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"First of its Kind": A Grower's Thoughts On The Flushing Study Everyone's Talking About
Takeaways, pushback, and suggestions for future research.
Earlier this week, I started to see posts about a new study on flushing.
But, before we can dive into the study...
Let's talk about what Flushing is.
Flushing can be used to mean a few things, but most commonly:
Intensive Media Flush - Watering in excess to purposefully rid the growing media of residual buildup
Input Cut-Off - Waning off the concentration (or presence) of nutrients as a plant makes its way through flower.
Plant Flush - Not possible. We'll start there. But there are some who will (incorrectly) state that excess watering will actually remove stored nutrients from the plant.
#3 is false.
No amount of watering is going to remove anything in the plant.
But, #1 & #2 can be used to encourage a plant to use up its reserves as it heads to the finish.
So, circling back to the study:
For our purposes, flushing refers to #2—cutting nutrients towards the end of a plant's lifecycle.
For starters, you can read the study in full here.
Here’s the short version:
A nutrient company conducted an experiment to determine if flushing made an impact or not.
To do so, they picked a single strain, in a single grow, running a single nutrient schedule, with the same processes—this is good because controls help, but limits the generalizability, more on that later.
Then, that strain was grown with 4 flushing methods: no flush, 7 day flush, 10 day flush, and 14 day flush.
Then, results were compiled and “judged”:
Total Yield (grams of biomass)
Quality of Trim & Extractability
Taste Test (Panelist of smokers) - Flavor, harshness, ash color
If you want to nerd out, it's worth a read, but here's what I got from it:
5 Main Takeaways (in the context of this specific study):
14 Days is too much. 2 Weeks without inputs starts to stress the plant out beyond the intention of "using up" stored nutrients to point of necrosis (dead tissue).
7 Day flush is really the max worth considering. The study's results suggest 0-7 days is the ideal flushing window.
Hash: 14 day led to higher yields but lower quality (based on nucleation) than 0-7 Day.
Chlorophyll & Micros: 14 Day results in less chlorophyll as plant becomes necrotic, but plants saw increased mineral concentrations (specifically Zinc & Iron) as a result of degradation and stress at end of flower
Ash: Based on this study, it would appear flush does not affect ash color as is commonly believed.
Now, with that said, even as I wrote this, I had to fight the urge to push back.
MAIN TAKEAWAY: There's some good insight here, but it is far from generalizable.
One-off: The controls in this study are also its biggest limitation. This is 1 genetic, growing 1 way, fed 1 nutrient line, in one setting. It's a great starting point, but not enough to be treated as a universal truth.
External vs Internal Comparison: This study examines the impact of flushing on plants using this system, and compares within that system. It does not address how flushing impacts other systems (different nutrient lines or grow setups). Again, good baseline, but we need a bigger sample size to make sweeping conclusions.
Residuals: If you feed light throughout a cycle, and then cut off nutrients, its more likely the plant may starve (and have negative changes as a result). But, if you're feeding heavy, you may have a growing medium full of nutrients, and if you don't cut it off, you could overload the plant. In that case, to what extent are you "cutting off" nutrients rather than letting the buffet you've assembled feed your ladies?
Transparency: I'd be remiss if I didn't mention another pushback that popped up in the Discord discussion of this study which is the lack of transparency about participants of the panel who "judged" the flavor and experience of these samples. There's no information as to whether these were—in alcohol terms—sommeliers or winos; were they connoisseurs or casual consumers? What credibility do they have for us to trust their evaluation of these variables?
Ok, so now what?
Without boring you about the process or topic, years ago I had to write my grad school thesis.
At the end of a thesis (and any peer-reviewed study), there is a crucial section.
In it, the authors discuss opportunities for doubling down on their research.
Sort of a "Where do we go from here" suggestion box for other researchers.
This does 2 things:
First, it challenges others to disprove the study. Scientists and researchers can get quite competitive.
Second, it offers a few ideas for follow-up studies that would improve upon the ideas and conclusions found.
Sadly, this study did not include the section.
So I wrote it for them:
It would start with something rather academic.
Something like, "Future research is needed to address the limitations...blah blah blah"
But the essence is:
OPTION #1: Replicate the study.
As is. No changes other than a different researcher.
Can they achieve the same results?
What about another? And another?
How often do these results hold true?
Every time? Much more generalizable.
But still, limited. Which brings me to:
OPTION #2: Same same but different.
Do the same study, with the same controls in place, but swap out methodology and nutrients.
Do it with New Millenium or Drop or Athena or Botanicare or half a dozen other bottle nutrients.
Do it with "organic" certified lines vs not.
Do it with Containers vs Shared Beds.
Do it in different growing media: coco vs soil vs hydro etc.
Do it in comparison to an amended/living soil bed (not being flushed).
Do it with the same setup but outdoors.
Do it with different genetics with different consumption patterns (some plants feed heavier than others).
All of these are ways that this study (and its conclusions) could be strengthened beyond a "You don't have to flush with our nutrient line" pitch.
When new information on hotly contested subjects comes out, it's natural to knee jerk in one direction or the other.
Skepticism feels liked you're being mean, perhaps.
But, good data will hold up to any skeptic.
For that reason, it's important to examine studies like this within the context of the sandbox they're presented in, and not generalize to every situation and approach.