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A critical look at preharvest flushing

Hydrologist

Plant of the Month: May 2011
Im startn' to see y people dont flush
 
I am new to the whole growing process, just got out of the Marine Corps and have only been smoking for about a year. But immediately fell in love, both for medical reasons and just because I enjoy it. Now I am about to buy a 4x4 reflective tent, growing w/ fox farm products. Has a 20 gal reservoir and a drip system. Now my question about flushing is this, does not flushing cause your finished product to be a less enjoyable high bc of all the chemicals that will be in it?
 
Did I screw this bunch up? The high is still good
As far as I'm concerned you've answered your own question. If it's a good high, everything is good. We all do this to get high, if I want to taste something nice, I've got a whole bunch of foodstuffs that taste much nicer than weed ever will. Let's face it, the art of cooking weed is largely about disguising the taste.
 

urmom

New Member
Wow give this man a hand :31: 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.
I am a big fan of organic soil growing and with a couple of feedings in veg (literally 2-3) and 2 in flower, with 2 compost tea feedings (1 each in veg and flower, both at the 4 week mark o the cycle respectively) I only use ph adjusted water in my plants. They grow nice and green and have incredible flavor and a super smooth smoke with mind-bending highs.
Excessive nutrients and flushing seems to be a waste of time and I have used high nutrient feedings in the past and have found no significant increase in yield and a definant decrease in flavor and quality of smoke (not the potency but how smooth the actual smoke of the burning plant material). Soil allows more terpins to develop and by allowing the plant to develop naturally with out significant additions of nutrients, you get a better expression of your genetics. Blueberry tastes fruitier, skunks hit your olfactory glands with a knockout punch and the look, taste and smell of the cannabis is much nicer overall. I hope that more people will lower the nutrient levels on at least one or two plants per grow to test for themselves. If you prefer the heavy nutrients after a few test grows then great! Enjoy your smoke! Either way, share your results so people can learn from our growing adventures...
 
Sorry I can't agree with this, we love eating cannabis here in Colorado. And we think we're a foodie state and look at strains as spices on the rack you need to try more experimenting kush and vanilla killer, sour d in your brown sugar based goodies, Durban Poison and Asian flavors, and lemon for you guessed it lemon. some stains are just more palatable than others. Whats better than a hippie with weed a hippie with weed and can cook.
 
As far as I'm concerned you've answered your own question. If it's a good high, everything is good. We all do this to get high, if I want to taste something nice, I've got a whole bunch of foodstuffs that taste much nicer than weed ever will. Let's face it, the art of cooking weed is largely about disguising the taste.
Sorry I can't agree with this, we love eating cannabis here in Colorado. And we think we're a foodie state and look at strains as spices on the rack you need to try more experimenting kush and vanilla killer, sour d in your brown sugar based goodies, Durban Poison and Asian flavors, and lemon for you guessed it lemon. some stains are just more palatable than others. Whats better than a hippie with weed a hippie with weed and can cook.
 

Budley Dorite

Well-Known Member
And nowadays there is bud washing (Doc Bud), supposed bring a smoother smoke & a more even cure. If you foliar feed a bunch, do yourself a favor and look into bud washing.
 
Anybody got any tips on flushing miracle grow soil? The kind with the time released nutrients. Obviously the true solution is to not use any MG or time-released nutes in the first place, but someone just HAD to do an experiment so the damage is done. Viable buds are ready to be harvested, but I'm way too paranoid about putting miracle grow in my bowls. I'm worried that the normal methods of flushing will only make the problem worse by releasing even more nutes.

I thought about cutting the plant down a few days early and sitting the main stem into a bucket of neutral water, like you would a rose or flower in a vase, so that it might live for a few days sucking up only clean water. This seems to me like my only option, and I'm not so sure about it.:Namaste:
 

TorturedSoul

Member of the Month: May 2009, Oct 2010, Sept 2017
I guess, in a perfect world, one would not overfeed one's plants - and then, they would have just used up the vast majority of their stored nutrients right before harvest. <SHRUGS> That'd be difficult, even with multi-part nutrients, however... Plants use different nutrients/micro-nutrients at different rates, but we don't purchase 23+ part nutes, lol; and different strains having different nutrient requirements complicates things even more.

Anybody got any tips on flushing miracle grow soil?
Yes: Be sure to take it out of the bag before you dump it in the toilet - otherwise, it's not going to flush very well :19: .

