when to stop fertilizing cannabis

A Guide To The Final Weeks Of The Cannabis Flowering Stage

  • 3.3. Ventilation
  • 4. Tips on keeping your noise level down
  • 5. The final 2 weeks of flowering
  • 5.1. Checking pistils and trichomes
  • 5.2. Ending nutrient cycle / flushing
  • 5.3. Trimming

After weeks of mounting excitement, the long-awaited moment for harvest is finally within arm’s reach. Plants have moved quickly through their first three weeks of flowering, undergoing a real transformation. White pistils have emerged, forming the foundation for the buds over which you have been drooling in anticipation.

After flowering for around five weeks, there are now solid cannabinoid-dripping buds found on your plants, which are still growing and expanding—but the biggest part of their development is already behind them. Your flowering plants should still be lovely and green as your ladies are firing up their THC production on all cylinders. The leaves around the buds are also becoming stickier, and the many THC-rich trichomes produced on them will later be used for making a nice lump of hash.

As harvest is just around the corner, it’s good to keep any fan leaves you trim from your plants as they too contain THC and can be later used to make hash or kief. More about this in our harvest section.


As long as you have taken good care of your plants, watering them well and not administering too many nutes, the very sight of your garden should be enough to get your mouth watering. No matter if you’re growing indoors under a grow light or outside under the sun, the many resin glands all over your plant sparkle as they dance in the light.

Flowering cannabis plants have additional demands for phosphorus and potassium, so it’s worth giving them PK 13/14 as extra stimulation for flower development. These substances are found in every basic nutrient mix, but in lower volumes. With some additional PK 13/14, you can get denser and more compact buds. If you are new to growing, it can be best to start with basic cannabis nutrients with NPK (nitrogen, phosphorus and potassium), plus a PK 13/14 additive for flowering and a root stimulator.

With these basic products, you will be able to grow yourself a great harvest. If plants are healthy, then you can apply a number of additional products like a bloom stimulator and/or boosters and enzymes. If your plants are struggling, however, it doesn’t make much sense to add these often-expensive products as they can only help as long as everything else is going smoothly. As newcomers tend to make mistakes, these products will in all likelihood be overkill and are not really necessary.

Enzymes take care of reducing the salt accumulation in your soil that can come from feeding additional nutrients. After using the same bottle of nutrients for a while, you will sometimes notice a white, crystalline deposit on top of it. These are the same salts that also accumulate in your container over time. By growing with organic nutrients, you can reduce the accumulation of these salts, as these contain considerably lower levels than mineral nutrients.


  • Can benefit bud development
  • Can maximise yield
  • Can benefit potency and taste
  • Not very useful for new cannabis growers
  • Not needed if you’re already using good basic cannabis nutrients
  • Can increase the chance of overfeeding
  • Added expenses
Easy Bloom Booster Tablet


The ideal pot size will depend on how many plants you are growing and the number of days/weeks of veg you give them. These two factors are often linked to each other: If you have a larger number of plants to pack in a small room, then you need to use smaller pots and give the plants less time in growth. For example, if you have 16 plants in one square metre and give them a veg period of between 5 and 9 days, the plants will be happy with 5 litre pots.

If you were to reduce that number to 10 plants and allow them a somewhat longer growth period of 10–14 days, then the plants will need 7–11 litre pots. The fewer plants you grow, the larger the pots you can use and the longer you can give them for growth. Ultimately, yields will end up more or less the same whichever method you use. The only difference is in the amount of time needed to reach harvest.

Someone with 16 plants with a common flowering time of 8 weeks will need about two months and five days from start to finish.

If you have 10 plants, then it will take you about two months and two weeks. In other words, it will take you longer to get the same yield. Commercial growers in particular find it important to get to harvest in the shortest time possible. That is why they often cram 20 plants in a square metre and only give them 1–3 days for veg, depending on the medium they are using. This way, they can harvest every two months if they use a strain that does the business in 8 weeks. In general, plants with more of a sativa influence take longer to bloom than those dominated by indica genes.

Your selection of cannabis seeds could be guided by their required flowering time, as well as other factors such as their taste or effect. If you prefer a more “up” high rather than a heavy body stone, you will likely want a sativa-dominant cross that takes a little longer to bloom, typically between 8–12 weeks. Even so, do not forget that every plant is unique. There can always be early-blooming, medium-long-blooming, and late-blooming individuals in your cannabis garden, even from the same strain and the same seeds.

If you read somewhere that a particular variety is “very uniform”, this means that there will be little difference in harvest time among the same batch of cultivars. In this case, you can expect 80% of plants to end up finishing around the same time. But there are also strains with less uniformity and larger differences in flowering time. You should take this into account when choosing a particular cannabis variety.


