To flush or not to flush?

Indoor Growing Hydroponics
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I have recently found myself talking about flushing on this and other sites.

I have a friend who uses the same clone as me. We sometimes compare our product to evaluate growing techniques and so forth, also a little healthy competition, hehe.

I do not flush, he is at the other extreme and does a three week flush, which I think is excessive even for flushing. In order to keep his plants healthy, just before starting to flush he has a PPM of aroudn 1600, whereas the highest I go is just over 700 (@1EC=500PPM). His grams per watt is 85% of mine and I know he knows how to grow.

When we smoke together, he always wants to know what special thing it is I do to mine to make it better than his, he says mine has a more complex flavour and is stronger, both are a smooth nice smoke. I previously have tried to talk to him about flushing and how it is just curing but he is positive he is doing the right thing. Fair enough.

My opinion is that when you flush you are just doing a partial light cure to remove chlorophyll, which is harsh to smoke. To get clean burning weed you need the right chemicals in your weed to burn. If you feed right, it will burn right. Once it is dry it will burn well, even if not flushed.

I know that if I get unflushed weed, then cure it for 6 week to 4 months, it will be stronger than when fresh, it will burn smoothly, it will be a smooth smoke. I would do the pepsi challenge with any flushed weed that was clones of my plant grown in hydro (apples to apples of course)

I noticed a post in a ‘should I remove fan leaves’ post,

Speeding Up The Flushing Process
Remove some of the fan leaves 14 days before harvest. It helps speed up the flushing process and or makes flushing unnecessary (Homemadepot, 03.13.2002).

In this case, you are allowing more light to reach all parts of the plant, which will cure it faster, there is no ‘flushing’ going on here. This why it can ‘make flushing unnecessary’.

I am of the opinion that flushing reduces yield, and strength, for a benefit which you can obtain with a proper cure. Then you would have more and stronger weed. All you have to do is not insist it must be perfect the instant it is dry and wait for it to cure. To my mind, flushing=impatience.

I have also noticed more of a link between excessive flushing and black unburnable weed than a link to not flushing. The reverse in fact, plants fed until crop then cured burn better than flushed plants.

What are your thoughts? Does anyone want to do a side by side with 2 clones?

I feel this is an urban myth which has taken over the community costing us yield and potency.

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I will repost a very interesting collection of knowledge from another site.

Of all the garden myths that exist, I hate this one the most! I’ll do my best to drive nails in its coffin, once and for all!

So first, I’m going to give you a little information, then, DO your homework! This can’t be answered in a paragraph, you need to get some in depth information.

I’m providing a link to a piece Read Spear posted on his website: Sativa Magazine. I This page includes links for reposting so this material is shareable. also emailed him today about posting excerpts from his book: Marijuana Cultivation Reconsidered. This page includes links for reposting

Science is Truth!

Roots and Fluids: A Primer
Read Spear | October 29, 2014 | Growing, Growing Tips, Technical

Roots do not work like sponges. They are picky about what they bring into the plant. Here is Chapter 8: “Roots and Plant Hydraulics” from my new book, Marijuana Cultivation Reconsidered.

Why You Need to Know: All manner of foolishness flourishes when growers fail to understand how roots take up water and nutrients, or how water and nutrients move throughout the plant. Don’t be one of the fools.

It is worthwhile to know a few things about ground tissues. Understanding how they work will save you money when you shop for products that claim to sweeten your buds or when you sort through the advice offered indiscriminately on the Internet.

The root is comprised of four nested layers of tissue: the outermost layer is the epidermis; just inside that layer is the cortex; this is followed by the endodermis; and finally, at the core, is the stele. There are also root hairs, which are extensions of the epidermis which serve to greatly increase the root’s surface area. And, finally, at the growing tip of the root (the meristematic region) are specialized cells known as the root cap. Some truly amazing things go on in the root cap. None other than Charles Darwin understood the root cap of the plant to function “like the brain of one of the lower animals.”32 33 34 The root cap can detect:

water
gravity
toxic chemicals
nutrient
light
pressure
bacteria
fungus
rock/inanimate objects
other plants
other members of its own species
its own roots
In short, this is an amazing part of the plant.

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Roots bring water and nutrients into the plant using osmosis to absorb water, and active transport to absorb nutrients. (Note the use of the word “absorb”–with a B–which connotes transport across a barrier and into another space. When we consider soil properties in a later section of this book, we will discuss adsorption–with a D–which means to stick onto, rather than transport into.) This fact is worth emphasizing: Roots use osmosis to absorb water, and active transport to absorb nutrients.

Counterintuitively, water and nutrients do not enter together with the ions that are dissolved in water. That’s an important point to remember. They enter separately. The second point to remember is that plants do not use nutrients in their organic form. They cannot use nutrients until they have been broken down into their inorganic form. We will get into that later in the book, just note it for now.

