A critical look at preharvest flushing

medme said:
i think its all in the trimming and the cure, for the good taste we like... i have NEVER flushed and i have gotten ALOT of good comments on my bud..... 30 day cure AT LEAST...
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Very happy I seen this. SO feed up until harvest.
 
DonBisso1 said:
i think another grower named jorge cervantes may have covered it in his videos and books
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Jorge Cervantes: The Connoisseur’s Harvest

World-renowned cultivation expert Jorge Cervantes lays out the details on drying and curing, including the best ways to maintain the fragrance, flavor and potency of your buds, in this excerpt from his new book, The Cannabis Encyclopedia. Read It and Reap: An Overview A bountiful harvest of...
hightimes.com
 
TheMaverickFarmer said:
I wonder if outdoors farmers flush their fields before they harvest their crops
.....
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There are a lot of variables that should be considered before deciding to flush or not to flush, method of grow, how the plants were fed, size of containers, etc......

Here is plenty of evidence that says to the contrary, but it seems that your comment here is a dig at people who decide to flush their crops.

The Importance Of Flushing Your Plants Before Harvest <-- Advanced Nutrients Blog.

Flushing 101: For Best Results, Just add Water – Green Goddess Supply
 
BAKIND said:
There are a lot of variables that should be considered before deciding to flush or not to flush, method of grow, how the plants were fed, size of containers, etc......

Here is plenty of evidence that says to the contrary, but it seems that your comment here is a dig at people who decide to flush their crops.

The Importance Of Flushing Your Plants Before Harvest <-- Advanced Nutrients Blog.

Flushing 101: For Best Results, Just add Water – Green Goddess Supply
Click to expand...

Lol...here we go. Hey growmie, no malicious intent here, whatever floats your boat dude
 
Here is a look at my latest harvest, G13, flowering time was 13 weeks. Grown in DWC with powder feed. I did a pre-harvest flush for 10 days, and 48 hours of darkness. Slow dried for 10 + days, at 70 degrees for first 3 days, then 64-66 for the remainder. I am disappointed with the overall size of the flowers, however, the smoke is good, and it stinks. since they have been in these containers for a few days, I can tell the smell has evolved some. Yesterday I rolled one and it stayed lit. Looking forward to curing 6-8 weeks at least. Last crop was not flushed, and the smoke would not stay lit even after 6 month of cure.


photo_2021-05-30_15-21-41.jpg
 
really really happy for you, good work mate!
 
Very good page, I find it very useful. However, in regards to flushing, I still think there are a lot of variables that should be taken into consideration when flushing or not.

I understand the argument that you can't flush nutrients out of plants, although I am not certain whether it's true or not.

The reason to :flush: is to withhold nutrients to the plants so they can start consuming nutrients within them, and not for the actual flush to remove the nutrients itself.
 
Smokin Moose said:
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.
Click to expand...
OMGoodness! NO! Flushing is not to remove minerals from the plant or to wash away anything. This is not the purpose. Feed your plants till the end just hold back on the (N). Flushing is watering without something added, in this case without nitrogen. This week or 2 allows the plant to consume the Nitrogen stored within the plant. Ever had pot snap crackle and pop? It's from excessive nitrogen. We are not needing to wash away all the goodness in the soil or media, this may be done with clear water for other reasons. The other issues of ashes and taste are a bit more subjective so I'll avoid it. As for me, I believe cutting back on NITROGEN then stopping all extra nitrogen for the last week to 10 days is the way yo go.. Story is told that nitrogen tastes bad, and doesn't burn clean.
 
Smokin Moose said:
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.
Click to expand...

The term "final flush" is actually a misnomer here. This is where some confusion starts. There are several types of "flushing". Remember chlorophyll, I'll get back to that. I think a big part of the reason that this is a controversial subject is because it is unclear how to do it. There is very little information on what might be another way.
Flushing all the good stuff out of that soil I worked so hard to build. All those billions of microbes and bacteria are paying a heavy price. Oh, the humanity. I can't take it any more.

