Flushing a Cannabis Plant
As a cannabis plant goes through its life cycle, it will consume nutrients, and store them up inside the leaves, and flowers. Flushing a cannabis plant before harvest will help remove these nutrients from the plant, giving you a cleaner, smoother product.
If a cannabis plant was suffering from excess or toxicity, then flushing the medium will remove built up salts, and refresh the medium to a more suitable level.
In this guide we will cover everything about flushing cannabis plants. By the end of the guide you will know how to flush, when to flush, and why.
If your plants are suffering with a nutrient excess or any cannabis plant problems, then ask for help in our cannabis plant infirmary. Just sign up to our cannabis growers forum.
- What is Flushing
- How Flushing Works
- How to Flush a Cannabis Plant
- Flushing a Cannabis Plant Before Harvest
- How to tell if you Have Flushed Enough before Harvest
- Flushing a Cannabis Plant to Fix Toxicity
What is Flushing
Your cannabis plant will get most of its nutrients from the roots. The roots will drink the water, and along with that, take in the nutrients that are available to it.
Flushing a cannabis plant is when the nutrients in the root zone are "flushed" out, by pouring lots of water through it. This is done for a number of reasons, but most growers will flush their cannabis plant as harvest time approaches, to remove any built up nutrients in the plant.
Flushing your cannabis plant is especially important when you have grown using salt based nutrients. These are chemical based nutrients, and can add a harsh smoke to your cannabis after harvest, if they are not properly removed.
How Flushing Works
Once nutrients are removed from the medium, a plant will not only start using the nutrient stores it has built up, but nutrients will also "leach" from the plant into the medium, where it can be flushed out again.
Using osmosis (high concentration to low concentration), if there is less food in the medium, than there is in the plant, nutrients will pass from the plant into the medium, in an effort to find balance.
As the nutrient stores are used up by the plant, and are also leached out into the medium, the amount of salts in the plant is reduced, and this will improve the quality of the final product.
Remember, you are removing the plants food here, and essentially starving it. You can only do this for a couple of weeks before the plant really starts to suffer. If the plant gets weak, so does its immune system, and it can be more vulnerable to mould and pests.
How to Flush a Cannabis Plant
There are many different methods of growing cannabis, so there are also different ways to flush. But no matter what method you are growing with, the principles are the same, you want to remove nutrients from the root zone, this will do 2 things:
1: Make the plant use up stored nutrients from leaves and flowers
2: Have nutrients inside the plant leach into the medium
You are going to need a lot of water for this, and it should be pH'd at the correct level for your medium.
Flushing a Cannabis Plant in Soil
When growing cannabis in soil, you have to be careful not to over water your plant. So, to make sure you have no problems like this after the flush, you should leave it a couple of days since your last feed before flushing the plants.
You will need three times as much water as you have soil in the pots, for example, if you have ten litres of soil in your pots, then you should use 30L of water to flush the medium properly
To make sure any left over nutrients after the flush are being used up, the water should be pH'd at the correct level for soil, which is between 6.5 and 7.0.
Pour the water slowly through the soil, and collect the run off as it comes out. Pour this away. After all of the water has been flushed through the medium, the flush is over, and the soil has a lot less nutrients in it.
Flushing a Cannabis Plant in Coco
Flushing a cannabis plant in coco is very similar to flushing a cannabis plant in soil. The main difference is, the pH is a little different. It is very hard to over water cannabis plants in coco, so you are not at as much risk of causing problems by flushing the medium.
You will need three times as much water as you have coco in the pots, so if you have ten litre pots, you should use 30L of water to flush the coco properly
To make sure any left over nutrients after the flush are being used up, the water should be pH'd at the correct level for coco, which is between 5.8 and 6.2.
Pour the water through the coco, and collect the run off as it comes out. Pour this away. After all of the water has been flushed through the coco, the flush is over.
Flushing a Cannabis Plant in Hydroponics
Flushing cannabis plant grown hydroponically is much easier than flushing in soil or coco. All you have to do to remove the nutrients from the root zone, is change the water!
Regardless of what type of hydroponic set up you grow in, change the reservoir water. The pH will always need to be set to 5.8-6.2 in hydro, but the EC level will differ depending on if you're flushing to remove excess problems, or flushing to prepare your cannabis plant for harvest.
Flushing a Cannabis Plant Before Harvest
For growers who have grown their cannabis with salt based nutrients, flushing before harvest is a crucial step to getting top quality buds. The salts will build up in the plant, and it is best to remove as much as possible before using the plant to smoke or make extracts.
The idea of flushing before harvest is to remove nutrients from the root zone so the plant is forced to use up stored nutrients to feed itself.
Also, because the medium is going to have less nutrients in it, nutrients will leach out of the plant on and into the medium again. The EC of the medium will rise, and the leached nutrients can be flushed out again.
It takes at least a week to fully flush your cannabis plant, and can go on for anything up to 2 weeks. This will depend on how big the plant is, and what medium you have been growing in.
