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A critical look at preharvest flushing

Emilya

Member of the Month: Mar 2019 - Grow Journal of the Month: Jan 2020
I'm pretty sure they were studying what "flushing" is considered to be by the Cannabis industry not what you are referencing.
Maybe so... High Times sold out years ago and is as much of an informational resource in my mind as some of these personality driven web operations have become. We here at 420Mag are at the leading edge of the cannabis growing science, and we are developing new insights and methods every time we document a grow with a journal.

I really don't recognize what the "industry" does as they attempt to mass produce pot to be the be all and end all of the science. These fools are still arguing back and forth as to whether it is possible to flush a plant. I would love to prove that article wrong, and it would be easy to do when they reference there being no difference in quantity or quality in starving the plants for various time periods. I am 100% positive that I could set up a side by side that would totally disprove this ridiculous claim.

Instead of pushing for the best quality, mass producers in the industry instead worry about the costs of water and nutes at the end of the grow. There is a huge difference between what they do and what I do. They are running a business. I am conducting scientific research. Not only that, but I have the influence of 4 generations of growers in my family, 3 that regularly visit my grow rooms. My grandpa is gone now, but Dr. Read in Wales as chief arborist is very well known for his theories in the garden, and guess who trained me as a little girl? I know for a fact that this modern tendency to think that it is a good thing to starve your plants at the end, came in with the industrialization of the emerging cannabis "industry" and I say again that to call this a "flush" is a perversion of age old horticultural technique.
 

TorturedSoul

Member of the Month: May 2009, Oct 2010, Sept 2017
High Times sold out years ago
Yeah, no doubt. They should have changed the name of that r(m)ag 40 years ago to Cannabis Times, because that's pretty much all it's about (now).

Other than that, the entity is a whore to its advertisers. But that seems to be a fully accepted practice in this industry :rolleyes: .

Speaking of accepted practices:

A study on whether it's better to feed nutrients right up until harvest time... ran by a nutrient seller... would not meet mine. (And I'm basically a "no-flush" guy.)
 

Emilya

Member of the Month: Mar 2019 - Grow Journal of the Month: Jan 2020
in response, I offer as proof of the need to feed, the organically grown end product, agreed to be the very best pot grown; and fed right up to the end, 24/7.
 

Elvin

Well-Known Member
Yeah, no doubt. They should have changed the name of that r(m)ag 40 years ago to Cannabis Times, because that's pretty much all it's about (now).

Other than that, the entity is a whore to its advertisers. But that seems to be a fully accepted practice in this industry :rolleyes: .

Speaking of accepted practices:

A study on whether it's better to feed nutrients right up until harvest time... ran by a nutrient seller... would not meet mine. (And I'm basically a "no-flush" guy.)
Coming from personal experience as well as actually taking a few agriculture classes (dealing with soils as well plants) so I have a very good understanding of how plants utilize nutrients, I do not "flush" nor bother to stop nutrients.
I do however also take any study with a grain of salt (because of inherent confirmation bias issues), even one as well done as this one that looks into areas that coincide with how plants actually use nutrients (not weed forum nonsense).
It needs to be replicated, and it needs to be done with other nutrient lines and methods
 

Weaselcracker

Nug of the Year: 2016 - Member of the Month: Sept 2015, Nov 2016 - Nug of the Month: Oct 2016 - Plant of the Month: May 2016
So as stated on this thread here Flush or don’t? I ran a side by side experiment on two Pineapple Chunk clones. The experiment was to feed one plant till the bitter end, and stop or greatly reduce feeding the other plant at the end as I normally do. I harvested those plants a few days ago.


I’m going to post this in that thread as well but it seems to have been dead for quite a while.
I just happened to be wandering through the daily posts and see that you guys are still uhhh arguing talking about the subject here- so the timing is good and I will post here as well.

The bud is in the dryer. I have been scratching my head over what to say about the plants.


I knew the results wouldn’t be ‘scientific’, but I did hope to be able to show some clear differences between the plant that was fed to the bitter end and the one that ended with a reduced diet. In my experience I’ve always favoured the ‘vine ripened’ thing where I let the plant feed off its own foliage for the last few weeks.