An additional mechanism is used within the plant to move water and other solutions around. That mechanism is called convection. So, that’s three ways of moving water into and through the plant: osmosis, active transport and convection (where the first two bring water into the plant and the third moves it around). Let’s have a look at each.

Understanding osmosis is critical to your understanding of roots. Please take the time to comprehend osmosis because doing so will serve you in many ways as a gardener. To begin, it will help to become familiar with a few terms:

Solute: This is the stuff that gets dissolved. Here, we mean mineral solids (nutrients)

Solvent: This is the stuff that dissolves the solute. In this case, we mean water

Solution: A combination of solute and solvent

Dilute: Lacking in, or having a low level of, solute

Concentrate: High in solute

Osmosis is the transportation of solvent through a semi-permeable membrane and into a concentration of higher solute in whichever direction will enable it to equalize concentration on either side of the membrane.

We always want osmosis to create pressure (which results in turgor, or the stiffening of pressurized tissue) in the plant cell. This causes the plant to stand upright.

Osmosis becomes a factor when the dissolved chemicals on one side of a semi-permeable membrane cannot pass through the membrane in order to equalize concentrations. Instead, water moves through to achieve the same result. Hence:

If the plant cell has a high concentration of solute (think dissolved nutrient) and the water outside the cell has a low concentration of solvent (think pure water), we describe that cell as being in a hypotonic solution and the plant cell will absorb water, resulting in turgid plant tissue.

If, on the other hand, the plant cell has a high level of solvent and the material surrounding the cell has a high concentration of solute, we describe that cell as being in a hypertonic solution and the plant cell will lose water, resulting in flaccid plant tissue.

When a plant cell is in a solution equal in concentration to the fluid surrounding it, it is said to be in an isotonic solution. This does not mean that no transport is occurring; it means that “water in” is in balance with “water out.”

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Active transport costs the plant energy; that’s why it’s called “active.” Active transport is the use of chemical energy (ATP in this case) in order to move solution against the concentration gradient. That means moving mineral nutrient ions from an area of low concentration into an area of high concentration, or, in other words, not equalizing the concentration but instead creating a greater concentration disparity–just in the opposite direction of osmosis.

Plants need to engage in active transport after they have exhausted their ability to absorb nutrient through diffusion. (In contradistinction to osmosis, diffusion is simply the tendency of molecules to evenly distribute themselves; for example, in the way an odor can become detectable at a distance from where it originated.) Since the plant’s need for ionic nutrient is greater than the supply available outside the plant in the soil, the plant must have a way to move that nutrient beyond equilibrium and against a concentration gradient.

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Plants are able to do this by using ATP to change the shape of proteins in the cell membranes. When the proteins change shape they are able to bind to ionic (charged) nutrients and deposit them on the opposite side of the membrane, which, in turn, causes another change in protein shape and results in another transport of ions, this time in the other direction. Simply put, there’s a trade. After the trade has occurred, an additional ATP molecule is needed to repeat the process.

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Plants use Hâș (hydrogen) and OH⁻ (hydroxyl) ions, which are products of plant metabolism, to trade for mineral nutrient ions that adhere to soil particles, such as NO3⁻ (nitrate) or Kâș (potassium). This mechanism is so effective that plants are able to move some nutrients against a gradient with a difference in concentration of 10,000 times!35 Plants do not secrete Hâș (hydrogen) and OH⁻ (hydroxyl) ions in equal measure; instead they tend to favor Hâș, which means that the plant is actively acidifying its soil (this occurs in addition to decomposition and consequent carbonic acid accumulation.) We will get into this a little more later on because it has implications when you mix or buy potting soil.

Convection is the loss of water through the leaves of the plant. This loss creates a lower pressure gradient at the top of the plant which the water moves toward in order to fill. This phenomenon of the plant pulling water upward is called the Cohesion-Tension (or C-T) mechanism and it is the primary mechanism for moving water through a plant.

Water sticks to itself because of its hydrogen bonds (cohesion). In the small tubes of the xylem, these bonds provide enough tension to enable the low pressure at the top of the plant to literally pull water great distances upward without ever breaking this tension.

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In Summary: Roots are amazing, almost “smart” things. Osmosis, active transport and convection carry water and nutrients into and throughout the plant. Plants do not absorb nutrient broth the way a sponge soaks up water from a countertop; mineral nutrient ions and water enter separately. Excessive concentrations of nutrient solution in the soil can reverse the direction of osmosis, killing your plant. Understanding how uptake and transport mechanisms work also resolves the question of whether or not you need to “flush” at the end of each growing cycle. Flushes are pure nonsense, as you now know. Only someone without an understanding of these transport mechanisms would claim differently.

