.
45 days, buds are fattening with the resin building up
From the 7th week of flowering the overall nutrient levels go down little by little, because the whole structure of the plant is already created, the buds are well formed and only the ripening of the flowers remains, allowing the trichomes to develop fully and reach the optimum point of maturation, at which time the plants can be harvested.
In the penultimate week of cultivation, or the 8th week for 9 week plants, the nutrients input is reduced or completely removed. It must be said that to go well, you have to reduce the nutrients little by little to avoid stress that could lead to the formation of small male flowers at the end of the cycle. If all the fertiliser is removed at the same time, as in the case of hydroponic grows, the EC levels between the plant and the substrate vary greatly and the plant will try to equalise both ECs (internal and external of the substrate) by evacuating all the salts it can, leading to an unstable metabolism and a consequent stress that could cause unwanted hermaphroditism in plants.
Mature bud after 9 weeks flowering
End of the grow and ripening of cannabis flowers
Now the plants no longer need high doses of nutrients to finish ripening, we’ll avoid adding more fertilisers. To know when to harvest your plants you must examine the trichomes carefully and wait until the vast majority of them have a cloudy white colour and some of them an amber, red or other colour.
Mature trichomes ready for harvest
Before being harvested, the vast majority of the cannabis plants leaves should be a yellow colour. This indicates a lack of nitrogen and other nutrients in them, which assures us that the buds will have a superior flavour and aroma since there will be no interferences caused by the salts accumulated in the leaves, stems and flowers.
It should be remembered that if the plants are harvested with a very strong green color, the taste of the flowers will neither be faithful to the variety nor pleasant to smoke, lacking the proper organoleptic properties, a great shame after weeks of hard work and dedication.
Without a doubt, flowering is one of the crucial stages in cannabis cultivation. In this article we tell you everything you need to know about the blo
Timeline of the Cannabis Flowering Stage (12/12 to Harvest)
Table of Contents
Week 1-3 – Transition to Flowering
Week 4-6 – Buds Start Fattening Up
Week 6-8 – Buds Ripen, Pistils Darken – some strains spend longer in this stage
Week 8+ – Flowering Ends, Final Flush, Harvest
Introduction to the Cannabis Flowering Stage
During the phase of life known as the vegetative stage (the first stage of life for marijuana), a cannabis plant grows about how you’d expect… like a weed! In the vegetative stage a cannabis plant only grows new stems and leaves, and can grow several inches a day with the added ability to recover from just about anything!
Even if you run into major problems in the vegetative stage, you can bring your plant back from the brink of death simply by addressing the problem and giving your plant some TLC.
In the vegetative stage, your cannabis plant only grows stems and leaves and is resistant to problems. It grows like a weed!
However, things aren’t so rosy in cannabis flowering stage. In the flowering stage your cannabis plant grows very differently, and is much more sensitive to problems. The tricky thing about the flowering stage is that you don’t have much room for error and big mistakes can lower your yields.
In order to maximize your yields, it’s important to know what to focus on during each part of the flowering stage. It’s also really helpful to know what to expect so you know when something is going wrong!
The Dance of the Flowering Cycle
This flowering stage “walk through” will explain exactly what to expect week-by-week while your plant is making buds, and it’ll tell you what you need to do to ensure you get to harvest with the best bud quality and yields possible!
Week 1-3: Transition to Flowering
When growing cannabis indoors, the flowering stage begins when you change your grow lights to a 12/12 light cycle (12 hours light, 12 hours darkness each day). Getting those 12 hours of uninterrupted darkness each day gives your plant the signal that it’s time to start flowering. In a way the plant “thinks” winter is coming because the days are getting short.
Note: It’s common to think that a cannabis plant getting 12 or less hours of light is what initiates flowering, but it’s actually uninterrupted darkness that does the trick! If the plant gets any light during the dark period, even for just a minute, it won’t make buds! In fact, a flowering plant may even revert back or express hermaphroditism if it gets any light at night!
