# Flushing cannabis plants? **URL:** \ **Category:** Factual Questions\ **Created:** [December 24, 2020, 2:05am UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781 "2020-12-24T02:05:03Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![therealhoax](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/therealhoax/32/5266_2.png) [@therealhoax](https://boards.straightdope.com/u/therealhoax)\ **Post date:** [December 24, 2020, 2:05am UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/1 "2020-12-24T02:05:03Z")
As a new (legal) cannabis grower, I’ve noticed a lot of people swear by “flushing” plants to remove toxins/nutrients and to provide a smoother smoke. Others say flushing is BS…and it sounds like it to me. What’s the straight dope on it? Definition of flushing (credit: [maximumyield.com](http://maximumyield.com)): “Flushing is the technique of ceasing to provide nutrients to a cannabis plant and instead only applying water. The process of flushing the marijuana plant usually takes place during the final transition phase of the plant’s life right before harvest. Ceasing to provide additional nutrients forces the plant to utilize its stored nutrients during its final week of growth. If a marijuana plant has excessive nutrients stored in its plant material, the built-up nutrients can affect the flavor of the plant and its overall quality.” ---
**Author:** ![SmartAleq](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smartaleq/32/163_2.png) [@SmartAleq](https://boards.straightdope.com/u/SmartAleq)\ **Post date:** [December 24, 2020, 2:12am UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/2 "2020-12-24T02:12:25Z")
It’s gonna depend a lot on what nutes you’re using but flushing will never hurt anything. If you’re using coco coir or soil there’s plenty of nutrients stockpiled in the medium and in the plant itself (in those big water leaves, mostly) to let it finish out with nothing but water the last week. ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [December 24, 2020, 4:10am UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/3 "2020-12-24T04:10:53Z")
Well, if you keep fertilizing until the day of harvest, the resulting product burns down to a black charcoal like ash. If you stop fertilizing at a prudent time beforehand, the resulting product burns to a fine white ash. The latter provides a clearer signal when it’s “cashed”, preventing folks from burning the darker charcoal like ash in an attempt to not waste what they paid for, even though it’s pretty much just making them cough. Fertilizing until the day of harvest will give you more yield, but honestly results in an inferior product. If your aim is to grow for smoke-able flowers, I’d personally advise on not fertilizing for a time before the harvest, the actual time that you would stop would depend on the medium you’re growing in. ---
**Author:** ![therealhoax](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/therealhoax/32/5266_2.png) [@therealhoax](https://boards.straightdope.com/u/therealhoax)\ **Post date:** [December 24, 2020, 4:16am UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/4 "2020-12-24T04:16:59Z")
Interesting…I recently bought some bulk cheap weed from a local store here in Canada and it burnt to a hard charcoal as you described…wasn’t sure if that was bad curing or something else. ---
**Author:** ![SmartAleq](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smartaleq/32/163_2.png) [@SmartAleq](https://boards.straightdope.com/u/SmartAleq)\ **Post date:** [December 24, 2020, 4:28am UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/5 "2020-12-24T04:28:38Z")
I think that’s more of a curing thing than a flushing thing–I’ve grown with and without an ending flush with the same clones and it never really makes a difference, but poor curing and drying, that will be a big factor. Poor curing and too-fast drying lead to harshness too, as does a less than tight trim job. ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [December 24, 2020, 4:28am UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/6 "2020-12-24T04:28:50Z")
Nah, that was almost assuredly fertilizing the plant to its limits and not giving it a break during the end of flowering. In my experience, the fertilizing controls a lot of things, but when you stop controls how well the end product actually burns. An improper cure will usually result in odd flavors - usually minty flavors in an under cure and musty flavors in an over cure. In between, there’s a range of cures to be had. Since I grew up on decades of brick weed, I personally prefer the smoother smoke of a long cure, but that’s not the fashion these days. Bright and flavorful is the name of the game, and I understand it, even if it’s not the smoothest way to go. ---
**Author:** ![Jackmannii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackmannii/32/311_2.png) [@Jackmannii](https://boards.straightdope.com/u/Jackmannii)\ **Post date:** [December 24, 2020, 5:14pm UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/7 "2020-12-24T17:14:10Z")
For plants (as well as animals and humans), anytime you hear a claim about flushing out ‘‘toxins’’, you can be almost 100% sure that it’s total bull (an exception would be medically-indicated chelation therapy for known toxic metal exposures). > [@scabpicker](#): > > Well, if you keep fertilizing until the day of harvest, the resulting product burns down to a black charcoal like ash. If you stop fertilizing at a prudent time beforehand, the resulting product burns to a fine white ash…Fertilizing until the day of harvest will give you more yield, but honestly results in an inferior product. My gardening experience does not extend to dope growing, but claims like these require actual evidence. ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [December 24, 2020, 5:21pm UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/8 "2020-12-24T17:21:28Z")
