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New Research Shows Flushing Plants Before Harvest May Be Unnecessary

In a blind taste test, researchers have found that many prefer cannabis that has not been flushed before harvest.

New Research Shows Flushing Plants Before Harvest May Be Unnecessary
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The results of a new research trial released this month show that flushing plants before harvest may not improve the quality of cannabis flower. The results seem to contradict the commonly held belief that flushing plants improves the taste and burnability of dried cannabis flower.

Under common, although not universal, cultivation practices, cannabis growers stop fertilizing their plants one to two weeks before harvest in an effort to improve the quality of their finished product.

“Flushing is important because it removes excess nutrients that are leftover within the plant,” explains High Times senior cultivation editor Danny Danko. “So it helps with the burnability of the flower by leeching out excess salts and nutrients.”

But in the trial conducted by RX Green Technologies, a manufacturer of cannabis nutrients and other cultivation products, researchers determined that those participating in a blind test tended to prefer cannabis flower that had not been flushed before harvest.

To conduct the trial, growers at the RX Green Technologies research and development facility in Colorado cultivated cannabis plants of the strain Cherry Diesel in a coco-based medium. During growth, the plants were fertilized with the company’s brand of nutrients. Four groups of 12 plants each were subjected to different flush times as harvest approached. Each group of plants was flushed for either zero, seven, 10, or 14 days.

Flower samples taken the day before harvest were analyzed for essential plant nutrients. Overall, there was no significant change in the mineral content of cannabis flower as a result of different flushing treatments.

After harvest, the plants were cured and tested for final trimmed flower weight, terpene, and THC concentrations. Lab analysis found no significant differences between the different flushing treatments for flower yield, THC potency, or terpene content.

Samples of cannabis flower that had been subjected to the various flushing times were also distributed to cannabis industry experts so they could rate them on smoking characteristics and flavor. Stephanie Wedryk, Ph.D., the director of research and development at RX Green Technologies, says she wasn’t sure what the outcome of the experiment would be.

“I did not know what to expect going into this,” says Wedryk. “I had talked to some growers I know and all of them had experience with testing flush times and not flushing and all of them only had negative experiences when they did not flush.”

Flushing Shows No Benefit

But when the data from the blind tests were analyzed, the researchers discovered that the participants tended to prefer the taste of the flower that had not been flushed at all, although overall, the duration of the flushing period had no impact on flavor, smoothness of smoke, or color of ash. In the results of the study, RX Green Technologies wrote that the trial indicates that “there is no benefit to flushing Cannabis flower for improved taste or consumer experience.”

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Wedryk says that while she doesn’t think that cultivators should overhaul their practices based on one trial, she does believe that growers should be open to trying new things.

“I would definitely recommend that growers play around and find what works best for them. I talked to a grower at the event who doesn’t flush and he’s perfectly happy with his product,” explains Wedryk. “Everybody has their own unique system and there are so many different components that go into growing. What works for one grower because of their unique set of circumstances might not work for the other grower.”

Danko agrees, noting that growers who are careful not to use too much fertilizer may need little or no flushing time for their plants.

“Really, flushing is an extension of the fact that most people are over-feeding their plants,” says Danko, advising growers to feed their plants lightly, in many cases at lower levels than recommended by nutrient manufacturers.

“It’s always easier to bump up the nutrients when you see a deficiency than it is to remove nutrients when you’ve overfed,” he says.

Although Danko still recommends that growers flush their plants prior to harvest, he supports efforts to examine commonly held beliefs in a controlled scientific setting. Wedryk agrees, explaining that “as more and more research comes out in cannabis, I think we have to question some of the things that we think we knew and see if these still really hold or is it a new day?”

Wedryk says that RX Green Technologies plans further research into common cultivation practices, although she declined to offer any specifics.

“Stay tuned,” she says.

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A.J. Herrington
Written By

A.J. Herrington is a San Diego-based writer and photographer covering cannabis and the environment.

40 Comments

40 Comments

  1. Avatar

    Matt Hutchins

    December 8, 2019 at 8:05 pm

    Wow this is some stupid shit.
    “Stephanie Wedryk, Ph.D., the director of research and development at RX Green Technologies, says she wasn’t sure what the outcome of the experiment would be.”

    You have a phd yet you are too dumb to know basic plant science. People like this end up with high paying jobs yet those of us who are real experts are shunned because we lack degrees, but yet have 30 years of experience with cannabis and infinitely more knowledge.

    • Avatar

      Kathy B.

      December 9, 2019 at 9:56 am

      What has been your experience then? Do you flush?

