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"Flushing" in Hydroponics: Essential Practice or Overhyped Tradition? · Hydroponics Daily

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  Hello and welcome back to Hydroponics Daily. I'm Dr Russell Sharp and this is
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  your go-to podcast for everything soilless cultivation.
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  It's still swelteringly hot here in the UK, 32 degrees Celsius.
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  So I've made the effort to close all the windows and turn all the fans off so
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  I may collapse halfway through recording this episode but hopefully I'll stick with it.
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  Today we are going from one extreme to the other.
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  Yesterday we talked about seedling establishment and today we're going to go
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  all the way through the growing cycle, all the way to flushing your crops just
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  before you harvest them.
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  So this is a topic that sparks a lot of debate amongst growers, flushing.
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  Whether you grow lettuce or legal cannabis, the chances are you've heard about
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  this practice, but is it essential?
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  What actually are in those flush products you might see in a grow shop and which
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  crops benefit from it? So let's unpack it.
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  What is flushing? So in hydroponics, Flushing refers to running clean.
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  Nutrient-free water through your system. So not a persiguation solution.
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  Usually a few days before harvest. The idea is that this removes excess nutrients.
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  Salts and residues from the plant tissues themselves, but also the root zone.
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  So in theory, what can this do?
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  Well, it's thought that it might improve flavour and aroma.
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  And it's particularly popular in legal cannabis, tobacco.
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  It reduces the harshness in the crop if
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  you're going to smoke it and that's the theory anyway before
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  you cure dry and cure it can also lower
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  residual salts in edible greens and so
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  for lettuce some people flush their
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  lettuce now why would you want to reduce residual salts
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  well you would particularly want to reduce nitrate levels
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  in your crop nitrates are
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  a contaminant in drinking water and in food they're
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  great for growing crops they're probably one of the
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  best forms of nitrogen for growing crops but in drinking
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  water and if too much soluble nitrates are in
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  your food then they can cause issues with your blood system
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  and especially your hemoglobin and if
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  you're pregnant then you your baby can be born with something called blue baby
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  syndrome sounds hilarious i mean you think of these sort of is it smurfs but
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  no it's actually not a good thing it means that the haemoglobin is damaged if your baby is born blue.
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  Good news is they can recover quickly but also there's other issues around nitrates
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  and things like that where you can affect your blood pressure and things like this.
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  Some areas in the world have very high nitrate levels in their drinking water and this is a problem.
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  Other areas it's not. But in the food you also want to reduce your soluble nitrate levels.
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  That's why you would want to wash any leafy greens that have been treated with
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  foliar fertilizers or foliar fertigation to reduce nitrate levels.
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  So flushing could potentially help. with that.
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  So, as I say, legal cannabis is the most commonly flushed hydroponic crop.
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  Growers typically flush 5 to 14 days before harvest, depending on the system.
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  I don't know of any hop growers that do this, probably because they're growing mostly in soil.
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  As I say, the goal is mostly to remove nitrogen, but other mineral buildup that
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  might affect the quality.
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  Now, normally, people are using plain water. They might use iced water as well,
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  just to stimulate the plant defences and the stress in the plant.
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  But you can also buy commercial flush solutions which are either ready to use
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  or you dilute them and we'll talk about them a bit later.
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  So lettuce and spinach are sometimes flushed for one to three days before harvest
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  especially in systems with very high electrical conductivity EC values when
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  you're targeting food safety standards.
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  Tomatoes and cucumbers these are rarely flushed. These
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  crops rely on consistent nutrient levels right up to
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  harvest and they are harvested continually
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  this is not one big bang and you're done like a lettuce or
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  legal cannabis crop you're and strawberries
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  tomatoes cucumbers you're growing you're harvesting them all the time
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  so you can't flush it's not you can fit that in with a
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  system so there what are in most flush
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  products well grow shop sell shelves
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  are full with products marked as flush lots of
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  the nutrient brands that will sell you 16 bottles in a
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  range will contain a flush product even though they.
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  Claim they simplify growing and they.
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  Won't often they won't say what's in these products however the.