SWIM used to grow some primo dope, and swore by flushing a week or so prior to harvest. This was in plants grown in a soil substitute and fed with nonorganic nute system. Flushing and curing definitely has an effect on final product, but I think a bad cure has more obvious effects (hay smell). ---
**Author:** ![Dallas\_Jones](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dallas_jones/32/3277_2.png) [@Dallas\_Jones](https://boards.straightdope.com/u/Dallas_Jones)\ **Post date:** [December 24, 2020, 7:08pm UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/9 "2020-12-24T19:08:21Z")
> [@Jackmannii](#): > > My gardening experience does not extend to dope growing, but claims like these require actual evidence. Well my gardening experience does include dope growing and I agree with you. This Flushing is all bunk and needs some evidence. This was posted after all General Questions. Plants will only uptake and use the nutrients that they need, the rest is left in the soil or other growing medium. The theory that they will continue to stuff themselves with unneeded whatever and that ends up in the product that you smoke is just woo. Thinking that the flowers are somehow getting too woody and will turn into lumps of charcoal when you burn it is misunderstanding basic plant biology. But there has been so much woo associated with marijuana cultivation that it remains an unfortunate problem. As this crop is moving into main stream production hopefully this sort of thing will fade away. Along with the ridiculous naming conventions which are not helpful and mean nothing. Flushing with water to clear out “stuff” is not science. Unless maybe you have chronic kidney stones. Now, if you will excuse me, I need to detox my liver with essential oils. ---
**Author:** ![Dallas\_Jones](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dallas_jones/32/3277_2.png) [@Dallas\_Jones](https://boards.straightdope.com/u/Dallas_Jones)\ **Post date:** [December 24, 2020, 7:27pm UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/10 "2020-12-24T19:27:07Z")
A little more. There is nothing special about a cannabis plant. It goes through the same stages as any annual flower. There is a seedling stage, a vegetative stage when growth is the goal, and a flowering or fruiting stage. You wait to fertilize until the seedling is established, fertilize and water during the growth/vegetative stage, and then leave it alone to flower or produce fruit. Maybe adding some bloom fertilizer late. If you have your soil/medium correct you won’t even need that. No different from a tomato plant or an azalea. ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [December 24, 2020, 7:36pm UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/11 "2020-12-24T19:36:08Z")
> [@Dallas\_Jones](#): > > Plants will only uptake and use the nutrients that they need, the rest is left in the soil or other growing medium. The theory that they will continue to stuff themselves with unneeded whatever and that ends up in the product that you smoke is just woo. This is correct. Flushing is a myth, as is much of the information surrounding cannabis cultivation and preparation. It’s an unfortunate consequence of abruptly going from illegal and popular to quasi-legal and incredibly lucrative. Anyone and everyone gets to be an authority in a space that science hasn’t yet really penetrated. ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [December 24, 2020, 8:58pm UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/12 "2020-12-24T20:58:26Z")
> [@Dallas\_Jones](#): > > This Flushing is all bunk and needs some evidence. This was posted after all General Questions. Heh, I didn’t even think this was really controversial. So, I checked Ed Rosenthal’s site. He just about covers it: > [@](#): > > **There are many reasons to question the efficacy of flushing:** > > 1. Eliminating or lowering the availability of essential nutrients slows growth at all stages, including the last weeks of flowering. > > 2. It is difficult to flush large plants that are grown in bags containing 200-300 gallons (760-1140 l) of planting mix. The plants’ roots have a reservoir of nutrients to draw from. Yet, these plants are savored by connoisseurs. > > 3. There are no double-blind studies that have been performed to test the efficacy of flushing. > > 4. Although certain stresses increase trichome production, it is doubtful that nutrient deficiency is one of them. Trichome and oil production is expensive; it requires the plant to expend energy. It seems counter-intuitive that depriving nutrients would increase cannabinoid/terpene production. > > 5. Some gardens using perpetual harvest techniques as well as aquaponic grow methods have no provision for flushing but produce fine connoisseur buds. > > 6. Large amounts of Calcium (Ca), one of the mobile nutrients, are required for cell division. To mature, plants must grow new cells. Without a constant supply, maturation slows. > > 7. Only