    • Avatar

      Huyton Freeman

      January 1, 2020 at 1:56 pm

      Stupid to use chemicals on your plants

      • Avatar

        John Kuhn

        May 25, 2020 at 3:16 pm

        Do you know what a chemical is? Did you know that oxygen is a chemical? That’s right. And Hydrogen too. Are you also familiar with the combination of two hydrogen atoms with one oxygen atom? Yup – it’s the chemical compound we here on earth call WATER. Good luck living without chemicals. Science – 1; stoners – 0.

        • Avatar

          Gerry

          July 23, 2020 at 3:52 pm

          I think the OP is referring to synthetic (or organically derived) mineral salts sold in plastic bottles and shipped from halfway around the world. And I agree, there’s no need for them.

    • Avatar

      Alf Mckenny

      February 21, 2020 at 8:55 am

      I think she was playing dumb cause, she didn’t want to ruffle any feathers so to speak , to all those out there that have been drinking the kool aide for to long.
      Tons of nutrient companies that have “ flushing agents “ money grab” I just never believed in starving a plant on its last weeks .
      Never did it in any the gardens I’ve grown food in over the years , why do it to my flowers 🤦‍♂️

      • Avatar

        Al

        May 13, 2020 at 5:34 am

        So do or don’t flush?

        • Avatar

          John Kuhn

          May 25, 2020 at 3:38 pm

          The jury is still out. It was only one experiment, using only one strain, grown one way in one medium. More research is needed. The current implied possibility from the test is that if you have truly dialed in your nutrient additions where no unnecessary nutrients have been added, you could in theory skip flushing entirely (or significantly reduce how long you flush). We are on a whole new frontier of learning, now that spreading legalization is making it easier for such research to occur. I look forward to new knowledge.

          • Avatar

            Ubel

            June 26, 2020 at 9:42 am

            Well said. I think it’s also important to note that this study is only surveying a relatively small group of people’s subjective opinions of taste from smoking. Smoking doesn’t not impart as strong or discernable of flavor as vaporizing does, so they really should have included that.

            Also, just because a bunch of stoned ppl don’t taste a difference doesn’t mean that there is not a health benefit to properly flushing. I know I’d rather consume buds that don’t have traces of these chemicals in them.

      • Avatar

        Daver

        July 7, 2020 at 5:30 pm

        Perhaps because nature has been flushing its own plants every fall since the beginning of time.

    • Avatar

      Pat

      April 11, 2020 at 3:09 pm

      Not as stupid as your comment, most Ph.D holders are not paid well, there are exceptions like with her being a R&D Director she likely earns good money. Most graduates go into teaching some write books or research. She also had to prove that she was an expert, with 8+ years of school, using science, writing a thesis, having it evaluated and then defending that thesis from attack by the thesis committee.

      You are probably shunned because you are a contemptuous arrogant overbearing know-it-all who hasn’t put the time, effort, money, or research into getting even a 4 year degree. Yet you feel entitled to pass judgement on someone who has. There is also a reason they say “indicates” if you’re such an expert pull up the paper and replicate the study and have it published, hell write a book too. You don’t have to have a degree to do any of those, until you do though I would sit down and shut the fuck up.

      In case anyone is interested here is what her thesis was on “Management Strategies for Weed Suppression during Transition to Organic Agriculture” and here is her linkedin which has some of her published studies in it. https://www.linkedin.com/in/stephanie-wedryk-70575542

      • Avatar

        nostrafckingdarmus

        June 15, 2020 at 5:21 pm

        organic farming science ambassador? 95% of poeple use un organic food, you can clearly see darker ash and in many cases not a smooth smoke when not flushed correctly, indicating your smoking something thats not present after fushing, why would you promote smoking non-organic material?

        • Avatar

          C.Dankens

          July 5, 2020 at 6:24 pm

          Your logic is straight up dunning ktueger

        • Avatar

          BonBo

          July 26, 2020 at 6:50 am

          You’re a dumb shit if you think flushing has any effect on ash color. Go research what causes the color of ashes, it’s temperature and duration of burning, has nothing to do with flushing. Dumb shit.

          • Avatar

            blablablabal5

            August 3, 2020 at 9:12 am

            @nostra, vaccines first off have been around longer than youve been born. Secondly, you linked a youtube which says to me you are gullible and easily influenced..kinda sad you base your crap off youtube.

            Maybe stop believeing everything online….its not always correct. Youre that mom that tries super hard to fine one little line that has negative effects, when theres millions of lines with positives.

      • Avatar

        holdy

        July 19, 2020 at 6:32 am

        fuck you pat i wasnt able to go to college

        • Avatar

          Naturabis_za

          August 11, 2020 at 1:06 am

          I have. But I’ve seen many dumb fucks there too. I know many who has great knowledge on certain fields and put some to shame with college degrees.
          Don’t ever feel bad Holdy regarding that. Deep down you know degree doesn’t necessarily makes you more knowledgeable. My 2c.

