One of the most controversial discussions in the Cannabis cultivation community, is the topic of “Flushing” the plants in the last two weeks of the flower cycle.
The underlying belief and the motivation for this entire discussion has been the theory that flushing washes out the “excess” or “bad” nutrients makes the flower taste better when you smoke it.
If you love to smoke cannabis, the smell and taste of the smoke are a HUGE part of it.
To give you and example, in a survey I posed recently on Linkedin, roughly 60% of participants said that the taste and smell are more important than how potent the cannabis is.
But why is that? You might think, “of course people smoke to get high,” but apparently it’s the experience of HOW you get high seems to be more important that just getting there quick.
Our sense of taste and smell are literally just nerve endings in your nose and mouth that create a perception in your brain of what the thing tastes or smells like. These sensors detect the combinations of individual molecules in the air with a very high level of accuracy.
The important thing to note here is that…
…the chemical composition is what CAUSES the taste and the smell.
Aside from this theory on Flushing, the only other thing we know about that absolutely affects the taste and smell of cannabis flowers – is Curing, and I think there will be little or no debate on that.
But what if I told you that normal curing is actually the second stage of the curing process.
After doing extensive research in the process of making my drying and curing course, I’ve come to believe that the taste and smell are heavily influenced by two separate processes – respiration and enzymatic fermentation.
Respiration is a vastly underrated part of this process.
As the plant nears the end of it’s life, and even several weeks after you cut it down, respiration keeps right on working 24/7.
Respiration converts all the products produced by the process of photosynthesis (photosynthates) – starch, sugar, etc. and it breaks them down into much simpler compounds.
Normally, we think of the curing process as if it starts when we cut the plants down.
We harvest the plants and then we cure for several weeks to several months and just like with fine cigars, meats, cheese, alcohol, etc, enzymatic fermentation keeps working for MONTHS or even YEARS to produce an aged product.
But what if the curing process could be started far in advance of the harvest?
~
Like as in, Flushing!
Let’s take a closer look at Flushing!
In my efforts to research this topic, I managed to dig up this 2017 study by Jonathan Stemeroff that seems to have gotten quietly filed away in the bowels of the university library without ever making it into the news spotlight it deserves.
From what I can see, the study seems extremely well done and thorough. I’m normally a skeptic, but after reading through this one, I can’t find any way to argue with the findings.
Here’s a direct link to the study…
Irrigation Management Strategies for Medical Cannabis in Controlled Environments.
Jonathan Stemeroff, University of Guelph
The study analyzed the effects of flushing on THC and Terpene content, as well as on the mineral content in the resulting dried flower buds. The experiment was set up as a Randomized Complete Block Design with four blocks, each containing two plants for each of the three irrigation treatments.
In this study, they made Six test groups… and they repeated the entire experiment 3 times.
- 10L Flush + 10L Flush, then plain water every 2 days
- 10L Flush then plain water every 2 days
- Plain water every 2 days
- Fertigated water every 2 days
- Plain water every 3 days (mild stress)
- Fertigated water every 3 days (mild stress)
The buds were then dried to common industry standards and analyzed using High-Pressure Liquid Chromatography.
The results….
Flushing causes NO SIGNIFICANT CHANGE in the chemical composition of the dry flower.
I know that will ruffle a lot of feathers, especially among the more “old school” growers who probably already have their fingers in their ears singing La La La La La at the top of their lungs.
If you simply don’t want to hear it, I’d recommend reading my other article about Dogma in the Cannabis Cultivation world and then maybe circle back when you’re ready.
If you have an open mind and you really want to learn about this topic like I do, I would encourage you to hit the link above and read the study yourself. On pages 58, 59 and 60 you will find graphs showing that no matter how they tried to “flush” the plants out, the tissues still contained statistically identical amounts of the various major plant nutritional elements N-P-K-Ca-Mg-S etc…
The following graph shows the six flushing groups.
They repeated the experiment three times so in the actual study you will see three graphs like this one, and they all look almost identical because each time they repeated the study, the results were the same.
