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.
Once the water has been prepared, and has stood to degas, you are ready to begin the flush.
Flushing in soil
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.
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27 thoughts on “Flushing your medium”
@macky ok I looked at a PH chart and 5.8-6 makes sense. 🔥🙌🏻🔥
@macky 👀 I was thinking you would really want the flowering nutes to be absorbed so 6.1-6.3 seemed more reasonable given 5.8 is listed for veg on your guide. 👀 😬
@greenplanet Why 6.1 and above? lol 😉 It depends on how long the flush is going to be really. After a week, it doesn’t really matter what pH youre watering the plants at as there shouldn’t be any nuits left in the medium. But up until the first week, there is nuits left in the medium and you want the plants to use them up if they are not flushed out
@macky with the final flush in coco can I ask why 5.8ph and not 6.1 or above?
@yoda OMG you got a whopper there. I am green envious
@james general rule of thumb I do think most growers should flush before harvest
If there not careful about the amount of nutes they use in flower.
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.
Aye thought you were in coco 😉
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
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.
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 ?
@jacks_farm and they follow what the bottle, or manufactures guide says. Its not their fault though, as ZN said, they think more food equals bigger plants.
@jacks_farmThey have the misguided opinion that loads more food means bigger buds 😀
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.
Never understood why some growers give there plants loads of food, mid flower (burn all there leaves) and then spend the next two week flushing it out before harvest
You betcha –> Is Flushing Worth It? (No!)
There is another study I linked to in that thread that shows the exact same results so, seems like decent evidence.
Good that this vid is posted here too now though. 🙂 👍
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. 🙂 br />
I am sure this must have been referenced previously because there are many followers of Shane from Migro.
@yoda i would buddy get a big syringe for run off , you risk it tasting like arse pal
@woolieback I’m in Coco as well. It’s just going to be very difficult to flush them because they’re roughly 1.7m- 2m in height. If I do try to flush, I’ll have snapped crowns and buds everywhere because they’re so big and heavy.
@yoda depends on substrate , im coco and it enhances it , no headache ect smooth as silk , catching when is the case in my tent ,
Will it ruin the flavour, THC make sick if you don’t flush?
Perfect Macky. cheers mate 🙂
@percysgrowroom very nicely written macky, all the correct information as usual,the pictures and diagrams are a nice touch also bro👏👏
Yes mate pretty much.
Flush in Hydroponics is just replacing the water inside of the reservoir, full of nutrients, to a water with no nutrients, right?
Definitely looks a lot cleaner and as always there an answer to everything just had to remind myself how to flush for harvest properly 😉👍 lol
Fixing up old articles 🙂 Looking much nicer on a simple format like this