  1. Darwin, C., & Darwin, F. (1897). The power of movement in plants. Appleton.
  2. Barlow, P. W. (2006). Charles Darwin and the plant root apex: closing a gap in living systems theory as applied to plants. In Communication in Plants (pp. 37-51). Springer
  3. Baluska, F., Mancuso, S., Volkmann, D., & Barlow, P. W. (2009). The ‘root-brain’ hypothesis of Charles and Francis Darwin: Revival after more than 125 years. Plant Signal Behav, 4(12), 1121-1127.
  4. Taiz, L., & Zeiger, E. (2010). Plant Physiology, Fifth Edition (Fifth ed.). Sinauer Associates, Inc.
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Nice article and read.

For any grower though the flush refering to a chemical flush here is a tool to have in the box.

The alternative tool is to use water, but when we refer to water here its a bit more in depth than just water.

You need to understand the article above, but also things like water chemistry.

Hard and soft water, acidic or alkali water, distilled water, de-ionised water.

I still believe flushes have a place in the m,arket place and growers tool box as many now also contain flavours/scents.

Do I flush my self at the end of each grow?

Answer is yes, in many different ways depending on medium and the reason. EG is the medium to be re-used.

Good read and thanks @MicroDoser

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As an organic gardener, I think flushing is for toilets. I feed my soil up to and past the harvest. Even without plants I still water/feed on the same cycle. I strongly believe if you don’t over feed, you won’t need to flush.

I like VaderOG’s solution to flushing. He still uses enzymes and sweeteners through flush, but stops with the NPK. He claims the black ash and crackling is from vitamins, particularity B2.

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Does he add vitamins like B2 or does he say they are present in the plant if you use normal NPK?

My opinion on black ash that does not burn properly is simply not having enough elemental chemicals in the weed to burn properly.

I almost do the opposite to VaderOG, I keep with the NPK but drop the additives.

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Don’t know, he runs Canna A+B.

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Surely something that leaves something in your plant matter which imparts a taste is doing the opposite of flushing?

As do I. I do not suffer from crackly burning and I do not flush. I think the cause of the crackle is not the Canna A+B.

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Dont get me wrong guys im speaking from the viewpoint of teaching and working with others, or pushing a plant to its limits to find out where the limits and best performancees are from changing feeding regimes using mono nutrients and ready made single, double or triple part nutrients of the shelf.

Shit happenes sometimes and flushing is required if I want to regain the plant versus loose it.

Sometimes I use only plain ph adjusted water from the tap, other times different methods.

A note to point out here is to test your base water before adding nutrients as it does change out of the tap from time to time.

My point been, bought/purchased flushes are a tool to have in your box just in case of not been a perfectionist every time :wink: But not everyone has the time or space to experiment with a control plant and clones from the same mother to test what food levels it performs best with :astonished:

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This is when flushing is the right thing to do, when you want to reduce the strength of nutrients at the root surface because shit happened and there is too much. When I talk about flushing, I talk about restricting nutrients at the end of a cycle in order to improve quality as something regular and normal that is done every time. Store bought ‘flushes’ to my mind are something else and emergency plain water flushes because of shit happening are something else again (and quite necessary on occasion)

I would agree with this 100%. If you are in the UK, it is law that the water company have to supply you with a water quality analysis for free on request.

Often they just put it up on their website and have more detail than you could possibly get from a home test.

Here is one from Warrington

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Your not far from me so probably similar water, think mine comes from the rivington and / or longridge reservoirs but united utilities head office is in warrington.

I keep koi carp too so have all that info :wink: @MicroDoser

Best days for water is wednesday least chlorine as they start to pump it in more starting thursday evening shift ending on sunday evening shift :wink:

Im not far from the crypto spiridium water works outbreak source :wink:

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I reduce nute strength towards the end of flowering so i don’t need to do much of a flush. Usually 1 week in peat based compost and a few days in proper hydro using tap water which obviously still has some mineral content. Never saw the need for an RO flush yet but some seem to. Maybe they go nuts with additives or something eh.

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Reminds me of the local chippy. Best time to get chips is when they get back from holiday because then they change the fat, hehe.

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i feed fairly heavily, ~2000ppm during flower give or take a few hundred. i have 10 females of same strain from seed and i’m going to test varying levels of pre harvest flushing again. i’m also going to rerun all 10 plants a second time from clone and will flush them opposite (least flushed will become most etc). will report back in 2 months and again in 5 months.