As you can see in the graph (copied directly from the study) each of the 6 different combinations of fertilizer and flushing showed only the tiniest differences which are well within the margin of error for a test like this.
This demonstrates that flushing causes no measurable difference in chemical composition, but is that really the end of the story, or are we just asking the wrong question?
Looking at the chart above, and seeing that there is no difference in the chemical makeup of flushed vs non-flushed bud… and knowing that there can be no difference in the smell or the taste unless there is a difference in the chemical composition of the material, it seems impossible to argue the issue further.
Even after seeing this…. some growers will still insist on flushing because they just know what they know.
Are they just so hard headed that they’re stuck in their ways, or is there really something more to this story?
Let’s take a closer look to see if there might be some evidence that flushing works?
In all the years I’ve been questioning this concept, I’ve never had a single grower tell me they actually did a test and accumulated some sort of evidence to support this concept.
The only reason they have for doing this is…
- Because some other grower told them to do it, or better yet,
- Because everybody else is doing it.
This whole thing seems to be based on the premise – “everyone else does it so it must be true” – which unfortunately relies on yet another premise which is that the first person who started it all actually knew what he was doing.
There are actually two completely separate reasons for flushing.
Theory #1
Flushing is done to remove nutrients from inside the plant…
According to those who believe this theory,
Running large amounts of water over the roots washes all the bad chemicals out of the plant’s system before harvest, thus making a much cleaner smoke.
In the study above, Jonathan Stemeroff very clearly shows us that flushing the nutrients out of the plant is not possible. It’s like thinking you could put a person in the shower to rinse the Iron out of their blood. You can’t rinse anything out of the plant tissues because not only does the plant have a very water resistant covering that protects it just like your skin protects you, but all those atoms are bound into molecules and compounds which are bound into proteins and amino acids and carbs and on and on… You can’t just rinse them apart!
This is basically saying that you think you can run water over the plant tissue and “undo” the molecular assimilation that went into the 6 month long process of binding all those trillions upon trillions of atoms together into each piece of each tiny cell… and you actually think that just running some water over it will take all that apart and give you a bucket full of N-P-K-Ca-Mg in the rinse water after you’re done.
If you believe this, you better stop showering immediately!
Theory #2
Flushing is done to remove nutrients from the substrate…
According to this theory, washing the substrate clean deprives the plant of nutrition for the last couple weeks of the grow cycle, the theory being that if you remove food two weeks before the end…
The plant will process all the bad chemicals out of it’s system before harvest, thus making a cleaner smoke.
Flushing the nutrition out of the soil – Yes, you can do that – especially in a hydro grow. In soil blends, not so much. Soil and soilless blends have a phenomenon called “cation exchange capacity” that essentially describes how strongly nutrients will bind to the substrate. So you can rinse a lot of the nutrients out, but you won’t remove anywhere near 100% of the old nutrients in soil substrates.
Even if you could wash the substrate clean, the question becomes, “Why would you?”
There are two potential answers – neither of which seems to make a lick of sense. One theory is that starving the plant will cause it to consume all the “bad chemicals” and they will somehow magically disappear as the plant starves.
Theoretically, by this late in the cycle (last 2 weeks) the plant already has all the nutrition it needs stored up in the roots and stems, so cutting back on food doesn’t hurt your yield.
When you cut a plant down, it dies slowly over period that may range from days to weeks depending on the temperature and humidity. During this time there is a gradual change in the forms of compounds as they break down in the plant, but they don’t disappear. All the elements found in the living plant will still be there when it dries out with the exception of some CO2 which is given off as a by product of Respiration. If you starve a plant for two weeks before harvest… that will not reduce the concentration of any constituent elements in the plant tissue. While the plant is alive it will take in CO2 and give off Oxygen during the day, and it will constantly (as in 24/7) use O2 and give off CO2 as a product of Respiration.
There is no other means by which a plant can expel anything!