Outdoors, it’s also the days getting shorter that cause a cannabis plant to start making buds in late summer, but outdoor buds develop on different schedules depending on the local climate. This tutorial is meant to explain how a cannabis plant usually develops when grown indoors, since that is done under controlled conditions, and plants tend to grow the same way.
For the purposes of this tutorial, the flowering stage starts the day you switch to 12/12
Autoflowering strains of cannabis don’t need special light periods to start flowering, however the cannabis flowering timeline in this tutorial is a good general guideline for indoor auto-flowering strains, too. Their “vegetative stage” lasts about 3-4 weeks, so as long as you start counting at week 3-4 from seed (when they start getting their first pistils) this flowering timeline will generally apply to autos too, though sometimes they finish up faster.
During the first few weeks after being switched to a 12/12 schedule, your plant will be growing like crazy and rapidly gaining height. In fact, a cannabis plant can (and frequently will) almost double in height after the switch to 12/12. This period of super-fast and often stretchy growth is sometimes referred to as the “flowering stretch.”
Example of flowering stretch – what to expect
Pre-Stretch – just before 12/12
Post-Stretch – 4 weeks after 12/12
Although your female plants will start sprouting lots of white pistils, they usually won’t start growing “real” buds with substance quite yet. If you’re new to growing cannabis, it’s very important to note that only female cannabis plants make buds.
Did you know you can figure out if a plant is male or female while it’s still in the vegetative stage?
If your plant is male, it will start growing distinct pollen sacs and should be removed from the grow room immediately to prevent it from pollinating your female plants and causing ‘seedy’ buds. Learn where to get feminized (all-female) seeds online so you don’t have to worry about male plants.
Remove any plants growing pollen sacs instead of pistils, because they are male and won’t make buds. Plus they can pollinate your female plants and cause them to grow seeds! What if my plant is growing both pistils and pollen sacs?
Female plants should be growing pistils wherever a fan leaf meets a main stem. They look like white wispy hairs emerging from the joints
During the first few weeks of the flowering stage, you will see bunches of single leaves forming at the tops of your main colas (like in this pic). Soon white pistils will start coming out of the middle of the bunches, and they will become your main buds!
During week 1-3 of the flowering stage, your plant will mostly be producing new stems and leaves as it grows taller. Right now your plant is still very resilient and can handle problems just like in the vegetative stage. However, it’s still very important to avoid problems and take great care of your plant!
As part of the stretch, your plant will be growing out its bud sites. Stunting growth at this point could cause the plant to make smaller and fewer bud sites than it would if it were healthy and growing fast.
If you have more room in your grow space under the light to spread your plants out, or if you are running out of headroom, it is important to gently bend stretching stems down and away from the center of the plant to help maintain a flat canopy (a technique known as low stress training).
During the stretch, gently bend new stems down to try to maintain a flat, even canopy
If you keep up with it during the stretch, you can prevent any one stem from getting much taller than the others
When stems are new, they are flexible and easy to bend, but they quickly harden up and turn woody. By keeping a close eye on your plant and bending any too-tall branches down and away from the center of the plant as soon as you can, you will maximize your yields since that flat shape will most efficiently use your grow lights. If all your main bud sites are spread out and about the same height, you can increase your yields by up to 40% or more!
Spreading out your bud sites and maintaining a flat canopy can increase cannabis yields by as much as 40%…or even more!
At this point, you only have a few weeks left until you lose the ability to do any further training, so don’t miss this last opportunity to control the shape of your plant, especially if you’re running out of room!
Week 3-4: Budlets Form
The mad stretching of the first few weeks will start to slow down in week 3-4, but your cannabis plant will still be growing upward. At this point you’ll actually start to see real buds instead of just hairs (I like to call them “budlets” during this stage) and all the pistils will be white and sticking almost straight out.
“Budlets” start forming where buds will be, with white pistils sticking straight out
Your plant is going to start getting a little picky about the environment and nutrients in week 3-4 so it’s important to keep a close eye on your garden. You need to make sure your plant stays healthy all the way to the end of the flowering stage, and you’ve still got more than a month to go so you don’t want your plant to run into any major health problems now!
