Flushing your medium

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Flushing your medium is the act of pouring water, through your soil or soil like medium to remove excess salts, and nutrient build up, or to prepare for harvest. Things will differ slightly, depending on your grow medium, and choice of nutrients.

If you’re growing cannabis organically, there is no need to flush your plants, as there are no salts to remove. You can learn more about organics in our organics section.

If you have any questions about flushing your cannabis plants, then feel free to ask us anything in our cannabis growers forum

Preparing water for the flush

To make sure you don’t shock the roots of your plants, water should always stand for at least 24 hours at room temperature before feeding. This allows the water to reach a good temperature (18c is optimal), and also allows chlorine to evaporate.

This is known as “degassing”. Degassing will also give the microbes in your medium a better chance of survival. Chlorine will kill a lot of the bacteria that are very important to the health of your medium. 

Adding an air stone to your water to help keep a good supply of oxygen is recommended whenever you let water sit. That air will be dispersed through the water and given to the plants roots. It will also keep friendly bacteria in the water alive.

The amount of water you will need, will depend on your pot size. For a good flush, you will need 3x water, as you have medium in your pots. For example:

4 plants in 10L pots. That’s 30L each, 120L for a full flush.  

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Flushing in Coco
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Flushing in Soil

Once the water has been prepared, and has stood to degas, you are ready to begin the flush. 

Flushing in soil

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When it comes to flushing in soil, drainage can be an issue. It is very easy to over water cannabis plants when they are grown in soil. To get the best results, after flushing, you should let your soil dry out for a few days before watering or feeding again. 

Use 3 times as much water, as you have medium in your pots. A ten litre pot will need 30L of water for a good flush.

Start by gently pouring water into the pot, wetting the medium thoroughly. Leave this to sit for a few minutes, to help any salts dissolve into the water. 

After a few minutes, continue to pour water heavily into the medium. The rush of water will push out the dissolved salts, and leave the medium fresh, with a much lower nutritional content. Remove all of the run off immediately, never let your plants sit in run off. 

Once the nutrients have been removed from the medium, your plants will start to eat the foods that have been stored up inside themselves.

After the flush

Depending on the reason of the flush, you may want to introduce new feed to the medium. Do this by feeding the plants straight after the flush, whilst the medium is still wet. Then let the soil dry out for a few days, so the roots get a chance to breathe. If you’re growing in coco, you should feed your plant, never give it just water. Learn more about growing cannabis in coco here:

If you are flushing because harvest time approaches, it is good practise to let the medium dry out for a few days to let the plants breathe. Give one normal watering when the medium is dry, and then let it dry again.

Once it is dry the second time, repeat the flush again, to ensure all nutrients have been removed from the medium. Leave the medium to dry, until the pots are light, before watering again. You should try and flush at least 10-14 days before harvest if you have grown using salt based nutrients.


Flushing in Coco

Prepare your water, 3x water as you have coco in your pots. 10L pots will need 30L of water. 

Pour the water gently into the pots, dampening the medium. Let this sit for a few minutes, as this will allow the salts to dissolve a little more.

After the medium has sat, pour the rest of your water through the medium, and remove all of the runoff. 

This will leave you with fresh clean coco, at a stable pH. 

After the flush in coco

Depending on the reason for your flush, you may want to introduce food again. If this is the case, re feed straight away, with a slightly lower EC than your previous feeds. Flushing coco is a great way to reduce excess salts in the medium. But if you are flushing to remove excess salts, you should refeed your plants straight away. Don’t leave plants growing in coco is a medium with no nutrients.  

However, if your har flushing because harvest time approaches, the is of course no need to reintroduce feed. But keep the water pH’ed at 5.8.

How to feed plants in coco

If you are flushing because harvest time approaches, then it is best to flush a second time, around five days to a week after your first flush. 

Continue to water your plants with pH’d water everyday, with at least 10% run off, which is drained away. Do this for five days to a week, until the second flush.

The second time round there is no need to pH the water, as you would prefer to let the plant use up any stored nutrients inside itself, instead of anything left in the medium.