Are there still any good reasons for Flushing in the last two weeks…
#1 Saving Money…
By far the most interesting and significant things that Stemeroff found in this study are economical.
For example: they found no significant bud weight loss, and no THC change from withdrawing fertilizer and reducing the water schedule by 25-45% for the last two weeks.
Two weeks with no fertilizer and 1/3 less water use – that could save a LOT of money.
This is a VERY significant impact on the bottom line – especially since lower water usage means less evaporation and transpiration in the room which also lowers your HVAC and humidity control costs.
#2 Increasing Secondary Metabolites…
Increasing Inflorescence Dry Weight and Cannabinoid Content …Using Controlled Drought Stress https://www.researchgate.net/publication/333438844
Dr Deron Caplan found that by withdrawing water in week 6 (of an 8 week flower cycle), and allowing the plants to wilt, but then recover with subsequent waterings, the plants would increase production of secondary metabolites by as much as 10%. This has HUGE potential for cannabis growers.
So what is it about flushing that actually works?
Flushing has been done all these years, because it “somehow” removes something that negatively impacts the smell and taste of the smoke.
Maybe this is a classic example of…
Correlation does not equal Causation!
Over the years, people have definitely noticed (correlated) that sometimes the pot comes out harsh and “chemicaly” tasting, while at other times it’s really smooth and tastes great.
The non-scientific explanation (causation) for this is to say that you are “washing all the bad chemicals out of the plant.”
However, when you flush, along with adding a lot of water, you also reduce the concentration of fertilizer.
It’s not possible to remove compounds from the plant by starving it, but it is possible that they can be converting into something else.
Think about this… removing food in the late portion of the life cycle roughly equates to the natural outdoor life cycle where nutrition (particularly Nitrogen) becomes depleted at the end of the season.
We all know that during this nutrient deprivation time, green chlorophyll loses it’s color and begins breaking down, which reveals the purple, yellow and red carotinoids that are always present but not able to be seen when chlorophyll is present in large amounts. So when the chlorophyll breaks down, the colorful carotenoids suddenly become visible as you can see in the image above.
Chlorophylls and carotenoids are two types of plant pigments that have the obvious difference of reflecting different colors, but they also have a completely different chemical structure. Carotenoids are not directly involved in photosynthesis, so any time you see them, you know the plant is no longer producing sugars through the normal photosynthetic process.
All those colors mean the plant is basically starving.
Chlorophyll is a type of compound called a chelate which consists of a central metal ion (Magnesium in this case) bonded to a large organic molecule, composed of carbon, hydrogen, oxygen and nitrogen as you can see in the image below.
Notice that Magnesium and Nitrogen make up the central core of the molecule.
Photosynthesis is the reaction where chlorophyll combines carbon dioxide and water, catalysed by sunlight, to produce glucose, and oxygen is given off as a waste product.
The chemical equation for photosynthesis looks like this:
Looking at the two chemical formulas above, the only things that might possibly be able to contribute to a bad taste are either Magnesium and Nitrogen, which are present in very small amounts because there is only one set of them in each chlorophyll molecule, or Glucose which is produced in massive quantities because each chlorophyll molecule keeps churning out this glucose compound all day long.
That sets up the high likelihood that Glucose might be our culprit here.
Many people have already come to the conclusion that chlorophyll is somehow contributing to the bad taste, but perhaps it’s not that the chlorophyll is the problem as much as it is that Chlorophyll is creating the glucose, and it’s the glucose that is the problem!
This hypothesis could explain how flushing works.
Maybe flushing has nothing to do with washing chemicals out, but it has to do with removing nutrition, which causes chlorophyll to break down, which stops the production of glucose.
Once the chlorophyll is depleted from a lack of Nitrogen and Magnesium in the feed supply, the plant’s stores of glucose begin to diminish rapidly as it gets used up in the process of Respiration.
Remember how Jonathan Stemeroff demonstrated that nothing is removed from the plant by flushing?
