Almost all of us who grow marijuana, especially if we grow indoors using hydroponics nutrients and pure hydroponics systems and substrates such as deep water culture or rockwool, have been advised to “flush” our marijuana plants at the end of bloom phase.

There are two generic methods for flushing marijuana plants at bloom phase termination. One is to stop all nutrients feeding and give only pure reverse osmosis water to plants during the last 5-14 days of bloom phase. Another is to stop all nutrients feeding, and give only reverse osmosis water plus a commercial flushing formula during the end of bloom phase.

It isn’t much of a surprise that people who most vigorously promote flushing marijuana are those who sell flushing formulas. Flushing promoters make several claims about flushing’s alleged benefits, including those summarized below:

  • Flushing purges “excess nutrients salts,” heavy metals, chlorophyll and other contaminants from plant tissues.
  • Flushed marijuana buds taste and smell better, are easier to combust, burn to a clean white ash, and are safer to use than unflushed marijuana buds.
  • Flushing stresses plants, which causes them to generate an unusual, intense burst of cannabinoid and terpenoid production that increases bud potency, scent, taste and value.
  • Flushing saves you money because you don’t use nutrients during the last days of bloom phase.

Almost every grower I’ve ever met, including some on the Growing Marijuana Perfectly team, believes in flushing and does it. I myself have always been suspicious about it. Why? Because I’ve seen what flushing does to plants while they’re still growing, and it’s bad. Simply put, flushing starves plants of nutrition during bloom phase.

When I used to do marijuana flushing (I haven’t done it in years), I didn’t like seeing the symptoms that appeared within 3-7 days of when flushing commenced. These negative effects included:

  • Sudden onset of leaf symptoms indicating nutrients deficiencies.
  • Sudden onset of resin gland degradation accelerating way beyond normal degradation.
  • Buds appearing to “shrink” rather than continue adding resin and bulk.
  • Bud hairs becoming brittle and twisted.
  • Leaves falling off.
  • Plant water usage decreasing to almost nil.

I never could convince myself these symptoms were inconsequential, that flushing wasn’t hurting my plants at a crucial time (the last days of their life cycle). It never made sense to me that starving my plants was a good way to increase end-of-bloom-phase bud bulking and resin production.

Arguments about flushing in the marijuana growing community overlap with another set of claims. These claims are made by advocates of organic or “natural” soil marijuana growing. They say marijuana grown using pure hydroponics and synthetic hydroponics nutrients are polluted, harmful, and don’t taste or smell as good as buds grown organically in fertile soil.

Organic marijuana advocates have a phrase for buds grown hydroponically: chemmy weed. It’s not meant as a compliment.

Hydroponics growers challenge the “organic bud is best” assertions by saying unflushed hydroponics buds might taste and smell chemmy, are hard to combust, and don’t burn to a clean, white ash…but flushed buds are just as clean, safe, easy to burn, and taste and smell just as good as organic marijuana buds.

Until recently, I only had anecdotal experiential evidence to challenge or judge the vehement, conflicting assertions about flushing, and about hydroponic versus organic buds.

Just one example of this came many years ago in beautiful Amsterdam when it used to be the only place in the world that had a “legal” cannabis industry and a Cannabis Cup event.

The biggest names in the Dutch cannabis seeds, cultivation and cannabis coffeeshop industries, along with famous cannabis heroes and connoisseur growers and consumers, were at an after-party at which the hosts set up a blind bud test.

The test was very fun and fascinating. Our hosts had 15 large bowls filled with buds of different strains. The bowls were labeled only with numbers. Each guest was given the opportunity to use a clean glass pipe and matches to sample the buds via combustion, and asked to fill out a taste, scent, potency, harshness and overall quality ballot for each sampled strain. The ballot also asked if the judge believed the bud was grown organically or hydroponically.

After 90 minutes of increasingly psychedelic partying and sampling, when we were about as high as you can get on cannabis buds, the ballots were collected. While we socialized, ate munchies and talked cannabis insider business and politics, our hosts were hard at work tallying the scores and analyzing the results.

A little past midnight, they called us to attention, which was difficult, considering that some of us had passed out, others were debating whether it’s safe to swim in Amsterdam’s canals, and a few had temporarily gone stark, raving mad.

The results were announced, creating a firestorm of good-natured stoner arguing, shouting, and bizarro pronouncements. Why? Because all but one of the strain types that scored the highest were grown hydroponically, not organically, and without flushing.

Let me put it another way: even the organic bud fanatics at that gathering had to admit they’d been totally unable to tell the difference between organic and hydroponic buds. And everyone was surprised that nobody could tell the difference between flushed and unflushed hydroponic buds.

I’ve seen similar bud judging results across the world. That’s why I have either scoffed at or stayed silent when people argued that organic marijuana buds are superior to hydro buds, or that flushing really works.

Problem is, anecdotal, subjective judging is useful, but it’s not “science.” And because until recently marijuana wasn’t legal enough for real plant scientists to do real plant science on marijuana crops, nobody had tested whether marijuana flushing is good, bad, or meaningless.

Recently, one relatively new but impressive player in the nutrients manufacturing industry, RxGreen Technologies, had the courage to start doing flushing research using marijuana plants, and publicly report the results.

The company has real plant scientists on staff, and they conducted research on clone-originated Cherry Diesel cannabis plants grown in coco coir. Some plants were flushed with pure reverse osmosis water for 14 days, and others for ten, seven and zero days.