Be especially aware of leaf symptoms, for example: discolored/yellow leaves, or if your plant starts rapidly losing leaves. It’s completely normal to lose a few leaves at this stage, especially leaves that aren’t getting light (which often look like they may have a nutrient deficiency and then fall off, but it’s just your plant cannibalizing the leaf since it isn’t getting any more light). That being said, overall your entire plant should still be lush and green in week 3-4 while your budlets are forming.
As your plant continues through the flowering stage, it’s normal to see a few yellow or discolored leaves near the bottom of the plant, especially in the places where the leaves are no longer getting light. This isn’t anything to worry about if it’s just a few leaves as the plant is putting its energy to the top of the plant and the buds.
But it’s not normal for your plant to be yellowing or losing leaves rapidly like this
Another thing to be aware of is nutrient burn. This is what happens when you give your plants too-high levels of nutrients – the tips of all the leaves actually get “burned.” While a little bit of nutrient burn won’t hurt your plant, it’s important to try to avoid it if you can. Your plant can never recover the parts of the leaves lost to nutrient burn, so if you accidentally give too much nutrients in the future, the burning will start “climbing” up the “fingers” of the leaves. Cannabis leaves tend to look much less appealing/pretty as more of each leaf gets burned. However, even cannabis plants with severe nutrient burn can produce good bud, so don’t give up if you run into thi problem!
Try your best to avoid nutrient burn (burnt leaf tips caused by too-high levels of nutrients), as it can only get worse as the flowering stage continues
When nutrient burn starts getting bad, it can actually start discoloring your sugar leaves (the small single-finger leaves emerging from your buds). If nutrient burn reaches the base of the sugar leaves, you won’t be able to trim it off at harvest so your buds will end up with yellow/brown spots where all the leaves were burned.
Nutrient deficiencies can also cause the same problem if left unchecked. This doesn’t necessarily affect the potency but buds don’t look as good as they could have.
So to grow bud you’re proud of, you’ll want to be aware of avoiding nutrient burn from the beginning. Since your plant isn’t really growing many more leaves, you need to really care for the ones it has left.
If they haven’t already, your plants may start to smell!
Some strains like Blue Mystic and Northern Light are known for having relatively low smells, but many strains can start getting pungent quickly!
Week 4-6: Buds Start Fattening
Your budlets are fattening and soon you will have buds with substance! They will still have nearly all white pistils sticking straight up in every direction, but the buds themselves will be getting fatter every day.
By weeks 4-6, the stretch is almost over and you no longer need to pay attention to training your plant. Instead of trying to keep the colas down, from now on you’re doing the opposite – trying to hold any buds up if they start getting too heavy for your plant!
If you’re having trouble fitting your plant in your space within a safe distance from your light, your training options can start looking very grim.
If your plant has grown into the light, you may have to consider last-resort solutions like supercropping (a high-stress training technique of forcing stems to bend at a 90° angle) which you normally should never do this late in the flowering stage.
Since you don’t get many more new leaves, you need to think of your remaining leaves as armor – insurance against any nutrient or leaf problems.
Although you don’t want an excessively leafy plant, and strategic defoliation (for advanced growers) can be helpful to expose bud sites, it’s important to make sure that you let your plant keep enough leaf coverage to power the growth of buds. It may need a little extra help if something happens!
Although defoliation may be used to expose buds sites, make sure your plant still has enough leaves (“armor”) to last until the end of the flowering stage to power the growth of buds, and as insurance against any possible nutrient or leaf problems.
Although most of the pistils will probably still be mostly white by the end of week 6, the buds are getting bigger and denser every day!
Week 6-8: Buds Ripen, Pistils Darken
From now on your plant won’t be making any new leaves or stems. It has completely switched gears away from vegetative growth and all its energy will be focused on growing buds from now until harvest.
It’s normal for some of the bottom leaves to begin to turn yellow as the plant continues to put its energy in the leaves and buds getting the most direct light, though the plant should still be mostly green from top to bottom even in week 6-8.
At this point, your plant may start getting much more picky and sensitive to nutrient problems, including those caused by incorrect pH at the roots. Now is not the time to slack off on caring for your plants!