Flowering cannabis plants require a specific set of climatic conditions to thrive. No matter the stage of growth, cannabis needs good ventilation to ensure robust health. In terms of humidity, flowering plants need significantly less moisture in the air than seedlings or early vegetating plants.


For the flowering stage, you should aim for a relative humidity of around 40–55%. The easiest way to achieve this, without getting an air humidifier, is to attach a dimmer switch to your exhaust fans. With a dimmer, you can vary the speed of the air extraction and therefore influence the climate in your grow room. Moreover, an exhaust without a dimmer always operates at full speed even if it isn’t needed. This will not only waste electricity, but also creates more noise than necessary.

With a simple dimmer, you can set up your exhaust to suck away less moist air during the first few weeks of vegetative growth so your young plants develop more quickly. As soon as the plants become bigger, you can let the exhaust do a bit more work as they start to produce more moisture. In fact, once your plants are in full bloom around the fourth week of flowering, you should make sure your plants receive no more than the 50–55% RH threshold.


Too high humidity in the last few weeks of growth can lead to the development of mould and mildew on your buds. This is because the moisture gets trapped within, creating an ideal breeding ground for a host of pathogens. It’s worth noting that strains that develop particularly hard buds have a greater risk of developing mould than those with an airier structure.

Once you’ve spotted mould on a bud, you can consider it lost, as the mould has long been wreaking havoc inside before it became visible on the exterior. When you have a well-developed bud and you see an odd, yellow-coloured sugar leaf sticking out from the middle, you are best off carefully pulling it out. If the leaf comes free easily, it is likely the bud is infected. So, you better make sure that you have good airflow around your plants in the last weeks of flowering. When in doubt, invest in a dehumidifier.


Aside from humidity level, moving air in general makes a big difference in the health of your plants. An exhaust system or a stand fan not only ensure that warm, damp air is removed, but can also make sure that fresh, CO₂-rich air is sucked into the space. They will need this fresh air to keep growing and blooming at an optimal rate. If you grow in a cupboard or a similar small space, you may not even have room for a fan, but can at least keep the door ajar to allow for some fresh air exchange.

With your exhaust connected to a dimmer, you can master the environment in your grow room much better for larger yields and a top-quality harvest.


If you are growing indoors, noise from your exhaust fans could be a problem. But no worries, there are a number of ways you can reduce this. As we already mentioned, the first thing you can do is get a dimmer to control the speed (and therefore noise) of your fans. Simply reducing them to 75–80% can make a big difference.

This also means that it’s always better to go with an exhaust that has a somewhat higher capacity than one with too little. If you have a large exhaust, you can just dim it down to half the speed so it won’t make much noise. If this doesn’t help, you can also try to put your fans inside an insulated box. When you connect a length of pipe to the fan, this can make a big difference as the sound is not immediately dispersed, but first runs along the pipe.

If all of these things still don’t help to keep the noise down, you can also fit a sound muffler to your fan. They look similar to a carbon filter, with a metal tube that you attach to your fan. You can then just fit a normal duct pipe to the muffler and should finally be able to enjoy greatly reduced noise.


If you grow strains with an average flowering time, the majority of bud development will occur by the 6th week of bloom. In the last two weeks, the buds will mostly be ripening and not really growing much more in size. At this point, the previously white pistils on the buds will now slowly turn amber-brown. Depending on the strain you are growing, as well as environmental factors, the ripening of the buds can take a variable amount of time to finish.


One way to get an idea of when to harvest is by inspecting the pistils. Once around 80% have moved from white to brown, it’s likely that your weed is ready for the chop. keep in mind, however, that other factors can influence the browning of pistils, so if you desire more accuracy, consider the trichome test. Checking the trichomes is the most surefire way to know when it’s time to harvest.

Trichomes are the tiny, sticky, crystal-like outgrowths on the buds and leaves that produce the good stuff from cannabis such as cannabinoids and terpenes. Trichomes are tiny, so you need a loupe or a microscope to see them. They look like mini mushrooms with a tiny bubble head on a stem.

As your cannabis is flowering, these trichomes change colour, moving from crystal clear to milky-white, and then finally to an amber colour. If the tiny “mushroom” heads are all still crystal clear, then there is still a ways to go. By the time most of the trichomes have turned a milky white colour, your cannabis should be just about ready. But know that when you choose to harvest can also affect the resulting high. Harvesting when trichomes are mostly milky, with just a few amber heads, usually results in more of an “upper high”. Wait for more amber heads to appear, and the effect will become more sedative. Many cultivators choose to harvest when around 15% of the trichomes have turned amber and the rest are milky-white.