In the equation above you can see that glucose, which is made up of nothing more than Carbon Hydrogen and Oxygen atoms, is broken down into water, CO2 and energy.
So nothing is removed from the plant but CO2.
There are no “evil chemicals” that must be washed out.
What evidence is there that glucose causes the taste difference?
Looking at studies done in the Cigar industry, I found this article…
Sugars as tobacco ingredient: Effects on mainstream smoke composition
In natural tobacco, sugars can be present in levels up to 20 wt%. In addition, various sugars are added in tobacco manufacturing in amounts up to 4 wt% per sugar. The added sugars are usually reported to serve as flavour/casing and humectant. However, sugars also promote tobacco smoking, because they generate acids that neutralize the harsh taste and throat impact of tobacco smoke. Moreover, the sweet taste and the agreeable smell of caramelized sugar flavors are appreciated in particular by starting adolescent smokers. Finally, sugars generate acetaldehyde, which has addictive properties and acts synergistically with nicotine in rodents. Apart from these consumption-enhancing pyrolysis products, many toxic (including carcinogenic) smoke compounds are generated from sugars. In particular, sugars increase the level of formaldehyde, acetaldehyde, acetone, acrolein, and 2-furfural in tobacco smoke. It is concluded that sugars in tobacco significantly contribute to the adverse health effects of tobacco smoking.
If this theory is true, it would essentially allow us to create an early beginning to the curing process while the plant is still alive.
Curing normally begins when the plant is cut down at harvest. In that early stage, the process of respiration continues taking place while the plant is drying out. If you dry very slowly, and in the dark, you starve out the photosynthetic process while still allowing respiration to continue using up the carbohydrates (glucose) that were previously created during photosynthesis while the plant was alive. That slow death is where you break down chlorophyll, starches and other compounds and the process continues as long as there is still enough moisture for those chemicals to flow around in the plant cells. Once you get the plant material dried out to a certain point (roughly 10% moisture), the lack of moisture begins to inhibit the movement of chemicals and Respiration grinds to a halt.
At that point, enzymes take over and the long slow process of enzymatic fermentation begins.
Enzymes convert terpenes and many other chemicals including glucose, from the original form they were in while the plant was alive, to the aged form of the mature cured cannabis flower. This process continues for at least several months and to be honest nobody knows how long it can go.
Some cigars are allowed to ferment up to 3 years!
These changes are created by terpene synthase enzymes and this is how a cured flower comes out smelling so totally different from a live flower!
Another example would be ripening fruit. We all know the difference between a barely ripe peach that you just picked off the tree, compared to the same fruit after it’s had another few weeks to ripen completely.
These are two examples of huge changes that we’ve all witnessed first hand.
If you’d like to know a LOT more about how curing changes the cannabis flower, sign yourself up for…
THE CANNABIS DRYING AND CURING MASTER CLASS.
How long can cannabis be cured?
If you try to cure cannabis at room temperature and in the presence of oxygen, the THC will decay into CBN at a rate of about 2% per month and within a couple years you could sell it as hemp.
To get the most benefit from respiration without causing your THC to be degraded, you need roughly 30 days of exposure to oxygen to allow respiration to process everything it can before the process stops completely.
After that point, be sure to use some form of sealed container, with an oxygen absorber, an your cannabis can last literally years with very little degradation.
10 thoughts on “Flushing Cannabis – Myth or Magic?”
Hello!
I really admire your scientific approach and what a great article!
I also have a quite a few questions.
First of all, how would one know if their plant even needed to be fertilized? Let’s say the soil was taken from the outdoors where lush grass grows. Would that soil even need fertilizing if the plant doesn’t show any signs of deficiencies? How often would one have to fertilize such soil?
Secondly, do you know of any actual scientific research conducted on symbiosis of cannabis and fungi?
Lastly, although slightly off topic from this article, could pulling aside and tying down the “light-blocking” fan leaves, be used as an alternative to defoliation (which according to some allows lower buds to receive more light)? Wouldn’t hemispherical light placement also be better, allowing the light to be radiated from all directions with similar intensity, thus allowing more light to penetrate the canopy?
