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How and when to flush cannabis plants

August 11, 2017
How and When to Flush Cannabis Plants(MStudioImages/iStock)
When starting out as a cannabis grower, “flushing” is a term you might be unfamiliar with. Although visions of frantically trying to shove a cannabis plant down a toilet may come to mind, flushing is actually when you stop feeding the plants nutrients and simply give them water. The process is generally as easy at it sounds; however, knowing precisely when to flush and how often is a more refined skill.

When to flush cannabis plants

There are three primary instances when flushing your cannabis is appropriate:

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Stages of the cannabis plant growth cycle

Changes in nutrients

Cannabis plants have different nutrient needs throughout different stages of growth; what it needs in its vegetative stage is different than what it needs while flowering. Clearing old nutrients during the transition into the new stage of growth will help kickstart the plant’s journey into flowering.

Think of it as someone changing their diet. You can fill your fridge with new types of food, but you might find yourself running out of room if you never removed the old food. By removing the old food, you are clearing a path and starting anew.

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What are the best nutrients for growing cannabis?

Nutrient lockout

Overfeeding cannabis, improper pH levels, and other stresses on your plants can result in a nutrient lockout. Whether it’s salt buildup from nutrients or pH levels that prevent absorption, a nutrient lockout results in a buildup of nutrients in the soil while the plant cannot access them.

Flushing your plants will remove excess buildup of salts and nutrients while helping to restore the soil’s pH balance. This will allow your garden to resume absorbing nutrients and grow at a healthy, successful rate.

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What Is Nutrient Lockout and How Does It Affect Cannabis?

Pre-harvest

Your final flush should occur before harvest. This will force the plant to use the nutrients stored within itself in the final week or so. If its nutrient reserves are not used by the plant or broken down, they will affect the quality of your harvest’s smoke and flavor.

Related

When is the right time to harvest cannabis plants?

How to flush cannabis plants

Once again, flushing cannabis simply involves running pure water though the soil or medium. When trying to stop a nutrient lockout or when switching nutrients, perform a flush by excessively watering your plants with water that has a pH level between 5.5-6.5 (for hydroponics) or 6.0-6.8 (for soil). Fully saturate your pots, and repeat 15 minutes later. The flush should clear any blockage and make room for your new feeding schedule.

To be certain a flush was successful, you can use a TDS (total dissolved solids) reader to determine how pure the water runoff is. You want the TDS reading of the water draining out of the pot to be close to the TDS reading of the pure water you are flushing with. This reading will ensure that the nutrients have been washed out of the soil.

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Diagnosing Cannabis Nutrient Deficiencies in Sick Plants

When looking to perform a flush before harvest, there are more factors to consider. Make sure your plants will be ready for harvest once the flush is complete. Flushing too early or too late will result in a lower-quality product.

Different growing mediums require different flushing time frames before harvest:

  • Soil: 2 weeks
  • Rockwool and coco: 1 week
  • Hydroponics: 1 week or less
Related

What Is Hydroponics? An Overview of Soilless Growing

By paying attention to the trichomes on your plants, you’ll be able to tell when your plants are ready for a flush. If flushing two weeks prior to harvest, you should begin when you start seeing the first trichomes turn milky.

When not to flush cannabis plants

The only time flushing is not encouraged is when you are growing in amended organic soil. This is because your soil already holds all the nutrients the plants need to thrive. By flooding the soil, you can wash away and damage the complex ecosystem that you’ve worked so hard to develop in the soil itself. Furthermore, these plants almost always receive pure water during waterings. The nutrient uptake by plants in this environment is natural, diverse, and easy for the plant to process.

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What is super soil and how do you make it?

Once you’ve properly flushed and harvested your garden, it’s important to dry and cure the cannabis correctly. It is through a quality flush and a long cure that your cannabis will be able to fully express itself.

Trevor Hennings's Bio Image

Trevor Hennings

Trevor is a freelance writer and photographer. He has spent years in California working in the cannabis industry.

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  • Jane Peters

    Your pot should be organic. Then you wouldn’t have to flush.

    • Rob

      You should always flush even if you grow organic.

      Chemicals are chemicals.

  • Charles Valiente

    When do you switch from Big bloomfertilizer to Tiger bloom fertilizer? Fox Farm that is.

    • E.L. Bl/Du

      throw it away and get some good organic fertilizer, like age old bloom and sweet finsih. Fox Farm fertilizers are NOT organic. Read the label. Switch to bloom when you start to see white hairs. Flusing is very important, and if you guys believe this article, then flush with Molasses in your water, this will still feed the microbes (which die over winter anyway) give the plant some energy during the flushing process, and gives the smoke a very nice smooth taste. I can taste unflushed cannabis, its very harsh and tastes like crap. These are “undetected” by the testing labs, but very important to the flavor and smoothness of your smoke. Flushing is CRITICAL at the end, regardless of your growing practices. Chemical fertilizers (poor farming practices) do not absorb completely and there is a great deal of run off into the waterways, streams and lakes, causing toxic algae blooms. Organic is the most healthy for our planet and our bodies.