At the end of flowering, there may still be plenty of nutrients left in the growing medium and leaves. To prevent harsh tastes and unsavoury chemicals making their way into your stash, you can simply stop feeding any nutrients in the last 1.5–2 weeks of flowering. If you are growing hydroponically, you shouldn’t stop feeding until the last five days as they will be growing in water only. But when you are growing in pots with soil or coco, there can be lots of nutrients left that we want the plants to use up.

Once you stop feeding, your cannabis plants will take up whatever nutrients remain in the growing medium. Then, they will start to suck their leaves dry to get the nutrients out from these as well. This is why your cannabis plant will turn yellow in late flowering. Not needing to feed your plant in the last couple weeks provides an extra perk—it saves you money on nutrient expenses.

During this two-week period, you can periodically flush your medium with pure, pH-balanced water to get rid of any mineral deposits lingering in the soil. Make sure to drench the soil to full capacity so it picks up the nutrients, then drench it again and let it all drain out the bottom for a couple minutes. However, be sure not to water too frequently, as you don’t want to hurt your plant in its home stretch to harvest.


When leaves turn yellow in late flowering, you don’t need to worry that something is wrong, as this is entirely normal. It is actually a good sign, as it shows your plants are using up any excess nutrients.

Some growers trim their plants slowly over the last weeks or days before harvest, removing some of the larger leaves to allow more light to reach the lower buds. As yellow leaves will normally shrivel and fall off on their own, this can save you some trimming work. Otherwise, in this late stage of flowering, feel free to trim off any yellowing leaves as they have fulfilled their purpose. This will also make it easier to harvest.

Part. 1: The Germinating Phase. Give your seeds the best possible start in life by reading our definitive guide to germination.

Part. 2: The Vegetative Phase. The germinated seeds peak out above ground and immediately spring up.

Part. 3: The Cannabis Blooming Phase. Just another couple of months of blossoming we will be ready to get our sheers out of the cupboard.

Part. 5: Harvest Time. Learn every step of the cannabis harvest and post-harvest process.

Learn all you need to know about the final weeks of the cannabis flowering stage, including ideal climatic conditions, how to know when to harvest, and more.

How and When to Flush Marijuana Plants

Flushing the marijuana plants before harvest can make all the difference in the best bud, or horrible hash. This small task is simple and super easy to do. Just add water! Be very careful, as the timing of the flush can play a critical role in this process.

The what, how, and why of flushing cannabis plants.

  • 1. What is flushing?
  • 2. The best time to flush your cannabis
  • 3. Prevent nutrient lockout before it becomes an issue
  • 4. Enzymes to the rescue
  • 5. How to properly flush cannabis
  • 6. How to flush hydroponic plants
  • 7. The outcome of flushing your cannabis
  • 8. When to avoid flushing your plants
  • 1. What is flushing?
  • 2. The best time to flush your cannabis
  • 3. Prevent nutrient lockout before it becomes an issue
  • 4. Enzymes to the rescue
  • 5. How to properly flush cannabis
  • 6. How to flush hydroponic plants
  • 7. The outcome of flushing your cannabis
  • 8. When to avoid flushing your plants

You have finally finished your grow, ending up with a beautiful plant covered in tremendous bud, now dried, cured and ready to go – yet something is not quite right. You can hardly get the stuff to burn, and when you take a hit, it feels like a mule kicked your lungs as you end up coughing for your life! The taste is harsh and disappointing. If this is a situation you have experienced, chances are your plants were not flushed properly before harvest.

This less than pleasant smoke is caused by nutrients and minerals used during growth still be present in the plant, altering the way it burns. Flushing removes these remaining nutrients, improving the quality of the experience. Fortunately, flushing your cannabis is an effortless and easy task, and will have you producing smooth and delightful bud in no time.


The act of flushing a plant is using plain water to actively remove any nutrients in the soil. A large amount of water is passed through the soil and drained away on a regular basis. Any minerals and nutrients present in the soil are washed away over time by the water, leaving the soil clean.

Why would you want to strip away all the minerals from the soil? Isn’t this hurting your harvest? It actually helps your harvest a significant way. When the nutrients are removed from the soil, it forces your cannabis plant to use up any remaining nutrients still present in the plant. It is a lot like the human body. We take in a lot of food and, what we do not use is turned into fat. In extreme situations where food is scarce, the body relies on this stored fat for energy.

As flushing forces cannabis to use up any remaining nutrients left in the plant, none should remain to taint the use of the harvest bud. However, if done to early, it can leave your plant unhealthy, so timing is key.