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Wow, that is indeed very heavy. Why do you go so high? I never see over 700 or so


Please remember though, IMHO the flush is just a partial cure to remove chlorophyll so the unflushed plants will need curing or they will be harsh as a result of chlorophyll. Also note that drying too quickly can lock in that harsh taste which can make it seem that the flush is the thing that makes the flowers smoke better. If you dry too quickly then you will not see any benefit from curing and believe that the flush is what made the cannabis better, instead of the light/UV cure that happens when you deprive your plants of nutrients during the heavy growth period.

I will quote some good words from Robert Connell Clarke about drying and curing which I have found make a difference to the quality of the final product and reversed my thinking on curing and flushing.

Floral clusters will dry much
more slowly if the plant is dried whole. This means that all of the water in the
plant must pass through the stomata on the surface of the leaves and calyxes
instead of through cut stem ends. The stomata close soon after harvest and
drying is slowed since little water vapor escapes.

Usually
about 10% water remains in dry, stored Cannabis floral clusters prepared for
smoking. If some water content is not maintained, the resins will lose potency
and the clusters will disintegrate into a useless powder exposed to
decomposition by the atmosphere.

As floral clusters dry, and even after they are sealed and packaged, they
continue to cure. Curing removes the unpleasant green taste and allows the

resins and cannabinoids to finish ripening. Drying is merely the removal of water
from the floral clusters so they will be dry enough to burn. Curing takes this
process one step farther to produce tasty and psychoactive marijuana. If drying
occurs too rapidly, the green taste will be sealed into the tissues and may remain
there indefinitely. A floral cluster is not dead after harvest any more than an apple
is. Certain metabolic activities take place for some time, much like the ripening
and eventual spoiling of an apple after it is picked. During this period,
cannabinoid acids decarboxylate into the psychoactive cannabinoids and
terpenes isomerize to create new polyterpenes with tastes and aromas different
from fresh floral clusters. It is suspected that cannabinoid biosynthesis may also
continue for a short time after harvest. Taste and aroma also improve as
chlorophylls and other pigments begin to break down. When floral clusters are
dried slowly they are kept at a humidity very near that of the inside of the
stomata. Alternatively, sealing and opening bags or jars or clusters is a
procedure that keeps the humidity high within the container and allows the
periodic venting of gases given off during curing. It also exposes the clusters to
fresh air needed for proper curing.

So, for unflushed plants which still contain all of their “chlorophylls and other pigments” it is important to dry them slowly, and cure them properly retaining some moisture to allow biological processes to continue. Then, even with no flush (although if you feed that heavily you may need a much longer cure) you should get “tasty and psychoactive marijuana.”.

I would say the minimum time to cure would be at least two or three times as long as the plants were flushed or 4 weeks or so.

Nice one for doing this test, Myself I am sure of my own results which have made me stop flushing but I am very interested in seeing how other people do.

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Measured radioactivity of popular nutrients.
https://manicbotanix.com/hydroponics-versus-organics/2/
I think I’ll continue flushing my hydro.

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You do whatever you like. As will I.

EDIT : Your linked site actually has a bit on flushing,the-flushing-cannabis-myth-busted, I have no idea why you did not link to that instead of whatever you were trying to show by talking about organic vs hydro


It says “flushing had no effect on reducing tissue nutrient accumulation between flushed crops and control (a crop that wasn’t flushed)” confirming my position.

One thing that you could have linked to which does contradict what I thought is that flushing did not reduce yields. They say it did not have a positive nor a negative effect on quality, which seems right if yield was not affected.

Your link confirms my position that flushing does not remove anything and is instead effectively a partial light cure and the only way to get smooth smoking weed is through a cure, either before you cut the plant, or afterwards.

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Everyone’s garden and technique is different. I personally never exceed 1.4EC but I know others who regularly feed over 2.0EC

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Enjoy your polonium gaggers :+1:

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flushing myth.png1619×587 71.3 KB

From your link.

Flush or do not, it does not do anything but if it makes you happy, then do whatever you like. Personally, I like to present facts to others so they can make better informed decisions.

Turn around clockwise three times, click alternate fingers 15 times, starting with the left hand of course, then jump up and down and say “bibble” before you cut your plant if you want, it will have the same effect.

None. Because it is superstition that is not based in fact.

Again, I ask what have I done to you to cause you to act like this?

You are not making yourself look good lately.

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I just like getting you all heatered up.

My point actually was that nutrient solution contains radioactive compounds.

Plants are still respiring in the final 2-3 weeks, and they are also mostly water or in your case solution by weight when harvested. You can harvest them full of water, or you can harvest them full of (albeit mildly) radioactive solution. I’m simply proposing to you that polonium, etc doesn’t “cure out”.

Label a jar of your stuff “Un-flushed Hydro”, take it to the dispensary, and see how fast it sells.

You’ve never proposed any benefit to watering nutrients in the final week. Please enlighten me.

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