This information suggests that flushing has more to do with forcing the plant to use up excess stores of starch, and reducing starch accelerates curing?
One of the most controversial discussions in the Cannabis cultivation community, is the topic of “flushing” the substrate in the last two weeks of the flower cycle, and disagreeing with this practice is a quick way to ruffle some feathers.
The underlying belief and the motivation for this entire discussion has been the long held 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 so this is no small deal in the cannabis world.
To give you an 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.
You might think, “of course people smoke pot to get high,” but apparently most smokers prioritize enjoying the experience of HOW you get high, and not just getting there.
In order to see if this flushing thing works, first let’s look at what we know for sure…
We know that we taste the chemical composition of the smoke.
We know the taste of the smoke changes when flushing is performed.
- We know that curing definitely changes the taste over time.
After doing extensive research in the process of creating my drying and curing course, I came to believe that the taste and smell are heavily influenced by two processes – respiration and enzymatic fermentation, but it turns out there is a third piece to this puzzle that also contributes heavily to the end result, and may in fact be the most important aspect of all.
I’m calling it, “induced senescence,” which simply means that you are purposefully causing the plant to grow old at the end of the crop cycle in order to take advantage of several things that happen when the plant slows down in it’s old age.
I say “induced” because you have the power to choose when this happens, or even to choose that it doesn’t happen at all, simply by manipulating the fertilizer. For example, choosing a 12-5-17 fertilizer ratio would be great in the VEG stage, but if you use that all the way through you will end up with very green and leafy buds that are still actively growing.
Switching to a ratio closer to 4-10-10 at the end, and then lowering the concentration, would effectively shut down active growth and allow the plant to age in a more natural fashion without completely starving.
What we didn’t recognize before, is that this step causes a cascade of chemical reactions that are extremely influential in forming the flavor of the smoke.
Let’s back up to the beginning so I can explain how it works…
Drying room biology…
In plants, the process of respiration works 24/7. The process doesn’t even stop when you cut the plant down because plants don’t die in the same way we mammals do it. They don’t take that one final gasp and fall over. It’s more that they simply dry out until eventually there isn’t enough water in the tissues to keep things functioning.
However, if you maintain the moisture levels, photosynthesis and respiration can keep going and that plant will stay very much alive – just as you’ve seen with a clone.
When you harvest a plant, if you dry the plant out slowly, respiration and photosynthesis could potentially keep functioning for several weeks, but you want to limit photosynthesis. Putting the cut plants in a dark drying room allows you to stop photosynthesis, and if you also keep the room at the right relative humidity (roughly 70% RH) you can extend the amount of time respiration keeps going.
Why would you want stop photosynthesis, but keep respiration going?
As you may remember from your high school days, photosynthesis produces glucose and CO2. The glucose is converted into starch which is a compound that can be stored within the plant – think potatoes.
Later on, respiration can convert the stored starch back into glucose whenever it needs extra energy to build flowers and seeds, or to keep itself alive if there is a sudden lack of fertilizer.
The stored glucose (starch) is converted at night when there is no photosynthesis, and it can also be used to build fruits and flowers in the fall.
So the purpose of stopping photosynthesis is to keep the plant from producing more starch.
The purpose of allowing respiration to continue is to begin burning off as much starch as possible while the plant is drying out because reducing starch accelerates curing.
These two steps are already commonly incorporate into the drying and curing process.
Now let’s back up to the original question….
Does flushing work, or is the whole thing just a legacy grower myth?
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.
Study:
Irrigation Management Strategies for Medical Cannabis in Controlled Environments.
Jonathan Stemeroff, University of Guelph
http://atrium.lib.uoguelph.ca/xmlui/handle/10214/12125
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 following graph shows the six flushing groups.
Remember, 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.
The results….
Flushing the roots causes NO SIGNIFICANT CHANGE in the chemical composition of the dry flower.
I know that will ruffle a lot of feathers for you “old school” growers and you may already have your fingers in your ears singing La La La La La at the top of your lungs.
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…
As you probably already know, our sense of taste and smell are literally just nerve endings in the nose and mouth that create a perception in the brain of what something tastes or smells like.
These tiny sensors detect the combinations of individual molecules in the air with a surprisingly high level of accuracy.
However, according to this study… Flushing causes no measurable change in chemical composition, so it can’t possibly be changing the taste or smell.
Looking at this study, it seems impossible to argue the issue further.
However, I’ve seen some very intelligent growers who absolutely insist on flushing and even after reading this, they just know what they know!
So…. I’ve had to ask myself,
Are these people so hard headed that they’re stuck in their ways, or am I asking the wrong question?
When I first got pulled into this discussion, the “flushers” stated two different reasons for flushing.
Theory #1
Flushing is done to remove nutrients from inside the plant…
Here’s a quote from one person who believes 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 pretty clearly demonstrates 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!