There were a couple minor problems that made it even less scientific than I’d like, at least when I comes to trying to post about the differences.

Problem #1 was that I left these two plants about a week longer than I should have because I got too busy to harvest them. So they both had a bit of a shabby look, especially the more ‘starved’ one.

Problem #2 was that I didn’t seem to be able to capture the difference between the bud on them with pics, with my phone. In fact I completely 100% failed. To the point where it’s barely even worth posting pics. But whatever...everyone likes looking at pics don’t they?


I put these two P Chunk clones into flower on the same day, and they had the same of everything up until the last three weeks or so. This PC pheno is a good yielder and grows large bountiful buds. Both were in ten gallon pots.

The ‘fed’ plant was fed right to the end at about 2.2 EC. It’s the smaller one on left. I’ll call that one the ‘fed’ one.

The other plant, the larger one on the right, was cut down to about 0.7 EC for the last three weeks. I’ll call that the ‘weaned’ one, since I can’t think of a better word right now.




First impressions looking at them were ‘Wow the one that was fed right to the end looks way better than the other one’. Also, I regretted leaving them both a bit late because the buds on the more starved one looked so ragged. You can maybe see in the pics how it’s quite a bit paler. I worried it was mostly going to end up going straight into the hash trimmings pile :( The bonus to that of course is that it saves a bunch of trimming ha ha- so I was happy about that. I admit I always really love being able to chop a plant in 10 minutes... I was looking forward to that :laugh2:

First I trimmed up the ‘fed’ one. It seemed decent, though not particularly amazing. The bud had a sort of dry leafy feel to it, not very sticky- a bit generic.

The fed one.

It wasn’t till I brought in and started trimming the other plant that I instantly realized the difference. So much for the ‘straight to the hash trimmings pile’ idea. In spite of the mangey looking sugar leaf, the plant that was weaned was far stickier than the other one. It was gooey and sticky, where the fed plant had been much more dry and crisp.

The unfed one




The fed plant was easy to trim -snip snip. It had a bit of a leafy vegetative feel to it.


The bud of the weaned plant had a soft gummy texture, was stinkier, riper looking, and all around much more impressive to handle, despite looking a bit ragged.


I tried hard to capture the difference with my crappy phone-camera but, IMO, completely failed, The pics just don’t show the texture. In fact they don’t show much of a difference at all. So... wtf... I guess.

What does it mean?

Too many variables involved to make any major judgements, even for my own self. And other people’s grows are going to vary from mine for zillions of reasons.

But the bottom line is- I poured almost three weeks worth more fertilizer into a plant that ended up with bud that looked distinctly inferior to the one that I barely fed at the end.

Could the bud that ‘looks worse’ actually be ‘better’ somehow than the stuff that looks better? Well- I very much doubt it based on my growing experience, but I suppose I’ve been fooled by bud appearance before.
Once it’s all cured I’ll see what the smoke reports say.

I will continue to repeat this experiment a few more times, and will post what I get. Maybe I can get some better photo examples. Also- the PC is a little too indica for my smoking tastes. I will run the test with some of my preferred sativa strains so I can smoke to my hearts content.

If the ‘fed till the end’ bud continues to be inferior I will stop experimenting, because obviously I don’t want to be wasting fertilizer, or producing inferior bud.

I don’t have any particular asked to grind on the subject and I don’t worry about how other people wanna grow their weed. I can only say what seems to work best for me, and that’s the subject I’m pursuing.
 
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Pinktiger777

Well-Known Member
I know what you mean. It took me an hour the other day with a toothbrush and iso and salt cleaning out my grinder. The dang thing was so gummed up it was unusable. Scissors work way MO better.
I soak my grinder in alcohol for an hour and it comes out clean. I’ve often wondered if I could use the alcohol later to get high, but I use iso and you can’t drink that. Lol
 

Nunyabiz

Well-Known Member
I soak my grinder in alcohol for an hour and it comes out clean. I’ve often wondered if I could use the alcohol later to get high, but I use iso and you can’t drink that. Lol
That just gave me an idea.
Our grinders have been so mired up in oil that we haven't used them in months.
The bud just turns to hash balls.

Been using scissors mostly.

