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Thread: A critical look at preharvest flushing

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    420 Member Medical Marijuana's Avatar
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    A critical look at preharvest flushing

    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.
    Last edited by Medical Marijuana; 01-01-2008 at 06:52 PM.

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    420 Member gmase22's Avatar
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    Re: A critical look at preharvest flushing

    Wow give this man a hand That was a great explanation of the plants transport system. I just read an article about how a preharvest flush can improve taste and such, but I gotta say after this you win. I am curious what you think of a preharvest flush with molasses or some carb source added or possibly just a thorough flush the day before harvest? Also what if you were using an organic soil? Wouldnt that continue to partially feed the plant even if given plain water? Again great info, and can't wait to hear your ideas.
    Last edited by Soniq420; 03-22-2009 at 01:16 PM.
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    Re: A critical look at preharvest flushing

    what if you were using an organic soil?
    not if you time it right,
    cannabis can drain the nutes out of most mixes in 3 weeks - 90 days.

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    Re: A critical look at preharvest flushing

    He wants to know how you would flush with an organic mix.
    You time it so you don't really have to,
    the plant has used all the Nutrients by the time it finishes flowering.

    That better ?

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    420 Member fastfor20's Avatar
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    Re: A critical look at preharvest flushing

    old post but great info ty

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    Re: A critical look at preharvest flushing

    When I started doing a very close Manicure and trimming as much leaf off as possible, I quit FLUSHING before harvesting, and my buds still taste great!. I did not FLUSH any of my last 5 or 6 grows and I never will again.
    An old friend from another site, a very experienced outdoor grower named Uncle Ben once told me, or asked me, how do you flush a 12 foot tall tree grown outdoors? He doesn't flush his outdoor plants andt hey still taste fine. He taught me that I can feed them the night before harvest, in Hydro, and still not get a bad chemical or fertilizer or clorophyll taste if I trim clsoe and dry and cure properly.

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    420 Member gunjababy's Avatar
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    Re: A critical look at preharvest flushing

    Quote Originally Posted by ------- View Post
    When I started doing a very close Manicure and trimming as much leaf off as possible, I quit FLUSHING before harvesting, and my buds still taste great!. I did not FLUSH any of my last 5 or 6 grows and I never will again.
    An old friend from another site, a very experienced outdoor grower named Uncle Ben once told me, or asked me, how do you flush a 12 foot tall tree grown outdoors? He doesn't flush his outdoor plants andt hey still taste fine. He taught me that I can feed them the night before harvest, in Hydro, and still not get a bad chemical or fertilizer or clorophyll taste if I trim clsoe and dry and cure properly.
    -------,
    define a proper cure in your opinion ?PLEASE

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    New Member Tulip's Avatar
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    Re: A critical look at preharvest flushing

    The very first post in this thread used many words to say the nutrients are moved through the stems to the leaves and not stored in the pistals or calyxes of the flowers or buds. The chlorphyll is also in the leaves and not in the pistals or calyxes of the buds.

    So, first step or point is MANICURE heavily to obtain the sweeter cleaner taste and avoid the chemical and chlorphyll taste.
    Secondly, We CURE pot, to make it taste sweeter, smell sweeter, to avoid bud-mold, to make it more Smokable or burnable, to get the chemical and clorophyll taste out, and to increase the potency. A GOOD cure takes 4 weeks, and some conisours (mispelled) cure it up to 6 to 8 weeks.

    The idea behind curing was learned from tobacco growers. Curing is a biological process of allowing the SUGARS and STARCHES to change into something MORE pleasant to the taste and smell. Normally the SUGARS and STARCHES taste HARSH and not so pleasant. To grow, Plants need SUGARS that convert into starches from Fertilizers and sunlight. Curing also removes alot of clorophyll or the clorophyll taste that is sort of a grassy leafy medicine chemical taste and leaves a sweet tastey pleasant taste.
    Also, we cure pot to avoid MOLD that can come within 30 days AFTER Drying.

    Proper Curing is done in jars, AIR TIGHT containers, in darkness, in a cool place. After being placed in the jar, we store them in a dark cool place, then we re-open the jar once a day, smell it, inspect it, air it, let it breathe for a few seconds and then re-seal it. IF we smell an unpleasant "nose pinching" smell, or see white growth, we need to immedialtey remove it from the jar and DRY it some more for a few more days.
    Proper curing is really just giving the buds time to let nature do what it wants to do and process remaining chemicals into somethign pleasant..
    When you first harvest the buds, save some moist large stems in the refrigarator, in a baggie. If you dry it too much, you can add a small piece of stem back, to remoisten it some.