    • Avatar

      WormwoodandWine

      April 20, 2020 at 11:03 am

      Wow, you seem knowledgeable, well-reasoned and not at all a resentful loudmouth who doesn’t understand how scientific research works at any level.

      • Avatar

        WormwoodandWine

        April 20, 2020 at 11:05 am

        And by the way, your 30 years of smoking weed doesn’t make you any more of an expert in plant science than drinking beer for three decades makes you an expert in organic chemistry.

        • Avatar

          lucien masse

          July 10, 2020 at 5:33 am

          parfaitement,,des gérants d estrades ils y ens a .lol

    • Avatar

      John Kuhn

      May 25, 2020 at 3:53 pm

      Let’s try to be less hasty with the nasty comments. They do not help anyone. She was being objective and signalling an openness to the possibility of an unexpected outcome. That’s exactly what you want and should expect from someone with a science Ph.D. If she already knew what to expect then why have an experiment in the first place?

      I remember years ago once saying that someone should come up with a simple and easy way for businesses and individuals to update the public in real time online (weekly, daily, hourly, whatever) on the status of whatever they were working on. People told me it was a dumb idea and not possible. A year later Twitter made its debut. Let’s all stay positive and remain interested and involved in whatever new developments modern research may provide us. Of course there will be some bad things, but we should always remind ourselves to work towards the greater good. Twitter is often annoying and lame, but the good it can do has in no way been diminished. I hope you’re doing well and stay safe and happy.

    • Avatar

      GrowyMcGrowface

      July 9, 2020 at 2:55 am

      Wow. Just wow. The worst thing that can happen to a study is a scientist with a set mind. It compromises all results. Like yours. You’ve been using the same method for years. Forgive my bold prediction, but I don’t think you’ve ever done a double-blind study on the subject. “I’ve had a feeling for 30 years” cannot be called science in any way. You have done the flushing, payed good monye for it, so you want it to be true. Take a step back, breathe and really think: Is what I’m saying a reflex-contradiction, or do I have any objective evidence to back it up?

    • Avatar

      Naturabis_za

      August 11, 2020 at 12:59 am

      This is exactly the kind of article what makes the people with no knowledge on this field confused. Where is the break down on what nutes we’re talking about? Organic or inorganic? I never flush my organic grow just stop feeding them 2 weeks prior harvest. However I would never smoke – or make any use of – cannabis which was grown with salts and there’s even a slight chance that I will smoke some chemicals with it.
      If anyone wants to kill themselves slowly, be my guest. I sure won’t.

  2. Avatar

    Adam Marsh

    January 10, 2020 at 8:12 pm

    If the duration of the flushing period had no impact on flavor, smoothness of smoke, or color of ash it seems to me that the benefit of flushing or stopping nutrients two weeks before harvest would be that your not paying for unnecessary nutrition. Why give it nutrients if its literally doing nothing if not to sell more nutrients? You want the plant to use up the nutrients it has stored, this is the idea that I understand behind stopping nutrients at the end of harvest

    • Avatar

      John Kuhn

      May 25, 2020 at 3:21 pm

      Good point. And like the article said, many people over-feed their plants, so they might have inadvertently created the need for flushing in the first place. But back to your point: if the plant doesn’t need the added nutrients for the final week or two anyway, you might as well flush if only to conserve the nutrients you paid for.

    • Avatar

      nostrafckingdarmus

      June 15, 2020 at 5:32 pm

      haha no brainer, same results basically with 2 option. free or spend money, if i pull one down that aint flushed i get moans off people, they can tell with out me saying anything which show me if you know your shit you can see if its been flushed, there are a few dodgy products (forcers) pest control sprays on the market, with carcinogenics that have banned, but people may still have them lying around that they could have in high concentration, last thing you want on your lungs

  3. Avatar

    Alf Mckenny

    February 21, 2020 at 8:50 am

    Here is another prime example of let’s ask Mother Nature …
    Does she flush everything in nature prior to giving you strawberries to avocados, to leaf lettuce? Hell no and never has , the nutrients are still right there in the earth 100% , so Danko and all the other folks can keep believing the misinformation they been peddling to us for generations.<blockquote>

    • Avatar

      Pat

      April 11, 2020 at 2:42 pm

      Actually most harvest times are in fall when rains are increased and with cannabis that is especially true. Harvest for Indica is generally in the monsoon season, increased rains, flushing. Seriously, check yourself, you are the one spreading misinformation. All of the plants you listed are not cannabis, they are not even in Cannabaceae, so any information regarding their growth habits or fruit formation would not be pertinent. I had to make an account just to comment because your ignorance defies logic. You’re the type of person to not vaccinate your kids or eat “GMO” because it is “unnatural” which isn’t even remotely true.