I just bought a Extractcraft Source Turbo on a black Friday deal, I think iam going to soak these grinders in 200 proof food grade alcohol and run that alcohol through a Büchner Funnel to filter it and add that to my first batch.
Has to be quite a bit of oil in there.
 

TorturedSoul

Member of the Month: May 2009, Oct 2010, Sept 2017
One assumes the alcohol will eventually evaporate. Butane, of course, evaporates much faster at non-Winter temperatures (but is more flammable).

Try sticking your grinder into the freezer for a few hours first, then quickly disassembling it and SLAMMING each piece onto a hard (unbreakable) flat surface. You might be able to scrape up a small pile afterwards, IDK.
 

Cotarakos

Well-Known Member
Pre harvest flushing is a controversial topic. Flushing is supposed to improve taste of the final bud by either giving only pure water, clearing solutions or extensive flushing for the last 7-14 days of flowering. While many growers claim a positive effect, others deny any positive influence or even suggest reduced yield and quality.

The theory of pre harvest flushing is to remove nutrients from the grow medium/root zone. A lack of nutrients creates a deficiency, forcing the plant to translocate and use up its internal nutrient compounds.

Nutrient fundamentals and uptake:

The nutrient uptake process is explained in this faq.

A good read about plant nutrition can be found here.

Until recently it was common thought that all nutrients are absorbed by plant roots as ions of mineral elements. However in newer studies more and more evidence emerged that additionally plant roots are capable of taking up complex organic molecules like amino acids directly thus bypassing the mineralization process.

The major nutrient uptake processes are:

1) Active transport mechanism into root hairs (the plant has to put energy in it, ATP driven) which is selective to some degree. This is one way the plant (being immobile) can adjust to the environment.

2) Passive transport (diffusion) through symplast to endodermis.

‘chemical’ ferted plants need to be flushed should be taken with a grain of salt. Organic and synthetic ferted plants take up mineral ions alike, probably to a different degree though. Many influences play key roles in the taste and flavor of the final bud, like the nutrition balance and strength throughout the entire life cycle of the plant, the drying and curing process and other environmental conditions.

3) Active transport mechanism of organic molecules into root hairs via endocytosis.


Here is a simplified overview of nutrient functions:

Nitrogen is needed to build chlorophyll, amino acids, and proteins. Phosphorus is necessary for photosynthesis and other growth processes. Potassium is utilized to form sugar and starch and to activate enzymes. Magnesium also plays a role in activating enzymes and is part of chlorophyll. Calcium is used during cell growth and division and is part of the cell wall. Sulfur is part of amino acids and proteins.

Plants also require trace elements, which include boron, chlorine, copper, iron, manganese, sodium, zinc, molybdenum, nickel, cobalt, and silicon.

Copper, iron, and manganese are used in photosynthesis. Molybdenum, nickel, and cobalt are necessary for the movement of nitrogen in the plant. Boron is important for reproduction, while chlorine stimulates root growth and development. Sodium benefits the movement of water within the plant and zinc is neeeded for enzymes and used in auxins (organic plant hormones). Finally, silicon helps to build tough cell walls for better heat and drought tolerance.


You can get an idea from this how closely all the essential elements are involved in the many metabolic processes within the plant, often relying on each other.

Nutrient movement and mobility inside the plant:

Besides endocytosis, there are two major pathways inside the plant, the xylem and the phloem. When water and minerals are absorbed by plant roots, these substances must be transported up to the plant's stems and leaves for photosynthesis and further metabolic processes. This upward transport happens in the xylem. While the xylem is able to transport organic compounds, the phloem is much more adapted to do so.

The organic compounds thus originating in the leaves have to be moved throughout the plant, upwards and downwards, to where they are needed. This transport happens in the phloem. Compounds that are moving through the phloem are mostly:
Sugars as sugary saps, organic nitrogen compounds (amino acids and amides, ureides and legumes), hormones and proteins.

Not all nutrient compounds are moveable within the plant.

1) N, P, K, Mg and S are considered mobile: they can move up and down the plant in both xylem and phloem.
Deficiency appears on old leaves first.

2) Ca, Fe, Zn, Mo, B, Cu, Mn are considered immobile: they only move up the plant in the xylem.
Deficiency appears on new leaves first.

