A critical look at preharvest flushing Added by: snoofer Last edited by: vaaran Viewed: 460 times Rated by 32 users: 9.16/10 Contributed by: vaaran Submitted: November 15th, 2004 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. Lecture Notes:, Transport and Nutrition in Plants, Bio 102 http://www.hort.wisc.edu/cran/Publications/2001 Proceedings/min_nutr.pdf The claim only ‘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. http://acd.ucar.edu/~eholland/encyc6.html 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. http://www.sidwell.edu 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. http://www.sirinet.net 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. http://generalhorticulture.tamu.edu 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. http://jeb.biologists.org.pdf 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. http://muextension.missouri.edu 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 modified: 00:07 - Feb 17, 2005 This could help me decide. I've still got a little time to decide. \t\t \t\t \t\t \t\t \t\t \t\t \t\t \t\t \t \t \t\t \t\t \t\t \t\t \t\t \t\t \t \t \t \t \t\t \t\t \t\t \t\t \t\t\t
Great read man. Thanks for posting this here. Very educational. Makes me look at flushing in a whole new light. Happy to read up on this because I don't see much on it besides the usual. Will surely help on my upcoming grow! +Rep
HIGH All, I mostly do Hydro and I Flush 2 weeks before I cut them down so I'm not worried about Stressing the Plant at this time...some don't Flush I don't know about Organics..I would think you wouldn't need to...would you Organic Growers Flush. So what happens when someone Overfertilize their Babies..would it be wrong to Flush the soil or would that just bring on more Stress.....excellent find daze...makes one Grow Hmmmmmm
I do not consider my technique as flushing. I usually just use water the last week anyway. I have always used molasses the last 4 weeks or so, and until doing further research, thought I flushed for 3 weeks when in reality molasses has many nutrients and minerals vital during the last phase of flowering which is full bloom. I think there may be a need for it in hydro since the concetrations and PPM's are much different and reach the plant direct, kind of a main line thing when compare to soil. As for problems, I tend to flush the first week of 12/12 just for the simple fact my expereince has showed me that Salinity usually occurs within the first 2 weeks of flowering, so I flush to lower this risk. Salinity, over fert, poor PH, these are all things which cause lockout and I feel a flush is very necessary in these cases. It is a very good read, and does make some very good points. less
I don't think he is arguing against flushing in general, there are many times when it is completely warranted. What he is saying is that there is virtually nothing gained in pre-harvest flushing vs. not flushing pre-harvest. I have two identical plants that I will be harvesting in a couple of weeks, I might flush one and not the other and cure them in the same manner, I'll post the results.
ohhhh please do this if you can. I've been going through tons and tons of reading here and at icmag and it seems there are just about as many people who think it's necessary as those who think you're robbing the plants of valuable nutes during the most important stage in bud and trichome development.
Overfertilizing will do two things to your plants; likely the soil will be at a very low ph (high acid content) which will prevent the plant from uptaking trace minerals such as magnesium (experience there) which will create all sorts of additional problems for the plant. Additionally, the excess of nutrients will burn the plant from the inside out causing the leaves to turn yellow/brown and die. These are just some of the problems that can come from over-fertilization, there are many more: salt build up, etc. Flushing is not necessarily bad for your plants at all, providing you are keeping your normal watering schedule and just flushing when you would normally water, the plants won't be overwatered or damaged at all. Flushing merely washes the grow medium clear of nutrients, allowing the plant to use up the excess of nutrients. Of course, flushing multiple times in rapid succession will drown your plants in the same way that overwatering will.
I had been hearing it on here that when you flush before harvest you still have the nutes available because the plant will continue to use up what is inside it and what is still lurking in the soil. So flushing allows the unused to be used up so they aren't starving for nutes. Even if it does cause stress that could be a good thing. indoor growing is all about tricking the plant into believing certain things are happening so at the end of flower the nutes do start drying out I would think the plant would see that as a sign it's life is coming to a close and go into overdrive trying to get pollinated, which as we know creates big fat sticky buds. That is not researched though.
Bump I've been reading threads on all different sites for 4 days on flushing, looking for info on if/when/how I should do it. This seems to be the most informative thread so far, I want to see more input from other experienced growers.
kinda in thought as what to do this run also and stumbled on this dug up thread...So what is the general concensus?