      • Avatar

        nostrafckingdarmus

        June 15, 2020 at 8:04 pm

        you are right it rains and you’re the type of person that would vaccinate your kids, do you know exactly whats in the vaccine you are pumping into your kids? https://www.youtube.com/watch?v=VgvyNo7IM-c here is a link to some very disturbing news my friend from a scientist called judy mikovitz saying otherwise! one piece of a VERY corrupt puzzle, just keeping watching the news everything will be ok

        • Avatar

          GrowyMcGrowface

          July 9, 2020 at 7:19 am

          Dude, check your facts. Her methodology was sloppy and when noumerous other teams were unable to replicate her results, she was fired. She was not arrested for her thesis (that’s pretty much impossible unless it directly harms someone) but for taking a laptop with sensitive data from the lab. Apparently she got pissed, or tried to milk the story, so she twisted a lot of stories.
          Anyways, how did we get to anti-vaxxers on a discussion about watering plants?

      • Avatar

        C.Dankens

        July 5, 2020 at 6:23 pm

        ” Actually” my ass, Gtfoh your backyard is not our model. Soil moisture conditions vary at harvest time in relation to geography. Soil growers who are sophisticated enough to be relying on soil biology arent about to kill off the microbes to shut down metabolism and nutrition cycling

    • Avatar

      nostrafckingdarmus

      June 15, 2020 at 5:36 pm

      you dont get excess salt build up in nature though bro

  4. Avatar

    bob

    March 30, 2020 at 1:31 pm

    flushing is for toilets

  5. Avatar

    Dennis

    April 5, 2020 at 6:50 pm

    Keep in mind these test were conducted in a controlled setting were there wasn’t any overfeeding. And second why do people ph there water for flushing isn’t that a waste of time being the whole point is for the plant not to pick up any more nutes?

  6. Avatar

    John Kuhn

    May 25, 2020 at 4:19 pm

    The importance of constant pH control is based on the knowledge that throughout the entire life of the plant there are always natural chemical reactions taking place that affect the final outcome of the grow. These chemical reactions can be helped or hurt by the pH of water and soil, so in order to allow for these natural reactions to take place unhindered, it is important to always maintain a desirable pH level throughout your grow.

    I’m sure you could get away with a certain amount of cutting corners, but there is a limit to how many shortcuts one can take before crossing the line from awesome to great; great to good; good to okay, and okay to not good. I have experienced numerous growers’ finished products and the results were always dependent on the knowledge, skill and dedication/patience/love of the individual grower. To paraphrase James Brown, you have to pay the cost to be the boss. Even those with natural talent have to work hard to excel in their field (no pun intended).

  7. Avatar

    Michael

    June 27, 2020 at 3:53 pm

    Since there is no real difference, besides a few people talking about taste. I think I’ll just water my plants the last two week. Yes the last two weeks I give my plants tap water at a pH of 6.5. Since I grow in soil, I don’t call watering my plants the last two week flushing, its just watering.
    Moral of the story: my weed just as good or better than yours and I save 2 weeks worth of nutrients.
    Last note your lights are feeding your plant not fertilizer. If all you do is grow in a good well balanced soil and water with water at a pH of 6.5 you will produce a decent sweet tasting crop.

  8. Avatar

    Regiside5385

    June 29, 2020 at 3:00 pm

    Where are the lab results? Ppl can say whatever they want w/o lab results it means nada

    • Avatar

      C.Dankens

      July 5, 2020 at 6:29 pm

      Ya so true. An analysis of the compounds released on combustion along with experiential data would be legit.

  9. Avatar

    lucien masse

    July 10, 2020 at 5:41 am

    je ne suis pas un scientifique,et encore moins un expert,j entretien aux 2 semaines,avec 15,30,15,de nutri,arroser avec amour,entretenue a tout les jours,(feuilles,insectes,ect.et je recolte en mi-sept,jamais eu de problemes,je n ai jamais,rincer,,.chaqun sa facon j imagine.je doit avouer que je ne connaissait pas cette facon de faire,et tant mieux si cela fonctionne pour vous.bonne journée,et amuser vous.lolll

  10. Avatar

    holdy

    July 19, 2020 at 6:37 am

    there is a obvious bias on the side of “use more nuitreients” proably beacuse these “scientific studies” were done by a NUTRIENT COMPANY” and a phd will not nessesarly make you anything at all sorry to the people who wasted tbeir time and money going to school for 8 years, your frustration clearly shows

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