The kind with the time released nutrients.
I'm not positive, but I suspect that that stuff releases nutrients at a higher rate when the soil temperatures are warmer. Obviously, you're not going to want to heat your plants up to 115°F, but you might release more of it if you allow your soil to run a little hotter (temperature-wise) than normal. Unfortunately, with increased temperatures, you'll also probably have to increase the airflow (for plant survival) and watering (so that they can transpire more moisture and help cool themselves), so... IDK how well that would work.

You could try transplanting them, I suppose, gently washing as much soil off of your plants' roots as possible in the process (possibly by placing them in a tub full of room-temperature water and gently "swishing" them back and forth). But I have never done this to large, mature plants. I assume it would be a lot of work (and stressful to the plants). So, again, IDK how well it'd work.

someone just HAD to do an experiment so the damage is done.
Well, the great thing about experimentation is that (if conducted properly) one generally only changes one thing and does so to a limited population, with a suitable control group in which that one thing was not changed, lol - so I assume that you'll still have plenty of the same kind of bud that isn't Miracle Nasty? Another great thing about experimentation being that the experimenter learns from his/her results, so whether the buds from the plant in question turn out decent or chemically-laden and nearly unsmokable, you will have learned whether or not it'll work for you in the future.

Viable buds are ready to be harvested, but I'm way too paranoid about putting miracle grow in my bowls.
I cannot blame you for that. My one and only experience with plants that had been grown with Miracle Grow products (the nutrients, IIRC, not the soil, but...) was bud that acted almost fireproof, lol - didn't even want to burn in a joint, and kept crackling/sparking. Also tasted nasty and did NOT "burn to a fine gray ash." I assumed that the grower overfed and did not flush on top of the poor nutrient choice, but... Yeah, I'd be "a bit" paranoid, too. I do wish you luck.

I'm worried that the normal methods of flushing will only make the problem worse by releasing even more nutes.
Hmm... If I'm correct in my suspicion about well .
more nutrients being released at higher temperatures, maybe you could let the plants get real thirsty, then lower the soil temperatures significantly (don't freeze it, lol) and do your flushing then? You couldn't go below a certain minimum temperature, because you wouldn't want your plants to go dormant (as they wouldn't uptake much water in that state), but it might be worth a try.

I thought about cutting the plant down a few days early and sitting the main stem into a bucket of neutral water
Speaking of being stressful to the plant :11: , lol. But it IS an interesting idea. If you try it, be sure to cut the trunk of the plant at a very pronounced angle in order to maximize the cut's surface area. I'd also suggest dimming your lights 50% (or more) as the plant - after it begins to recover from the stress - will most likely be focused on root production, which doesn't require sunlight-at-noon-in-August level intensity. And change the water often (every few hours) - IIRC, plants can excrete waste-products via their root system as well as uptake nutrients/water, and it may be that they'll do the same thing (to some extent) in that situation. Adjust airflow/temperature as needed to help prevent wilting whilst still encouraging the plant to uptake water.

I'll be interested in learning what you ultimately decide to do - and how the buds are after drying/curing. Please return and let us know, if you would be so kind.
 
Wish I knew this sooner, I always thought flushing would stress my plants,but I was told it was better, now that I've read this thread I'm opting for no flushing,I really don't like stressing my plants so..heh thanks !
 
I also just found this. My problem is I have added to much silica I think to a organic soil that may be overly amended and am having nute burn and stunted growth. I think maybe if I can do a partial flush one time it my let my plant recover for the last 4 weeks of flower.
 

TorturedSoul

Member of the Month: May 2009, Oct 2010, Sept 2017
When I added - a small amount of - a silica product (General Hydroponics' Rare Earth) to a DWC reservoir one time, I saw a pretty extreme pH shift by the next morning. Did your runoff pH change significantly from normal after you used the product?
 

Kingjoe83

Grow Journal of the Month: July 2017
Thanks for your opening post and sharing great info right there [emoji120]
 
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 post...very informative and valuable information. On my previous grow, I did an experiment with my Blue Dream plants. I flushed 3 plants after week 6 of flower. I did not flush the other 3 plants but only fed them ph'd water. There was no noticeable difference in quality or taste. The flushed plants finished 3 days earlier and had slightly smaller buds. My plants were in soil in 7 gallon smart pots. I flushed with 30 gallons for each plant. That's 90 gallons of water used to flush 3 plants. The other issue I dont like is that flushing saturates the medium with so much water that it can take 7-8 days for the plant to dry out...the overwatering condition definitely does not help with final bud size and quality.