A completely different concept of flushing. Focus on one single nutrient, Nitrogen. This is what we should be interested in "flushing". Flushing is not a good term here because it implies we are washing something away, that's not going to happen. I'll keep is simple, I am not writing a book here. Chlorophyll smokes like lawn grass.

Keep feeding your plants but, STOP adding extra nitrogen for dirt growers. Hydro folks cut back on the N and I think you need to switch to NH4 or ammoniacal forms and wean off. The plants will stop making chlorophyll and will switch and use all that stored nitrogen (from chlorophyll) for other needed metabolic processes. Sure, the microbes will still be feeding the roots nitrogen but, at a much much slower rate. You can literally see the chlorophyll leaving the lower branches first and working its way up the stems.
"Finally, silicon [silica] helps to build..." trichomes. That's right Silica is needed to make trichomes. You will have lots and lots of silica if you use vermiculite in your soil. Also, you can buy silica blast liquid products for hydro or soil. Don't forget to add sulphur compounds for stinky tasty buds.
p.s. Chelated minerals save your plant energy to do other things like GROW. Chelated minerals combined with inoculated dirt: $15 adds 15% buds.
 
You get rid of the chlorophyll by long slow drying and curing.
In living Organic Soil there is nothing to flush and the plant fades as it naturally should.
In coco you just feed flowering nutrients right up to harvest.

Final 2 week flushing does nothing whatsoever except give you about 10% less yield.
And any mobile nutrients used by the plant and turning leaves yellow just goes straight to the bud so even the theory of flushing is flawed.
 
Nunyabiz said:
Final 2 week flushing does nothing whatsoever except give you about 10% less yield.
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believe it can destroy the final 2 wk bud swell too. lotsa growers either miss or it doesn't happen.
 
Nunyabiz said:
You get rid of the chlorophyll by long slow drying and curing.
In living Organic Soil there is nothing to flush and the plant fades as it naturally should.
In coco you just feed flowering nutrients right up to harvest.

Final 2 week flushing does nothing whatsoever except give you about 10% less yield.
And any mobile nutrients used by the plant and turning leaves yellow just goes straight to the bud so even the theory of flushing is flawed.
Click to expand...
I agree the concept is flawed. Rightly or not I have "flushed" before because I made a big mistake and my runoff was hot reading about 4650 ppm, I was aiming for 1750 ppm. I freaked out and started watering to rinse off some nitrogen.

It just never made sense to me to shut down plants just when they are at full throttle for weight gain and trichome development. So, I spent about 40 hours looking at the different ideas. Probably another 40 trying to figure out what the first 40 was about.In the end I learn why I am doing the things I do to my plants

Personally, I wean off nitrogen toward the end. I have come to a better understanding of the process of chlorophyll degradation in living plants. I got goofy with phosphates and sulphates, and stuff from the medicine cabinet and the kitchen cabinets and the refrigerator. Somewhere in this I must open a benevolent version of Pandora's box.

When these plants are allowed to senesce or late into flowering they begin to conserve their sugars. The plants roots stop bleeding sugars to the microbes. The microbes run out of energy and stop feeding the plant minerals and stuff. The plant makes seeds (or not) and runs out of energy due to the shortage of stuff from the microbes...I gotta stop some where, and all the players wait till next season. Flush twice, it's a long way to Chicago.

Happy Scientist
 
MAK1 said:
Personally, I wean off nitrogen toward the end. I have come to a better understanding of the process of chlorophyll degradation in living plants.
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it really is the n that all this junk was about yrs ago. this began with tomato nute growers and early hydro apps sometime in the late 70's early 80's. pre-internet.

there were little or no cannabis specific nutes available and folk relied on hard to source books, which had their own flaws, or gossip. the latter hasn't changed.

obviously it never was a thing outdoor. it has it's roots in the beginnings of indoor and the struggles with early tech.

everything we're doing is past that now.






MAK1 said:
Flush twice, it's a long way to Chicago.
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i am so gonna shorten my reply on the topic to something very similar in the future.
well done, sir. :)
 
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