Storage organelles:
Salts and organic metabolites can be stored in storage organelles. The most important storage organelle is the vacuole, which can contribute up to 90% of the cell volume. The majority of compounds found in the vacuole are sugars, polysaccharides, organic acids and proteins though.

Translocation:
Now that the basics are explained, we can take a look at the translocation process. It should be already clear that only mobile elements can be translocated through the phloem. Immobile elements cant be translocated and are not more available to the plant for further metabolic processes and new plant growth.

Since flushing (in theory) induces a nutrient deficiency in the rootzone, the translocation process aids in the plants survival. Translocation is transportation of assimilates through the phloem from source (a net exporter of assimilate) to sink (a net importer of assimilate). Sources are mostly mature fan leaves and sinks are mostly apical meristems, lateral meristem, fruit, seed and developing leaves etc.

You can see this by the yellowing and later dying of the mature fan leaves from the second day on after flushing started. Developing leaves, bud leaves and calyxes don’t serve as sources, they are sinks. Changes in those plant parts are due to the deficient immobile elements which start to indicate on new growth first.

Unfortunately, several metabolic processes are unable to take place anymore since other elements needed are no longer available (the immobile ones). This includes processes where nitrogen and phosphorus, which have likely the most impact on taste, are involved.

For example nitrogen: usually plants use nitrogen to form plant proteins. Enzyme systems rapidly reduce nitrate-N (NO3-) to compounds that are used to build amino-nitrogen which is the basis for amino acids. Amino acids are building blocks for proteins, most of them are plant enzymes responsible for all the chemical changes important for plant growth.

Sulfur and calcium among others have major roles in production and activating of proteins, thereby decreasing nitrate within the plant. Excess nitrate within the plant may result from unbalanced nutrition rather than an excess of nitrogen.

Summary:
Preharvest flushing puts the plant(s) under serious stress. The plant has to deal with nutrient deficiencies in a very important part of its cycle. Strong changes in the amount of dissolved substances in the root-zone stress the roots, possibly to the point of direct physical damage to them. Many immobile elements are no more available for further metabolic processes. We are loosing the fan leaves and damage will show likely on new growth as well.

The grower should react in an educated way to the plant needs. Excessive, deficient or unbalanced levels should be avoided regardless the nutrient source. Nutrient levels should be gradually adjusted to the lesser needs in later flowering. Stress factors should be limited as far as possible. If that is accomplished throughout the entire life cycle, there shouldn’t be any excessive nutrient compounds in the plants tissue. It doesn’t sound likely to the author that you can correct growing errors (significant lower mobile nutrient compound levels) with preharvest flushing.

Drying and curing (when done right) on the other hand have proved (In many studies) to have a major impact on taste and flavour, by breaking down chlorophylls and converting starches into sugars. Most attributes blamed on unflushed buds may be the result of unbalanced nutrition and/or overfert and unproper drying/curing.
Great read I've tested over and over, one plant flush, the other plant, bring down nutrients slowly til harvest, my buds taste better and are fater when fed to the end..my opinion feed them puppies.
 
Drying and curing (when done right) on the other hand have proved (In many studies) to have a major impact on taste and flavour, by breaking down chlorophylls and converting starches into sugars. Most attributes blamed on unflushed buds may be the result of unbalanced nutrition and/or overfert and unproper drying/curing.
yeah ive done a test on people (my friends) and not a single one of them could pick that i i didnt flush my weed AT ALL. the harshness from smoking is usually due to left over sugars and carbohydrates. people talk about built up nutrients but ive never seen flowers show nute burn, only the leaves in them and that is because plants sacrifice leaves as waste dumps in those situations to preserve the flowers, so how the hell can you "flush" these nutes out? all your ndoing is starving the plant inthe final push when it needs the most nutrients. i looked at tobacco growers and they do the exact opposite to us, they give more nutes not less at the end. also remember correct curing and drying processs will cause the plant to continue to use up its stored energy (carbs and sugars) as long as it doesnt get to dry. I never flush, at all, and ive had people tell me how its grea that they finally found someone who flushes properly. i roll my eyes in my head but whatever.
 
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