After harvest, they lab-tested the plants for mineral elements, cannabinoids, terpenoids, potency, and harvest weight. They also distributed blind samples of each flushing duration to cannabis connoisseurs who evaluated the buds for taste, potency, combustibility, and harshness.

The results? Lab tests showed no significant differences in plant tissue mineral elements retention, harvest weight, taste, scent, potency or terpenes between flushed plants and plants that weren’t flushed.

And here’s a gentle kick in the teeth for flushing advocates: buds that weren’t flushed at all received the highest favorable rating from cannabis connoisseurs as regards taste, scent and harshness! Not only that, buds flushed seven days had the least favorable rating, and the 14-day flush buds also rated poorly.

But wait, there’s even more intriguing info from this study. The Cherry Diesel sugar leaves were sent to a professional extractor to measure extraction yield and preferred characteristics of cannabis concentrates made from the test crops’ resins. The results:

Cannabis flushed for 14 or ten days had the highest extraction yield compared to the seven or zero day flushes. However, the visual appearance and overall quality of the concentrate for the zero-day flush was much preferred over the other flush period concentrates because the zero-day flush concentrate was the clearest, with the least amount of nucleation and discoloration.

The extractor stated that increased flush time resulted in higher extraction yields, but the finished product was not as desirable as the finished product made from unflushed buds.

And it didn’t surprise me that the study’s scientists saw that the leaves of cannabis plants flushed for 14, ten and seven days showed yellowing, curling and other signs of nutrients deficiency, and had slightly less chlorophyll, compared to leaves of plants that had not been flushed.

Reps for companies that manufacture hydroponic flushing products argue that this study is fatally flawed because it used only reverse osmosis water, absent any flushing products made by some hydroponics nutrients companies, who claim their flushing products allegedly include chelates, mineral elements, and other materials that purge bad taste, scent, salts and compounds from crops while maintaining or even increasing production of cannabinoids, terpenoids and harvest weight.

I’ve asked the plant scientists if they intend to do another flushing study to test the effects of flushing products, and am waiting on an answer. I’m also starting a test run of the RxGreen Technologies nutrients line. If they work as well as I anticipate, you’ll be the first to know.

I’d also like to point out that this topic includes a cultivation strategy promoted by the late, great Subcool and other soil marijuana growing advocates. Subcool was an evangelist for growing in “supersoil.”  He even provided a public recipe you’d use to make supersoil yourself.

When mixed and aged correctly and supplied with beneficial microbes to create a living root zone, supersoil contained enough nutrition to give most marijuana strains all the nutrients they needed with no need for adding any fertilizers—until the last 2-3 weeks of bloom phase.

What happened in many supersoil gardens when bloom phase was longer than 5-7 weeks is the plants had used up all the nutrients elements by that time, and began showing the same nutrients deficiency symptoms as flushed marijuana plants show. Subcool advised growers to just let the deficiency symptoms happen. Don’t give them any more food, he said.

You see, Subcool believed in flushing, but he didn’t like hydroponics growing. He felt that the “autumn colors” of end-of-bloom-phase supersoil-grown cannabis leaves were a beneficial “fade,” similar to flushing, that meant nutrients elements and chlorophyll were leaching out of plant tissues, leading to the superior taste and scent he believed supersoil-grown buds have when compared to hydro weed.

I tried to explain to him that the only reason organic buds could taste and smell better than hydro buds is that organic buds grown in “live soil” are enhanced by beneficial microbes in the organic root zone. These microbes create a symbiotic relationship with cannabis roots that spurs hormone signaling and sugar exchanges that result in increased terpenoid and flavonoid production, leading to better-tasting, better-smelling buds.

It isn’t flushing, or the fact that your supersoil plants run out of nutrients elements to create a natural “fade flush” at the end of bloom phase that makes buds taste and smell better, I told Subcool, it’s the interaction of beneficial root zone microbes with marijuana roots and metabolism causing increased production of compounds that enhance bud taste and scent.

He replied that this just shows why he believed organic cannabis is superior to hydro cannabis. But I reminded him that hydro growers can also install beneficial microbes in their root zones, leading to hydro buds with enhanced taste and scent.

I also want to encourage you to give your plants bloom phase base nutrients if you’re growing in supersoil or any other type of substrate and your plants show obvious signs of nutrients deficiency near the end of bloom phase. Starving plants DO NOT give you your heaviest, most potent, tastiest harvests!

Sadly, Subcool is no longer around to debate “the fade,” organics versus hydro, or flushing with. And I know I’m gonna make a lot of hydroponics nutrients companies mad when I say this, but when you flush your marijuana plants at the end of bloom phase, and especially if you buy flushing products to do it, you’re just flushing your plant health and money down the toilet.

I also want to make clear that flushing your root zone substrate with reverse osmosis water for one watering cycle, to clear excess nutrients salts from substrate, is a valid form of cannabis flushing.

Don’t believe what you just read about marijuana flushing at the end of bloom phase as being useless and even harmful? Ok, I get that. Then do yourself a favor. Properly install beneficial microbes in your hydroponics root zone during your next grow op season. Flush a couple of your plants at the end of bloom phase just like you always do, and don’t flush the other ones. Watch what happens to your plants and buds. Then have a bud judging party like the Amsterdam party I described earlier.

I have a prediction: after you do that test, you’ll never flush your marijuana plants again!