Thanks for reading! You can find more information on anything related to growing in our cannabis grow guides. All of our guides are written by experienced growers, and members of our cannabis growers forum. If you have any questions, sign up. It is free, and we are always happy to help!

24 thoughts on “Flushing your medium”

  1. Posted by: @james
    Everybody that checked the differences between flush and no flush knows flushing does make a difference.

    Quite the opposite actually. Anyone who has actually looked into this scientifically has found the exact opposite. This blurb is from that U of Guelph study back in 2017 where he looked at this exact situation and found there was no difference in the amount of N,P, and K in the flushed tissue compared with tissue that hadn’t been flushed. Check out the actual study (see Section 4.4) for the methods, analysis, and figures, if interested.

  2. @jacks_farm
    I actually flush before flipping to 12/12 but I am on Coco using Bio Pellets at first time (3 weeks) and salt based later on.
    2 weeks before harvest I simply do not feed anymore. I just give PH`d water.
     
    And to give a plant exactly when it needs what is sth I think only nature can do.
    Thats why I think flushing may not be needed really if you grow on soil. Those nutes are not directly available.

  3. sorry @James I kind of understand what your saying about 
    “conentration of salts in water with membrances equalizes” but doesn’t really answer my question.
    just saying if you feed a plant what it needs at the right time, you don’t need to flush 
    IMO flushing a plants can be very stressful to a plant in late flower and really increase the humidty in the flower room, seen growers snap buds and there not sure when to water next, can really throw things off 
    Think my plants about 3 weeks in flower (12/12) under a net and its settled and happy getting its 2ltrs of water every 2-3 days ( start adding honey soon)
    don’t want to move them till harvest or cause them too much stress

  4. Posted by: @jacks_farm
    why do you need to flush the root mass ?

    1. I do not think if you work organic its needed to flush. But I do not know that.
    2. It does not matter what part you flush as the conentration of salts in water with membrances equalizes.
    You can see this with 2 eggs just put salt on the yellow of one and salt on the clear part of the other.
    2 hours later you will see an obvious difference.
    Means if you flush one part of a plant nature will take minerals out of the other to equalize the molecules per litre.
    Other then with the egg salt can pass the roots membrane.

  5. Posted by: @james

    Posted by: @basssman7
    Science says flushing makes no difference to yield, THC or terpenes or taste.

    Everybody that checked the differences between flush and no flush knows flushing does make a difference.
    Also it is known that a generic Medical is chemical same to the original. Anyway generic and original work different for people.
    I do not think we are able to analyze the important parts of it. Just remember we do not know 90% of anything energy or material.

    I think that is the case for a lot of growers because they need to flush out excess salts in the medium before harvest.
    Nitrogen encourage the canopy and buds to strech and Bush out and it very high ec (salt content)
    so I don’t want any nitrogen in my root mass by week 6 (12/12) and want to see the lower canopy drain the nitrogen from the leaves and as the leaves fall off it helps to reduce humidty and reduce humidty helps to increase the amount resin the plant produces, letting the plants really dry out a few times at this stage seems to increase resin production 
    when the plants in mid/late flower it uses more sugar to mature the buds 
    so use something like (top max), (honey or brown sugar in hot water)  to make the buds sweet and dense 
    if you feed a plant what it needs at the right time why do you need to flush the root mass ?

  6. Posted by: @basssman7
    Science says flushing makes no difference to yield, THC or terpenes or taste.

    Everybody that checked the differences between flush and no flush knows flushing does make a difference.
    Also it is known that a generic Medical is chemical same to the original. Anyway generic and original work different for people.
    I do not think we are able to analyze the important parts of it. Just remember we do not know 90% of anything energy or material.

  7. Science says flushing makes no difference to yield, THC or terpenes or taste. It seems like a legit study. If you want to look at it backwards it means you can save money on nutrients for the last two weeks.  I hope they do one in the future regarding darkness before harvest. :)I am sure this must have been referenced previously because there are many followers of Shane from Migro.

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