Flushing is usually commenced two weeks before harvest. If the plant has an eight-week flowering period, the flushing will need to take place six weeks after the start of the flowering stage. It is best to take a close look at the trichomes on your plant to assess when your cannabis is likely to be ready for harvest. If the tiny trichomes are just beginning to turn from clear, to a cloudy and milky colour, this could be a good indication that the plant can begin flushing. It should be timed so that the majority of trichomes will have fully changed to the desired colour for harvest after two weeks – this gets easier with experience, so stick with it!

Flushing can also be a good way to reset the soil while a plant is in the vegetative state. Sometime, growers will accidently overfeed their cannabis, causing the tips of the leaves to begin changing colour and shriveling. This is called “nutrient burn”. Flushing the soil can remove the excess nutrients, helping avert the problem. However, it is a drastic measure at this phase of growth, so be sure that the problem is nutrient burn, and not something else first.

Flushing isn’t exclusive to harvest time. The technique can also be used in the vegetative phase to remove excess nutrients from the soil. Sure, nutrients keep your plant healthy and ensure optimal yields, but too much can cause nutrient lockout—a state where plants can’t access nutrients.

Nutrient lockout can be caused by both salt buildup and incorrect pH levels. It can be solved by flushing affected plants with plain water. The fluids push nutrients out of the soil and wash away the buildup, allowing roots to once again freely uptake nutrients.


Nutrient lockout can be an inconvenience at best and devastating at worst. As the old saying goes, prevention is better than a cure. It’s best to take steps to avoid nutrient lockout as opposed to tackling it later down the line.

Preventing nutrient lockout can be accomplished via routine flushing. By flushing your plants once before flowering begins and once halfway through flowering, you’ll minimise the chances of nutrient buildup.


After flushing to counter nutrient lockout or before harvest, you might still notice your plants are dark green in appearance—a sign of excess nutrients. In this case, some growers elect to add enzyme-rich formulas to the soil.

If you don’t remember anything from biology class, enzymes are proteins that catalyse reactions. They help to flush out the soil by breaking down starches, carbohydrates, and nutrients. There’s a variety of products on the market that contain effective enzyme formulas.

If water isn’t doing it for your plants, these small proteins will make lingering nutrients budge!


Flushing your cannabis plants is a straight forward process. Whenever you would normally feed, you flush instead. Untreated tap water is all you need to use for flushing, just be sure to make sure the pH is at a safe level for cannabis. Most well water contains a healthy pH level and will not need treatment, but if it is necessary for you to add treatment to adjust the pH of your flushing water, feel free to do so. The pH adjustments will be the only thing you will need to be concerned about.

Flood the soil with as much fresh water as it can hold. Leave this for a few minutes to allow all of the nutrients to be picked up, the flood the soil again to flush it all away from the plant. If you are indoors using pots, notice the colour of the water that is draining from the bottom of the pot. It will be stained and look dirty. This is where a TDS (total dissolved solids) meter will come in handy. If you were to collect and measure the TDS of the “drained off” water, it would be around the measurement of 1300ppm, which is pretty high. It is important to keep flushing the plant until this number drops to a level of 50ppm, or at least until it is close to matching the TDS of the fresh water that you are using to flush with. The colour of the draining water will lighten up and appear to be cleaner. You want to get as much of the dissolved minerals away from the plant as you can.


Flushing hydroponic plants is much easier than removing nutrients from a soil medium. Hydro growers can simply drain their system and replace the water with plain pH-balanced water instead.

Flushing hydroponic plants is also a much shorter process. Once the water supply has been switched out, hydro plants won’t have access to any external nutrients. Because of this, you’ll only need to flush plants for two days.


After the bud is harvested, take the extra time to cure the bud to its highest potential. A proper cure will cut back even further on that harsh edge, removing aspects like excess chlorophyll. You will be amazed at the difference this small effort can do to your product. All of your hard work will show in that first inhale of that silky smooth smoke that hits the back of your throat as soft as honey. This is nature at its smoothest. You can increase the quality of your cannabis by just adding water!


The only time it’s recommended to avoid flushing is when you’re growing in an amended organic soil or super soil. This medium is carefully developed over time to harbour beneficial microorganisms such as bacteria and fungi. This delicate biodiversity can be washed away and damaged by flushing.

Then again, the lack of flushing shouldn’t be an issue as no synthetic or external nutrients are added to this medium. Instead, plants rely on microorganisms to break down organic matter and deliver it to the roots.

The HI-98107 pHep pH tester provides fast and accurate pH readings. The easy-to-use device is designed for non-technical users, and can help both novice and advanced growers measure water pH.

HI-98107 pHep pH tester provides fast and accurate pH readings. The easy-to-use device is designed for non-technical users, and can help both novice and advanced growers measure water pH.

Flushing cannabis before harvest can makes the difference between a smooth or horrifically harsh smoke. Here is how to do it.