Theory #2
Flushing is done to remove nutrients from the substrate…
According to this theory, washing the substrate deprives the plant of nutrition for the last couple weeks of the grow cycle, the theory being that if you remove food…
The plant will process all the bad chemicals out of it’s system before harvest, thus making a cleaner smoke.
At first I had issues with this statement because there are no “bad chemicals.”
However, if you consider that most of the people who say things like this are not the most highly trained chemists, maybe there is another way to interpret what they are observing.
Flushing the nutrition out of the soil – Yes, you can definitely do that. Maybe you won’t be able to get every little scrap washed out, but you could certainly get most of it, and some elements like Nitrogen are very water soluble so they would wash out easily.
But even if you could wash the substrate clean, the question becomes, “Why would you?”
The explanation the “flushers” originally gave me is that starving the plant will cause it to consume all the “bad chemicals” and they will somehow magically disappear as the plant starves. The scientist in me knows you can’t make anything disappear. However, by removing nutrition, you could definitely slow or even halt photosynthesis.
Then the question becomes, “can late season starvation be doing something that changes the taste?” And even if it changes the taste for the better, is this going to hurt your yield?
According to research done by Deron Caplan (see #2 below), reducing the food supply late in the cycle (last 2 weeks) does NOT reduce yield. Apparently the plant has already stored up all the starch it needs for flower and seed production, and it has that starch supply waiting in the roots, ready to go.
Still, the idea that you’re trying to clean the plant out always bothered me.
When you cut a plant down, it may dry out slowly, but all the elements like N, P, K, Ca, Mg, S, Fe, Cu, Mo, and Zn that were found in the living plant, will all still be there when it dries out, with the exception of some CO2 and H2O that will be given off as by products of transpiration and respiration.
Plants can’t pee or poop so there simply isn’t any other way for anything to leave the plant.
So what is it about flushing that actually works to change the flavor of the smoke?
Thanks to Brendon Roberts for this image and for helping me hash out my thoughts on this topic.
Starting back at the beginning…
Flushing has been done all these years, because it somehow removes something that negatively impacts the smell and taste of the smoke.
The non-scientific explanation for this is to say that you are “washing all the bad chemicals out of the plant.”
However, when you flush, not only are you adding a lot of water, but the standard practice is to also reduce the concentration of fertilizer down below half, or even down to zero in a soil grow.
What happens when a plant begins to starve?
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 stops function and loses it’s color as the Nitrogen gets used up. This reveals the purple, yellow and red color pigments called carotinoids that are always present but are 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 all plant pigments but they have a completely different chemical structure. Carotenoids are not directly involved in photosynthesis, so any time you see them, you know photosynthesis has shut down and the plant is no longer producing sugars.
So those fall colors tell you that 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 the Magnesium and Nitrogen, or the glucose. Magnesium and Nitrogen are present in very small amounts because there is only one set of them in each chlorophyll molecule, but glucose is produced in massive quantities because each chlorophyll molecule is constantly churning out this glucose compound as long as the sun is shining.
That sets up the high likelihood that Glucose might be our culprit here and at first glance this makes a great starting hypothesis.
However, glucose is only present in fairly small amounts in the plant tissues before it gets moved and stored as a more stable form called starch, and plants typically have a LOT of starch.
How does it all fall in place?
Once Nitrogen and Magnesium are removed from the feed supply, or depleted from the soil, the chlorophyll molecules are soon depleted to the point that they can no longer produce glucose. Once that happens, the plant’s stockpile of starch (stored glucose) begins to be used up as the process of Respiration must draw on that stored supply to keep the plant alive.
How do plants break down starch?
Note how Photosynthesis (above) produces glucose on the right side of the equation, while this equation for respiration uses that same glucose (stored starch) to create energy to keep the plant alive.
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 in this process except CO2 and water vapor.
There are no “evil chemicals,” but there are compounds that are being broken apart and transformed into other compounds.
What evidence is there that either glucose or starch causes the bad taste?
To keep from having to reinvent the wheel as I tried to answer this question, I started off by looking at studies done in the Cigar industry because while tobacco is certainly a very different plant, they are still studying the compounds in the plant that create the taste and smell of the smoke.
The following are a few excerpts from the studies.
Sugars as tobacco ingredient: Effects on mainstream smoke composition
https://www.sciencedirect.com/science/article/abs/pii/S0278691506001773
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.
Once you get over the fact that they are pointing out all the hazardous properties of burning sugar while also mentioning the benefits of the sweet taste and how it attracts young kids to smoking….. you can see that sugars can improve the harshness and make the smoke taste better, but you don’t want too much of it because it also releases carcinogens.