Flushing Before Harvest in Soil
After flushing your plants with 3 times as much water as you have soil in the pots, you should let your medium dry out for a few days. This will give the plant time to take in some of the water and start breaking down the built up salts and nutrient stores.
When the top 2 inches of the soil have dried, you can water the plant again, as normal, so you get around 10% run off. Do not add nutrients to any more of your feeds! Just give the plant water from now until harvest time.
Around 7 days after the first big flush, you should do a big flush again. This will remove any extra salts that have leached from the plant, and give the plant its last watering before harvest. After this big final flush, you should leave the plant for 3 or 4 days to dry out, and then harvest.
Flushing Before Harvest in Coco
A good flush in coco will take about 10-14 days. This will depend on the size of the plant. The bigger the plant is, the longer you should flush for.
Start off with the big flush of 3 times water as you have coco in your pot. This will remove all nutrients from the medium, and allow the plant to start using stored nutrients in the leaves and buds.
This flush should be pH'd at 5.8-6.2. Allow the coco to sit for 48 hours, and then water the plant with just water, every other day. This will keep the coco saturated, and constantly flush out any salts that have leached from the plant.
After 6 or 7 days, do one more big flush. There is no need to pH the water this time, as you want to encourage the plant to use whatever is left in its nutrients stores, by locking the nutrients out in the medium.
The plant will begin to turn yellow, and can be harvested after about 3 days after the second big flush.
Flushing Before Harvest in Hydroponics
Flushing a plant in hydroponics before harvest is relatively simple. It can be done in about 7 to 10 days. Begin by emptying your reservoir and replacing the nutrient solution with just water.
Keep topping up the res over a few days with just plain water. You're not going to feed your plants again, so from now on, you will only give your plants water with no nutrients added.
After a few days, empty the reservoir again, and top it up with some fresh water. This will remove any salts that have leached from the plant into the water.
It won't take long for the plant to use up their stored nutrients and within 7 to 10 days, and couple of res changes, the plants will be ready for harvest.
How to Tell if you Have Flushed Enough before Harvest
If you are monitoring the EC of the runoff or reservoir, you can tell when the plant is ready for harvest. As a plant leaches nutrients into the medium, the EC will rise. So by monitoring the changes of the EC compared to your background EC, you can tell if the plant has stopped leaching or not
When the EC has stopped rising, and sits at a stable level around your BG EC, the nutrient stores have been used up, and your plant is just a few days away from harvest.
Another way to tell if the plant is ready, is by tasting the liquid that seeps from the leaves. Break a leaf off, and taste the liquid that comes out of the stem. If it is bitter, there is still too much salt build up in the plant, and you should flush for longer
If the liquid is sweet, or fruity, and lacks that bitterness, the salts have been used, and your plant is ready for harvest.
In some cases however, a grower will not flush because of harvest, but because of a salt build up, or to fix a cannabis plant problem like toxicity or excess.
Flushing a Cannabis Plant to Fix Toxicity
Cannabis plant excess and toxicity occurs when there has been too much food available to a plant. Just like when a medium has less nutrients in it than the plant, the nutrients leach from the plant into the medium, the opposite occurs.
When there are more nutrients in the medium than there are in the plant, the medium will force nutrients into the plant to try and restore balance. This forces the plant to take in more nutrients than it can use, and it begins to store them around the plant.
A plant can not be forced to eat more than it needs, and it can be easy to over feed a cannabis plant. The best way to fix an overfed cannabis plant is by flushing the medium to remove salts, and gradually reducing the feed until it is at a more suitable level.
Flushing to Fix Toxicity in Soils
When flushing a cannabis plant to fix toxicity, you have to be careful not to reduce the EC of the medium too quickly, as this can shock the plant. Gradual changes are needed to make sure the plant isn't too badly affected.
Begin by giving your plant a heavy watering, with no nutrients, just water. Use twice as much water as you have soil in the pot ( 10l pot, 20l of water). This will remove the excess nutrients from the soil. Then allow the medium to dry out for a few days.
Continue to give your plant just water for its feeds, until the symptoms of the excess begin to fade and the plant returns to a healthy shade of green.
Flushing to Fix Toxicity in Coco
Coco doesn't hold on to nutrients as well as soil, and flushing is a lot more effective. You always need nutrients in your coco, unless you're flushing to harvest your cannabis plant.
If you're flushing to fix a cannabis plant problem in coco, then you can't flush and leave the medium void of nutrients, it will be too much of a shock for the plants.
Instead, give your plants a heavy flush, with twice as much water as you have coco in the pot. Remove the run off, and then feed your plants again, but this time lower the EC by 0.2 points, and see how the plants react from there.
Lowering the EC might be enough to get things started again. After a few days if there are no improvements, reduce the EC of the feed again by 0.2 points. This will gradually bring the plants into the correct range and fix the excess problems
Flushing to Reduce Toxicity in Hydroponics
To fix a nutrient excess problem in full hydro set ups, you do not need to flush, but simply change the reservoir levels to a more suitable EC.
Reduce the EC of the reservoir by 0.2 and watch the plants for sudden changes. If after a couple of days the problem hasn't start to fix itself, then reduce the EC by 0.2 points again. Slowly, you will bring the EC down to the most comfortable range for your plant.