      • Legalize420

        I agree with this 100%. I have because of finances struggled with ONLY the fox farm trio, and molasses. My results really spoke for themselves. But it’s also due to the uncontrollably hot and humid small room I live in there’s like barely any air it sucks. Very stuffy.

        Anyway I a month ago or so finally for the first time all year got to the Hydro shop. I have been growing on and off mostly off but half/half but I’d say for a decade or so now. I admit I don’t know the soil food chain or understand microbes but I’m going to give organic growing a try and see what the results will be like. I got some M3 Michigan Mix or whatever Super Soil, some Papas perfect poop and a bottle of Mammoth P. This is for my 2-3 legal medical MMJ plants. I am pretty excited about using the new super soil but if I have a worse turnout than with Happy Frog and the trio+molasses(ima use molasses with the super soil also). So I’m pretty excited to have my own organic clean medication. My last plant was a small auto and sadly it’s cola got so big a brown single bud leaf appeared and some brown calyxs so I chopped it out and cut down the plant as it was ready at 60%+ amber anyway. I am very excited to grow organic but at the same time it was so costly I can not do it again after it runs out. Hopefully I can grow my 86 Williams Wonder IBL in the Super soil and get great results. First I’m trying autos. Hopefully Organic and Mammoth P is as good as they say!
        Keep up the good work and let people know the best medicine is clean organic medicine even if your ghetto dirt poor! 👍✌

      • Godsgreengifts

        Hello Leafly community! I want to start growing and I am looking for some comprehensive learning resources. Would anyone mind pointing me towards some excellent information sources for growers?… especially for beginning growers trying to learn what growing methods, systems, and equipment to begin investing in. Thank you in advance!

        • E.L. Bl/Du

          Ed Rosenthals “Marijuana Growers Handbook” Its the text they use at Oaksterdam University and its pretty throuough. Another good one that Leafly will be carrying is Dustin Fraser “Outdoor Performance Cannabis” Even an experienced grower learned something (17 yrs) and covers Soil to harvest, what we need to know. Keep in mind he was a tree farmer (organic) and has a fancy watering system. We made ours out of old hoses and junk, and a few new soaker hoses (fasioned after his fancy one) and it works like a dream. Another good one I read was “Marijuana Horticulture The Indoor/Outdoor Medical Growers Bible” by Jorge Cervantes who also has a YT channel. Keep in mind, hes a little spacy, but has allot of great knowledge to pass along. I hope that helps and HOPEFULLY you decide on outdoor, it leaves less of a footprint, tastes better, higher yield, and can get it stronger if you do it right. Best of luck, and just remember to keep an open mind, every year you will learn something more. Its the ones that “know it all” that end up with smaller plants that arent very big or strong. You will also hear allot of controversy about how amber the trichomes should be for harvest. I go by the book and just agree with the so called “experts” Once they turn milky, and a few (20% are amber) time to get them down. After that they start to deteriorate and decrease in THC and CBD/CBG/cbn. You have about 72 hrs to get your crop in before they start to deteriorate. This is why I plant different strains, so they dont all come off at once, and you have time between plants to get them in. Happy growing 🙂

      • Caleb

        Fox farm big bloom IS organic tiger bloom is not they do have some other organic products but I have not tried them so I will not speak on them

        • E.L. Bl/Du

          too many variables for me. Fox Farms products were really good when they first came out, and that was all that was available. THings are different these days, and many better products out there for the same amt of $. the ONLY thing I like is their Ocean Forest soil for indoor starts, b/c its sterile. (NO microorganisms which can be added w. nutrients) I have not had the plant and bud size and bud density that I get w/ AO products when using fox farms nutrients. (although their labels are really cool) FAR from it. I have only been extremely disappointed GREATLY w/ Fox farms products, and it is far too time consuming to repeat the same mistake again in ANY subsequent grow season. Fox farm will never produce a 5 pound plant, but Its pretty regular using AO and excellent soil. All depends on how much you want to get out of your investment.