If your leaves are already turning yellow in week 6-8 it’s too early! Early leaf yellowing is likely caused by either a nutrient problem or light burn (which are both much more common in marijuanas flowering stage). React quickly to problems so you don’t hurt your yields!
Another common problem to watch out for at this stage: if you see a whole new bud or “spire” emerging out of the side of an old bud that’s already developed, it’s usually a sign of heat or light damage.
“Foxtailing” like this is caused by too much heat or light – it’s not normal bud growth! If you see this it means you need to control your temperature and light levels to prevent further damage!
From now until harvest it’s extra important to avoid too-high levels of light or heat because (in addition to foxtailing) this can discolor/bleach/burn your buds and may even “evaporate” away some of the THC / potency.
If things are going well, your buds should be really hitting their stride at this point. They will grow in size significantly over the next few weeks!
Week 8+: Flowering Ends, Final Flush, Harvest
Home stretch! You’re so close! To make sure things go smoothly until harvest, treat your plant like a movie star and attend to its every need! Very few strains of cannabis are ready to be harvested before week 8, but now we’re at to the point where some short strains are getting close to being harvest-ready!
Many growers do a final flush, which involves giving only plain water to your plants (for a few days up to a few weeks) before harvest.
Once you’ve reached week 8, buds are fattening quickly. Trichomes and pistils are maturing, though new pistils may continue to develop on the buds as they grow.
You are basically just maintaining your plant until harvest. Different strains are ready at different times, but from now on you’re going to pretty much treat them all the same. Keep a close eye on the buds, pistils and trichomes as a whole to help you figure out the best time to harvest to get the effects you are looking for.
Now is Probably the Best Time to Take Bud Pics!
Quick Tip: Want to take better bud pics? Try taking a picture of the bud in the dark with your camera flash on. Learn more tips for taking great bud pictures!
Just around 8-10 weeks is when you get to see the buds in their full glory. It’s also when the smell of cannabis often starts to get overpowering!
Your plants are probably STINKING up everything around them!
At this point it’s completely normal for your plant leaves to start yellowing, sometimes rapidly. As long as the yellowing isn’t affecting your buds and you’re very close to harvest then it’s completely normal. You probably can’t prevent this type of yellowing no matter what you do with nutrients because this is just what a cannabis plant naturally does as it’s wrapping up the flowering stage.
After Week 8 it’s normal to see leaves turning yellow, in fact there’s not much you can do to prevent it. As long as it’s close to harvest and the yellowing is not affecting your actual buds it’s ok!
Raising nutrient levels at this stage is not recommended as it won’t stop the yellowing and can possibly prevent your buds from fattening up as much as they could have (cannabis wants relatively low levels of nitrogen in the flowering stage for proper bud growth).
If buds start getting too heavy and fall over, special tools known as plant yo-yos (pictured to the right) can be hung from the ceiling and will hook around your buds to gently hold them up without damaging them.
Many growers choose to give their plants a 2-week flush before harvest to help make sure the plant has used up any additional nutrients that may affect the taste or smell of the buds.
These buds are ready to start flushing – white pistils have nearly all darkened and curled in
(learn exactly when to harvest so your buds produce the right effects)
Sometimes you’ll need to harvest your plant early due to life situations, or because the plant is unhealthy and buds are starting to look burnt or discolored. If your buds look completely done, and you’re seeing leaf symptoms getting worse, it’s often better to harvest a little early to ensure the best possible quality given the situation.
You may want to harvest your marijuana buds early if they’re starting to get damaged by nutrient or other problems. Sometimes it’s better to cut your losses than let your buds continue to get beat up! If you harvest your plants too early you can improve many unwanted effects by curing them. For example, these buds probably should be harvested before the buds get any further damage.
Harvest buds early if they’re getting damaged!
Harvest day is the best day!
(well, until the day you try your buds for the first time!
You can maximize marijuana yields by focusing on the right factors each week of the flowering stage. If you know what your cannabis should look like week-by-week, you'll also be able to quickly tell if something is going wrong!