    I have CURED pot one week in jars, and tasted it, and then Cured it 4 weeks and tasted it. If you will try the same experiment, or ask any experienced grower, you'll learn (taste) the difference. It is much more potent, and much sweeter tasting, and smells much better too.

    Flushing and NOT Flushing is arguable, debatable and all growers seem to take a strong opinion for or against. If you're into smoking leaves, you ought to FLUSH them good. I don't smoke the leaves, and I don't flush. Just my humble opinion.

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    420 Member Wingman420's Avatar
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    Re: A critical look at preharvest flushing

    ------- not as verbose, but also very informative, a good read! Educated, and experienced.

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    Re: A critical look at preharvest flushing

    Quote Originally Posted by wingman580 View Post
    ------- not as verbose, but also very informative, a good read! Educated, and experienced.
    Thank you,
    I've never been accused of being VERBOSE, thats for sure.

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    New Member Tulip's Avatar
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    Re: A critical look at preharvest flushing

    Great post, Mile Hi Dave.

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    Re: A critical look at preharvest flushing

    Quote Originally Posted by Nook View Post
    Thanks Medical Marijuana, interesting thread. Thanks for the counter position on flushing, -------. My manuel is growing fast, shame about the seed delivery.

    I'm just wondering if it wouldn't be an answer to use sparkling mineral water for the last few days. This would contain a low but present amount of magnesium, calcium, sodium and potasium as well as a nice amount of CO2, minus the nitrogen so chlorophyl production is low using up the remains in the soil. It would be a decent "final meal" for the plant while keeping it defoxified. What do you think???
    It would be well worth trying, as an experiment. Especially in a SOIL grow.
    I just don;t know for sure.

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    Re: A critical look at preharvest flushing

    Quote Originally Posted by ------- View Post
    When I started doing a very close Manicure and trimming as much leaf off as possible, I quit FLUSHING before harvesting, and my buds still taste great!. I did not FLUSH any of my last 5 or 6 grows and I never will again.
    An old friend from another site, a very experienced outdoor grower named Uncle Ben once told me, or asked me, how do you flush a 12 foot tall tree grown outdoors? He doesn't flush his outdoor plants andt hey still taste fine. He taught me that I can feed them the night before harvest, in Hydro, and still not get a bad chemical or fertilizer or clorophyll taste if I trim clsoe and dry and cure properly.
    Ditto. Back in the 70's and 80's we didn't know what flushing was. My plants went with spike fertilizer for fruit trees the last two months. As long as you got a good dry and cure you were solid.

    -------,I don't think most growers know how to work leaf anymore. I don't know of it's the switch over to indoor growing and the lack of light causing THC production in the leaves not to be what it is outside or something else

    IMO,it's harder to cure leaf properly than bud but it's the same process. You've got to watch not letting the leaf dry out too quickly or the chlorophyl is locked and you're stuck w/ some harsh tasting leaf.

    I always liked the jar method for leaf and the paper bag cure for bud after a short hang dry. YMMV.
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    New Member Tulip's Avatar
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    Re: A critical look at preharvest flushing

    Quote Originally Posted by Firestone View Post
    Ditto. Back in the 70's and 80's we didn't know what flushing was. My plants went with spike fertilizer for fruit trees the last two months. As long as you got a good dry and cure you were solid.

    -------,I don't think most growers know how to work leaf anymore. I don't know of it's the switch over to indoor growing and the lack of light causing THC production in the leaves not to be what it is outside or something else

    IMO,it's harder to cure leaf properly than bud but it's the same process. You've got to watch not letting the leaf dry out too quickly or the chlorophyl is locked and you're stuck w/ some harsh tasting leaf.

    I always liked the jar method for leaf and the paper bag cure for bud after a short hang dry. YMMV.
    + REP, Bro, good post

    I never thought I'd have so much bud,that I could not smoke the leaf, but I do. Now I sort my leaves and make Oil from the large ones and Hash from the trim leaves.

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    Re: A critical look at preharvest flushing

    If you can find a mid-70's copy of Ed Rosenthal's Marijuana Growers Guide,there are a couple of nice detailed methods laid out on how to dry leaf vs. bud. That book was worth it's weight in gold when it came out.

    Well cured leaf can be a joy to smoke in a joint or bowl and you're not on the couch for the day afterwards. It's nice buzz for bs'ing and goofing around.

    IMO,using food dehydrators as a drying method causes a lot of the harshness in today's weed. It retains a powerful buzz,but who wants to smoke something that uses a drying method that leaves the final product tasting like two day old hay?

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