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Pre-Harvest Flush?

Discussion in 'Organics' started by Los Muertos, May 26, 2011.

    Los Muertos

    Los Muertos Active Member

    I'm sure this has been answered before, but is there any need to do a final pre-harvest flush when using organics? Seems I've heard somewhere that it's not necessary.
    Billy Piper

    Billy Piper Active Member

    I have a heard a couple of things..apparently if you use exactly the right amount of nutes the plant uses everything leaving your weed tasting good.
    Personally i flush as i only grow for myself.
    Los Muertos likes this.
    Los Muertos

    Los Muertos Active Member

    Yeah, I guess not flushing after all that work is kinda lazy. Might as well, right? Thanks BP!!

    shifty1 Well-Known Member

    If you watered correctly you never need to flush. It has been proven time and time again that pre hravest flushing isn't needed it if the buds are dried and cured properly. This may interest you
    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.

    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.

    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.

    Also recommend looking through threads from uncle ben here at roll it up he has answered this question and in my opinion has given one of the better answers as to why pre harvest flushing is not needed.
    South Texas and Los Muertos like this.

    skiweeds Active Member

    yes i always flush even with organics. for even better results get a flushing agent. it absorbs and drains your nutes left in the soil. a couple examples are royal flush by humboldt or clearx by botanicare. your smoke will taste much better. i've tested this several times. everytime i skipped the flush, my friends and i who are all heavy smokers, can tell. especially when using a vape. also another trick for better smoke is put your plant in the dark for 2-3 days before harvest. when you plants are in light, the start uptaking nutrients. when in the dark the nutrients go backwards into the roots and soil. you dont have to flush and you especially dont have to put in the dark. neither of them will effect potency. but imo it will taste much better and smoother! good luck.
    Los Muertos

    Los Muertos Active Member

    I'm still learning and I know plenty of mistakes feeding/watering were made so I'm gonna flush her, but that's really interesting shifty1. I'll leave not flushing to the pros for now.

    shifty1 Well-Known Member

    @los Muertos you want your plants look like this at harvest: this is a strain called ape kush @ 10 wks; in the last two week
    I give a half dosage of nutes but you want to keep your leaves green, green leaf= healthy plant yellow leaf not so much. You work so hard to keep leaves green during the grow it seems rather silly to flush all that away.
    If you were growing tomatoes would you flush, probably not so why is mj any different.:peace: Bottom line do wahtever you like but trust me it is a wasted step and you will lose out because of it.

    Attached Files:


    cowboylogic Well-Known Member

    Absolutely and positively no reason to flush. Complete and total myth.......

    malignant Well-Known Member

    you must grow some horrible tasting grey ashing vomit inducing harshly unsmokeable shit passed off as buds..

    [email protected] Well-Known Member

    have you ever smoked a tomato?

    Oldreefer Well-Known Member

    Flushing...why not do a search and get the million of answers that always show up in every manner without starting a thead where the same thing is said and argued over and over and over.....and over...

    [email protected] Well-Known Member

    I wait to see if my plants will fade on there own near the end of flowering, if they don’t fade I flush them, you want to dissipate as much of stored chlorophyll out of the leaves prior to harvest.

    shifty1 Well-Known Member

    Flushing used to be part of my routine for over a decade then I deiced to try without it and I couldn't tell the difference with the finished product. Try it yourself you will be surprised I know I was. And as the op wrote he is organic so with that being said flushing is not needed. You never flush with organics there is no salt build up so you do not have to.

    Nullis Moderator

    Yah we definitely don't smoke tomatoes (only on the Simpsons, but that was really Tomacco). AFAIK there are certain elements that lend to a harsher smoke including K, Mg, Zn and other 'heavy metals' and this makes sense to me. But, in real organic growing we don't really force feed our plants nutrients as with synthetic fertilizers where you almost always have synthetic chelates. The most popular synthetic chelating agent is EDTA (made from sodium cyanide and formaldehyde, there are others which are more expensive) and it has a particular affinity for heavy metals, which it shuttles (forces) into the plants along with other nutrients.

    Really you just don't want to be feeding heavy for the last few weeks; many use a mild organic fertilizer solution or none at all for the final couple weeks. Plain water the last week before harvest. No need to run gallons and gallons of water through the soil in preparation for harvest, that's all.

    Maybe. Or perhaps you just don't know as much as you think you do.
    Los Muertos likes this.
    Los Muertos

    Los Muertos Active Member

    Ok then, sorry. I did plenty of searching but couldn't find a straight answer; I understand now that there's not one.
    I really just wanted a little advice from people that I respect. Forgive my stupidity, I'm still learning.

    Perfect..that's exactly what I wanted to know. I know it's frustrating for a lot of you guys to keep hearing the
    same shit constantly and I apologize.

    Nullis Moderator

    Well that's life isn't it? Same shit constantly; sounds like life to me.

    Wetdog Well-Known Member

    That is one of the most ignorant, stupid things I've heard yet.

    Guess you've never seen any of CBL's nasty assed buds, have you?

    Flush away, if that's all your growing skills amount too.


    malignant Well-Known Member

    enjoy your chlorophyll, iron, and mag residuals.. peace

    krok Active Member

    This thread is funny.
    There are always discussions about this topic - it's never settled, which I find amusing - as all you have to do to find a FINAL answer is to TRY BOTH for yourself.

    I mean, if you're happy with your results, continue. But if you're curious, like me, you'll try both.

    I grow in compost. I flushed 2 plants, and fed 2 plants to the end. My friends can't tell the difference, nor can I (you have to do a blind test, where not even you know what you and your friends are smoking). Also, you should do this several times to get enough samples (and calculate the results). I don't remember what % we ended up with, put it was pretty random (the guesses).

    Then you can decide.

    I don't know why I (and two friends) could not tell the difference, maybe it's only needed in hydro.

    shifty1 Well-Known Member

    Well said krok. @los muerto Wish you the best with your grow and if things do not turn out as you wanted do not get discourage it takes time. Just try and learn from your experiences and apply that knowledge to your next grow.

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