Final Thoughts on Flushing Cannabis Plants
Flushing is easy, all you need is water, and some pH up or down. The medium you grow in will make the flush differ slightly, but really, it's all about removing the nutrients and starving the plant.
If you have a cannabis plant that is near harvest, why not enter it into our Plant of the Month competition before harvesting? There are free cannabis seeds up for grabs, sign up to our cannabis growers forum, and enter your plant there.
Thanks for reading Percy Growers, for the best result, wait until your plants have really turned yellow before chopping. The more salts you can get out, the smoother that smoke will be.
Good luck with the rest of your grow!
Nice article mate? Learnt a few things there and have been flushing all wrong. I need more buckets for my next flush that’s for sure!
If your plan is for one year, plant rice
If your plan is for ten years, plant trees.
If your plan is for one hundred years, educate children.
Thanks Pete Bit of a mission to write, but i think it covers everything
Solid write-up mang. Very thorough. ?
Hmmm there's been a lot of talk about flushing over the different forums etc recently and theres is now a bit more evidence that it doesn't do what we want it to do.
This is mostly on flushing before harvest
It's something I've never done
The water in the plant and the medium will hold some dissolved salts in, it all about trying to get them out, buy either having the plant use them up, or flushing them out of the medium.
There Is plenty of controversy behind this, just like most things in growing lol. Some people think you don't need to flush at all. IMO, you do, if growing with salt based nuits, organics is fine, but you want to get some of those unused salts out of the plant before harvesting it.
Flushing does remove salts from the plant mate, otherwise the EC of the medium would not rise in the medium after the flush 😉 Something definitely gets leached from the plants into the medium
Great article and right on time ill be ready to flush here in the next couple weeks
Sativa to change the things I can, Indica to accept the things I can’t.
That's a very interesting paper - the first I've ever seen. 2017 too, before legalization. Makes sense though... Buddy was in cahoots with ABCann (an LP), and he's a MED patient, and it's sweet that it's as a MSc thesis too, so that's cool that this research is now being done. U of Guelph has actually been in the news a lot (relatively speaking lol) regarding cannabis research lately. I'm actually probably going back to Uni (of Sask) for a degree in Horticulture Science/Microbiology, and gonna look into these exact issues... but I digress.
Some interesting results he came to.. but if you open the actual paper, he has a bunch of 'unrecoverable errors', described in the amendment right at the beginning.. The errors don't refer specifically to the part of the paper regarding the effects of flushing (sections 3.9, 4.4 & 6.2) though (at least not directly), but it does throw a bit of uncertainty into any of the conclusions he comes to. He also refers to flushing with 'clear water' which I assume is pure distilled (ph = 7.0, TDS < 20) water but.. can't be 100% sure ?. He also only records the concentration of N, P, K, Ca, Mg, and S as '% concentration by mass', which only gives you the overall masses of each of those elements, but doesn't necessarily separate the available/absorb-able nutrient versions (e.g., For Nitrogen, NH4+ and various nitrates are what the plant actually absorbs - not just pure N, sulfur is in sulfate form, not just pure S, etc.), from the N that is contained within the cells and other plant systems in different molecules. Same goes for all the other elements there too I think. He does make a good point that I probably agree with though - we need to be sure that it actually is residual salts that cause cannabis to 'burn black, taste harsh', etc... I'm personally not totally convinced that it is. My super soil has 2000ppm runoff when I harvest, so my plants are eating right till the end, and my shit burns whiter than a $30 piece of avocado toast. A study of some established-to-be black burning shit weed vs. nicely white burning smooth smoking weed needs to be officially done. I'm not sure what instrumentation you need to be able to compare the differences in specific molecular weights of various compounds, rather than the total elemental weights (my girlfriend is an analytical chemist and this is exactly what she does - but for uranium/potash/base metal/gold ore instead - but she isn't home right now). Then, isolate the anomalies, figure out what they are, and if they fuck up your burn. If the only difference is the shit weed comes back full of residual fertilizer compounds, then maybe we can assume it is that causing the bad smoke. But I bet we would be surprised...
Just 2 cents from someone who has read a lot of scientific literature. Hopefully that makes sense.. ?
All that said, flushing to clear your medium of excess build-up/pH issues absolutely works though. A total different reason for flushing, however.
It's more chlorophyll that will make your buds burn black, if the buds are dried to quickly right? cutting out the nuits towards the end will stop the plant produce so much chlorophyll, and reduce how harsh the smoke is.
Difficult one to test really. Maybe i should just chop a plant without flushing it and see the difference lol
Yeah, improper drying definitely contributes... and your point on chlorophyll is probably on the right track. Chlorophyll's molecular chemical formula is C55H72O5N4Mg, a huge fucking molecule, with A LOT of carbon (and some N there too). My girlfriend informs me that large molecules like that are extremely difficult to break down (besides in the lab with strong acids), and I guess the more complex the molecule, the less efficient it will burn, and therefore leave more residuals.. makes sense.
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