thanks for chiming in mates....:smoking: Hey
Seb'-- first, let's make a clarification on terminology.... Flushing is used a little loosely, though it's the "industry standard term".... Around here, we found it useful to to distinguish between flushing and cleansing... flushing usually refers to a period of time that the plant is not given any more nutrients, and is forced to use up internal reserves, and if applicable, what's left in the grow medium,... this is what cleansing is defined as too... But, adding to some confusion in general, flushing also refers to the pouring of copious amounts of water through the medium to purge it of excess/build up's, sometimes to fix badly off pH...
... as mentioned, the flow of nutrient ions does not go in reverse, absolutely wrong! Nothing is "removed" from the plant by active transport of such ions out through the roots,...some of the nute elements are not mobile once metabolized in any case,... The basic idea behind flushing is to stop/limit further nute' uptake so the plant can use up what's stored in reserves and clear out what's in the "plumbing" as unmetabolized (not yet converted to other forms and/or stored yet) nute elements,... much of the controversy hinges on just how much is in there to begin with and if it's even necessary... In organics, bottom line, it's absolutely not... with synthetic nutes, because of their being in form readily available for uptake, it becomes a conditional thing IMO... first, keep in mind, the uptake on most nutrient ions is not under any sort of selective control by the plant,.. water and nutes uptake are linked; N is a great example of this, P too,.. the plant takes these in whether they need it or not; this is how troubles like N toxicity start... the plant can only use and store so much before it becomes overwhelmed and the build up of excess start to cause harm and show symptoms,... usually, it's cumulative build-up over time and multiple feedings that cause this, but a big blast of too much will too, and worse!

.... ion's going in don't do it passively, they are taken in by swapping out a like charge equivalent in H+ or OH-; the plant is in constant management of balancing both internal pH and charge,...
....so, why the flush/cleanse? Without a doubt, a plant still chock-full of nutrient ions, N/P/K in particular, will make for nasty tasting harsh smoke! This is typically caused by heavy feeding right up to the end, hence the flush period you see on all these nute company's feed schedules,... we hear lots of back and forth about these schedules, usually being too heavy, and all a conspiracy to get you to over-use nutes,... my take, that schedule is a suggestion, a generalization, not written in stone,... also, if they wanted you to use and waste nutes so have you to buy more and more, why would they bother with that flushing period in the first place? Consider, if you did feed heavy all the way to harvest, and ended up with fouled buds, you very likely would blame the nute company for this, and never use their products or schedule again, right? ...truth is, this flushing thing is also done with other ag' products,... veggies grown hydroponically do the same thing, they uptake and store just like cannabis, and that includes the "plumbing" still loaded with goods in transport,.. certain crops need less time to do this,... Now, with skilled experienced growers, they know to not feed too heavy in general to avoid burning and causing off-pH issues, especially during that final ripening stage,... as you see above, some cut way back on such inputs anyway,.. plant demands are tapering off during this period, which includes water,... and if you have fans leaves showing the normal fading/dying out, that's a good sign that the plant is having to scavenge within for those resources, clearing out the stores,... and as such, by harvest, they're "cleaned-up",... I think it's possible to overdo this too, particularly if you've feed lightly, and have had defc. issues during bloom,... starving out a plant too soon, too much, will also compromise quality,.. for example, K and S are important for terpenes, and significant defc.'s of them will make for poorer aromatics,... cut out too much P, and that final fattening up may stall out,... and as always, the x-factor of individual strains is in play here, not all will finish and behave the same,...of course, grow env./medium/nutes specifics play their role too,... it's a fine, kinda fuzzy line when dealing with all this!
So, whether to "flush"/cleanse or not is something of a subjective call, but certainly depends in part on how heavily you feed, if you had defc. or toxicity issues, the type of nutes, and what the plant is telling you via the leaves and bud ripening,....

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---tried to put this in a nutshell, but it don't fit!