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  Most of the time there's a lot of water in the
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  product the mostly water 95 97 water
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  probably they might contain or the most of them normally contain
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  citric acid citric acid is doing a number a
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  couple of things there they are it is reducing the ph and
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  it's also chelating some of the nutrients it
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  can remove some of them it'll bind and remove residual minerals
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  that wouldn't otherwise be removed from the water alone other
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  options that could remove edt remove nutrients chelate
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  them are edta and other similar synthetic chelating
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  agents i would avoid this because you don't be adding too
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  many synthetic chelates to your system if you
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  can help it especially if you're just about to harvest now but
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  so then normally what they contain is citric acid don't mix seed
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  citric acid and edta together because edta is
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  not stable at the low
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  phs that citric acid creates so if you
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  can manufacture one make sure it's one or the other there might be
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  sugars and enzymes in there to support microbial breakdown
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  or sweetened fruits though the evidence for this
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  is limited however these are normally not in flush
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  products they're normally in like hardening off products the sugars and then
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  the enzyme products for breaking down grow media and then low ph stabilizers
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  that balance water chemistry as the nutrients are removed but this is this again
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  is probably going to be citric acid or something similar you probably don't
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  want to be using lactic acid or.
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  Acetic acid because these are not as
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  good at chelating acetic acid and they're harsher
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  give off fumes and things like this so always probably stick
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  to citric acid and people say
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  that these products don't strip the plants but citric acid it's
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  probably going to act like acid rain does so i would never put a flush product
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  on the foliage only ever on the soil because it's going to be stripping calcium
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  out of all and other nutrients off the root system as you go so you don't want
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  to be flushing too early because citric will strip calcium phosphorus,
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  other nutrients like this away from your plant so be careful i would say they do strip the plant,
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  if you're using citric and then you've got to ask yourself a question do you
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  want to buy an expensive flush product or do you want to buy just some citric acid.
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  Does now does flushing actually work and that's where the debate really
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  hots up some studies suggest no measurable difference
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  in active ingredients in the
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  crops have been tested and certainly no difference in
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  flavor between flushed and unflushed product in
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  in that's in edible crops and in legal cannabis others
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  reports a subjective improvement in
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  smoking quality smoother taste cleaner ash
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  things like this so it's very much up for
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  debate and there's lots of different studies but subjectively people
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  do claim and often people have very strong opinions
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  on things like this in leafy greens short flushing can
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  reduce nitrate build-up improved taste and shelf life
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  and that's been studies have shown that so it's definitely one to look at in
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  commercial food production flushing isn't about taste it's about food safety
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  and mineral management and whereas in legal cannabis it's about perception and
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  subjective opinion often so how do you do it if you decide to flush, as I say,
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  illegal cannabis 7-10 days before harvest, leafy greens 1-2 days,
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  avoid long flushes because you could damage the roots, and.
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  You probably want to remove your fertigation solution from when you're flushing,
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  if you're using a flush product, because it will reduce the pH and nutrients
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  will crash out of the solution and it's a waste of nutrition.
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  You want to be removing that nutrient and the plant should senesce,
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  so go yellower over the period or then when you're flushing.
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  If you've got very high EC levels, then it might take longer to flush.
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  Monitor your EC levels of your material you're adding.
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  If it's water it should be next to zero and then
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  but especially monitor the ec if you run off and see how that
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  progresses over time and see if you can get
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  it as close to zero as you get to the time where
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  you're about to finish flushing and harvest so
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  it's up to you to decide whether flushing is
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  right for you in your context so depends on
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  your crop your goals and your system and your previous experience and
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  what you want to say about your crop a lot of people might be a
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  unique selling point so that it was flushed for a long time it was
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  flushed with this certain product it was flushed with an edca free product it
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  was flushed with organic acids things like this so that's
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  been hydroponics daily for today flushing if you
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  know somebody who flushes their crop or has strong
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  opinions on flushing please share this episode with them if you've
• 9:07
  had this episode shared with you then you
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  owe the person who shared it with you a pint and as it's 32 degrees
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  celsius and if you're listening to this today on friday or over the
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  weekend you definitely owe them a drink so get down the pub it
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  could be an alcoholic one could be non-alcoholic but you definitely have
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  them a drink they've got to stay hydrated if you
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  have found it useful then please screenshot
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  this episode and share it on social media instagram tiktok linkedin facey b
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  do you remember facey b when we used to call it facey b when it was cool to
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  be on facebook not anymore is it i've been waffling for too long i'm dr russell
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  sharp the inventor of liquid gold as it's called in the UK or Goldleaf in the US.
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  And I will see you again tomorrow where we'll talk about another subject to
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  do with hydroponics. Thank you very much for listening. Goodbye.