Carbohydrates-Key Players in Tobacco Aroma Formation and Quality Determination
Article in Molecules · April 2020 DOI: 10.3390/molecules25071734
2.3.1. Starch
At the end of processing, almost all starch is converted into water-soluble carbohydrates, and subsequently into aroma compounds. Although starch is necessary for aroma formation, its presence in tobacco products can cause bitterness and flavor changes. Moreover, it can affect changes in color and odor of cured tobacco. Therefore, complete degradation of starch is one of the important steps in tobacco processing.
Changes during Processing of Tobacco Leaves
During the process of curing, which includes aging, yellowing, browning, drying, and fermentation, different chemical changes occur in controlled conditions of temperature, moisture content and ventilation. The first phase of curing is yellowing, marked with many chemical changes and changes in color. During this phase, chlorophyll starts to degrade and the green color of leaf is changed into the characteristic yellow color of tobacco. Burley’s yellowing phase is prolonged for a few weeks at relatively low temperatures (up to 40 C), where almost all sugar is decomposed.
Although starch is necessary for aroma formation, its presence in tobacco products can cause bitterness and flavor changes [30]. Moreover, it can affect changes in color and odor of cured tobacco [11]. Therefore, complete degradation of starch is one of the most important changes during leaf processing.
3.1
Opposite to other carbohydrates (starch, cellulose, pectin), the presence of reducing sugar has a positive influence on tobacco-smoking properties, improving flavor and aroma [45].
3.2
Degradation of starch starts in the first phase of curing and is predominantly caused by amylolytic enzymes. Those types of enzymes are very sensitive to temperature and moisture content. Temperatures above 70 C and low humidity can completely stop their activity in tobacco leaves [30]. In the research of Abubakar et al. [43], most of the starch degraded within the first 80 h of the yellowing phase. Yamaguchi et al. [42] proved starch degradation in tobacco leaves during the curing process to be predominantly caused by amylase enzymes.
3.3
Maillard reactions are a complex system of non-enzymatic browning reactions that have been gaining more and more attention during the past two decades. It is well known that Maillard reactions generate aroma and flavor compounds during thermal processing in the food and plant industry. Those types of processes include transformation of aroma and color precursors (sugars and amino acids) into aroma and color compounds. Examples for such process are roasted meat, baked bread, roasted cocoa, and nuts [46]. Moreover, many specific beverages such as coffee, beer, whisky, and tea owe their aroma properties to the Maillard reaction.
As already mentioned, tobacco aroma profile originates from two main processes, degradation of carotenoids and Maillard reactions. Degradation of carotenoids mainly results in floral aroma compounds such as damascenone and megastigmatrienone. Maillard reactions result in di erent aroma profiles. Those compounds give tobacco products a characteristic woody, caramel and baking flavor.
This one statement from this study becomes a key element of how to manage the cannabis drying and curing process…
…at a moisture content between 10% and 27%, changes in tobacco are conditioned primarily by enzymatic reaction; above 27% moisture, microbiologic reactions are dominant, and at moisture content below 10%, all enzymatic reactions stop.
Summary:
If this theory is true, we’ve all been wrong about flushing and how it works.
I was wrong in thinking that it didn’t work at all, while the old school flushers were wrong about what was actually happening.
According to this theory, it’s not about the water at all, and instead, the observations people have been having all these years are caused by the tapering or removal of nutrition at the end, which begins reducing starch and accelerates curing.
This new insight points to starch as the cause of the bad taste.
Cannabis farmers have always generally agreed that there is a need to taper of on Nitrogen in the flower cycle, but we never had a reason for it except that it causes your flowers to grow foxtails and extra leaves. This information provides a whole new look at why we should drop nitrogen drastically in the last three weeks, and then remove roughly 60% or more of the food supply and 30% of the water in the last two weeks as we taper down to nothing at the end.
Reducing starch accelerates curing
This produces what I call…
Senescence is another word for the process of aging that all creatures experience as they near the end of life.
Using this concept essentially allows us to create an early beginning to the curing process by halting photosynthesis which causes the plant to begin pulling from stored starch reserves to keep itself alive. If we do this early enough, the plant will use up a significant portion of that starch and create a much smoother smoking product.
After harvest, the drying process is modified to push this process even farther my removing light to stop photosynthesis, and then retaining as much moisture as possible so the dying plant can continue respiration (which burns up starch) as long as possible during the drying process.
Are there any other good reasons for flushing?
#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…
In this study from the university of Guelph…
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.
And as if that little bombshell was not enough, this study also showed that by week 6 of 8, the plant already has all the necessary nutrition stored up so that a high stress event, like severe drought stress in this case, won’t cause a decrease in yield.
Don’t try that in week 4!
If you’d like to learn a LOT more information (with a similar level of detail) about how your growing, drying and curing techniques can all work together to enhance the smoking experience, sign yourself up…