Best regards,
Chris
Wow that’s a lot of questions. I appreciate your enthusiasm. You’ve definitely got a lot of research to do.
I’m going to recommend that you look into using Google Scholar to find resources for all these questions.
Some people also try using Sci-hub to get the articles.
Cannabis Dogma is real! There’s too much Bro/Master Grower Science out here.
Now with Social Media finding actual information is nearly impossible.
I fell for flushing then went down a rabbit hole and realized I was sold BS.
Thanks for sharing I hope people find this because you’re spot on.
Next try to tell people that’ll light made with the same exact components at half the price is the way to go
Thank you!
And yes I’ve read one study that compared LED street lights to expensive grow lights and found them to be identical.
and its not like putting a person in the shower that would be equivalent to just misting the plant ,it would be like a person drinking something instead because the plants drinks from its roots, this is a very bad comparison ,it you spry you skin with acid just you skin well burn but if you drink it you well affect the health of your whole body, same with a plant if you spray the plant with acid the leaf well burn and the rest of the plant well be fine but if you put acid in the water you feed it you well kill the plant !
Ok, so if I use your acid analogy…
I can spray acid on my skin and I can wash it off.
If I drink acid it will move throughout my entire body and I can’t wash it off.
Sounds to me like you just said the same thing I was saying.
Once you get something inside your whole body, you can’t just wash it off!
I totally agree with you!
this is bs !!! grow 2 plants at the same time and bring one of the plants foods ppm to 2500 and don’t flush it and see what happens! iv grown for 20 years now thanks to my brother, and my brother uses and ton of nutrients like crazy amounts im like wtf are you doing man lmfao, and their is a huge difference, but only if you run high ppm! i keep mine low so i don’t need to flush even though i do just because i know it takes the chemical out because i don’t want to smoke all that
My first question is… did you read the study?
I’m guessing you didn’t because you’re not making any reference to anything in the study.
Your main point seems to be – a cannabis plant will accumulate nutrients that it is exposed to.
However, the point of this article is that once you get the nutrients in there, you can’t get them out by washing or flushing.
When you say that you use a low dose of nutrients and then you wash it…. I’m wondering, did you have anything in there that needed to be washed out?
If you didn’t put in excess nutrients, why do you think you need to wash something out? And if you don’t have anything that needs to be washed out, then you wash it, why do you believe that washing actually did anything?
What you are basically saying in your comment is that…
1) Your brother overfeeds his plants and the nutrients stay in there and ruin the smoke.
2) You feed moderately and there is no weird taste from excess nutrients.
I agree with you on both counts.
What I won’t agree with – because this study proves it pretty conclusively – is that you or your brother could overfeed nutrients and then wash it out by running water over it.
That very clearly doesn’t work.
You are obviously interested in all this stuff… Please take the time to read the study and I’d be happy to discuss further.
Hi, what about chem rational around mystery “flushing solutions” probably edta chelating agents or surfactant?(soap)
And why sometime ash is completely white and smoke smooth and sometime the opposite?
I mean of course different clones growing in different ways.
I really felt in love with your scientific way to treat lots of topics. Sorry for my eng. Regards!
Hello Mio,
All of the “flushing solutions” on the market are designed to remove elements from the soil. None of them claim to do anything to a plant. Some are made with Saponins and some are chemicals that bind with the elements and others are simply acids that break compounds apart and allow them to be more easily washed from the soil. Many of these products are great to use when you’ve over fed your plants, or if you want to clean out the soil between runs.
As for the harsh smoke, try vaporizing instead, that way you get all the benefits without the smoke.
If you have to burn cannabis, and it burns slowly, try drying it more or less, grind it up better, and try a hotter flame. All of those are variables that affect the burn. Chemical content has nothing to do with it unless the mature plant was sprayed with something that was allowed to dry on the outside of the plant just before harvest.
Thanks for reading and writing a comment. I hope this helps. Glen