          • Caleb

            This is the first time hearing of AO farms I just looked them up. Thank you for turning me on to them 😎

          • E.L. Bl/Du

            Caleb, I have had the BEST luck this yr using thier products (and excellent soil which I cannot stress enough) HOwever, if you add fish/seaweed and Kelp to the mix (grow/bloom) you can get excellent results w/o all the nutrients they recommend on their advanced feeding schedule…. We have some colas this yr like I have NEVER seen in 10 yrs of learning (and 17 yrs of being a master gardener). If any problems, their customer support is a wealth of information. Its actually called Age Old Organics (vs farms) Many of the grow stores carry it these days. I found a cheaper source online @ planet natural. I wish you happy growing of healthy plants. If you’re unsure, they have a small quart size you can try, and if your lucky, they give out samples every so often (4 oz) if you can get your hands on some at the grow store. I tried to upload an image but it wont let me.

    • Caleb

      First sign off flowering add tiger bloom but also keep big bloom running.

      • Stacy Federico Neil Walker

        Arjan and Franco the kings of cannabis flush their plants and wait until the fan leaves are yellowing, this is how you know that the plant has run out of nutrients, so it starts to use the nutrients stored within itself, it has logic behind it, the taste of the smoke in this case will be improved as there are less unwanted chemicals in the plants if using synthetic tomato plant fertilizers or organic alike, the companies will try to fool you though, for example fox farm schedule says to add a certain amount of big bloom or grow big or tiger bloom etc per litre or gallon but in my experience even maintaining the correct PH levels and EC / PPM (TDS) between 5.5 and 6.5 for a hydroponic setup, this scheduled amount is too much nutrient, and will end up causing toxicity which always leads to nutrient burn and other signs of over feeding, if you have nutrient burn on the tips of your leaves you should flush to remove nutrients from your roots, this resets your plant health and it clearly works so the evidence to suggest that flushing your plants before harvest is beneficial to the grower is 100% clear in my mind, i agree with flushing the plant and removing as much as i can before harvest even if it means to weaken the plant right at the end of its flowering and trichome development stage (cloudy to amber trichome’s) i do not flush with and flushing products as this is another additive after all, i rather just use ro water with a tds under 30 ppm and i do it for 5 days, my reservoir always matches the tds in my ro collection bucket and once both reservoir and ro container are the same it means that your reservoir has no added nutrients in it anymore so from that point on i remove all water and lighting for the next 24 – 48 hours just to remove as much water from the plant as possible before i cut the main stem and harvesting the bud’s, stress added to the plant will sometimes make the plant jump into survival mode and gives you a slight boost in bud growth in the last 24 hours, too much stress before this stage and your plants might even revert the flowering and you will start to diminish thc and cbd etc from your buds and sugar leaves and in some extreme cases the plant has even tried to change sex and become a male and grow pollen sacks as a last resort, i have heard this happened to many people but i have never experienced this before myself because of flushing before harvest so i seriously see no problem what so ever with flushing, in fact i can only see beneficial results.
        I have tasted bud which has not been flushed before harvest and the ash of the weed was black not gray, gray ash means there is no nutrients being burnt only biodegradable product, but black ash on the other hand means that nutrients have been burnt and there is more in your harvest than just bud THC, CBD etc this is also evidence to suggest that flushing does eliminate unwanted nutrients from your final product.

  • les

    It never ceases to amaze me that people perpetuate this myth. It shows you know nothing about plant biology, which shows you can only starve a plant to death, you CAN’T flush anything it has taken in, and you do it right when the plant needs its nutirents the most.

    Also, it show that most people know absolutely nothing about how plant nutrients are made. Of course the same people buy all those phony “bud boosters” that either do nothing at all, or are loaded with illegal hormones. Do some reading before you say a word, you might just be enlightened!

    The only thing you should flush is your toilet!

    • justadbeer

      I agree 100% – This is what I refer to as “Stoner Science” . Ever see a farmer flushing his crops before harvest? This is a myth perpetuated by companies that want to sell you a flushing agent such as Clearex. They even go as far as saying that flushing with plain water will cost you yield’s. I have fallen for, and fed into this nonsense at one time. Not any more. I’ve done side-by-side comparisons and found zero difference (in taste ) There is no question that flushing removes nutrients from the root zone, so the key question is whether flushing the root zone has any influence on the rest of the plant’s biomass — In truth, (and science) Nutrients are locked in the plant, and an external flush cannot undo the complex biology that locked them in. Of course everyone is free to do what they will.

  • Kelly Ritter

    My indoor gets fed daily so even though I use an amended organic soil I feel it is good to flush with clean water the last week pre-harvest.

  • Alien Wired

    You mean the same Jorge Cervantes that tells us to PH our water before adding nutes???

    You mean THAT Jorge??

    You mean the Jorge Cervantes that isn’t even his real name, and has written a book at the height of marijuana criminalization to exploit people like you and get filthy rich?

    THAT “expert” Jorge?