a small amount of Boron (B), another mobile nutrient, is required for plant growth. Without it in sufficient quantities bud maturation slows or stops. So, long flushing periods are likely to reduce yields. > > Despite the lack of peer-reviewed studies regarding efficacy, the overwhelming majority of marijuana growers flush. > > The consensus is that depriving plants of nutrients during the last phases of flowering results in a higher quality bud. Obviously, I’m on the side of what he considers the consensus. Yes, the plant _will_ take up nutrients it cannot use. It will take up nutrients until it dies from it. Over fertilizing your plants will kill them pretty quick. But that’s not even the argument here. The argument is that the extra nutrients make the end product not as good to smoke. As of yet, I haven’t seen any evidence that this isn’t the case. However, I went poking around and found this study (warning, PDF). [https://atrium.lib.uoguelph.ca/xmlui/bitstream/handle/10214/12125/Stemeroff\_Jonathan\_201712\_Msc\_with\_erratum.pdf?sequence=8&isAllowed=y](https://atrium.lib.uoguelph.ca/xmlui/bitstream/handle/10214/12125/Stemeroff_Jonathan_201712_Msc_with_erratum.pdf?sequence=8&isAllowed=y) > [@](#): > > 6.2FLUSHING NUTRIENTS FROM GROWTH MEDIA > The practice of flushing is a current common industry practice but there is no evidence in published literature of its effectiveness in reducing nutrient concentrations within > the bud or even whether or not this is a desirable result. After testing the nutrient concentrations from each treatment from three separate experiments, there were no significant differences in nutrient levels between any treatments within each experiment. This result showed that the intended purpose of flushing to reduce nutrient concentrations within the bud has no effect. These data show that for the last two weeks of the flower cycle for cannabis, it was possible to use no fertilizer water for irrigation with no significant impact on yield while saving input costs on fertilizer. Which is interesting, because it seems there’s no need to fertilize in the last two weeks, regardless. Save your money. ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [December 24, 2020, 9:06pm UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/13 "2020-12-24T21:06:29Z")
> [@scabpicker](#): > > Yes, the plant _will_ take up nutrients it cannot use. It will take up nutrients until it dies from it. Missed the edit window, but this is partially incorrect. Over fertilizing normally just keeps the plant from being able to absorb water from the environment. It’s not normally from it taking up too much nutrients. ---
**Author:** ![Jackmannii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackmannii/32/311_2.png) [@Jackmannii](https://boards.straightdope.com/u/Jackmannii)\ **Post date:** [December 25, 2020, 3:16pm UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/14 "2020-12-25T15:16:48Z")
Well, I’ve learned something from this thread - mainly that cannabis growers are susceptible to the same sorts of dubious claims and questionable science that affect other specialty crop growers and generalist gardeners.\* The idea that over fertilization of herbs (including M.J. as an herb in the discussion) leads to lush nitrogen-fueled growth and lower concentrations of active/tasty ingredients potentially has merit (again, specific claims require solid evidence of altered concentrations of active substances, not “I swear it tastes/smokes better”). Stopping fertilizer a week before harvest doesn’t provide enough time for desirable chemical concentrations to build back up in the plants, nor would “flushing” steer overfertilized plants back to health in such a short period (i.e. reversing stunting or replacing fertilizer-burned foliage with healthy new growth). For plants grown in coir-based soil/soilless mix, I’ve seen recommendations to “flush” out the media before using it to lower undesirable salt concentrations, though the coir I use hasn’t presented any problems in that regard. \*the array of wildly overpriced fertilizers, additives and fancy lighting setups in hydroponics stores is something to behold. One of the latest “growth” businesses involves “beneficial bacteria”. What they actually benefit in real-world applications is mostly guesswork at this point. ---
**Author:** ![SmartAleq](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smartaleq/32/163_2.png) [@SmartAleq](https://boards.straightdope.com/u/SmartAleq)\ **Post date:** [December 25, 2020, 8:17pm UTC](https://boards.straightdope.com/t/flushing-cannabis-plants/928781/15 "2020-12-25T20:17:01Z")
Well, I can attest that the use of _Bacillus thurigensis israelensis_ does cut down on the likelihood of fungus gnats and the attendant flyers in an indoor environment. Growing indoors does present its own unique set of problems requiring sometimes very creative solutions because you don’t have a whole biosphere in there to keep things in check. I agree on the silliness of the whole range of super duper overpriced nutrient “systems” out there–me, I use JR Peters Jack’s Pro inorganic salts, cheap, simple, absolutely foolproof if you know what the fuck you’re doing and aren’t one of those rash reactionary idiots who throws spaghetti at the wall every time there’s a perceived problem. I’ve given up arguing over nutes with people, it’s like a fucking religion to them and we all know how successful THAT is.