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How To Flush Cannabis Plants

Created by
JoshuaHolt JoshuaHolt
Added 27 October 2020

Flushing Cannabis

Flushing is a term we hear often in cannabis growing communities. You may have been told that flushing is essential for your plants but still don't understand how to do it properly. Flushing is easy to perform and can make your roots healthier, as well as leave you with a much tastier product after harvest. This article shows you how it's done. 

What is Flushing?

The concept of flushing isn't any different to how we normally understand it. Flushing uses water to remove a build up of a material. Growing healthy cannabis normally requires us to add fertilizers, many of which contain mineral traces that build up in the grow medium and plant throughout the cycle.  

Concept Of Flushing

The process of flushing helps to remove any excess nutrients by 'washing' the roots. Pure water is given and allowed to rush over the roots until it drains. Any mineral and salt residue is flushed out of the growing medium, meaning the plant can use up its remaining store of nutrients before harvest day.

Flushing typically takes place two weeks before the end of the flowering period, but it depends on the grow medium and how heavy the feeding schedule is. The only time we do not recommend flushing is if your are growing with organic super soils, which contain loads of beneficial microbes and organisms that can get washed away from adding too much water.

Why is it Important To Flush?

Cured Cannabis Buds

Although some people might disagree, flushing your cannabis plants is essential if you have been feeding them chemical nutrients. Cannabis that hasn't been flushed properly can taste rough and deliver a harsh smoke that does not burn nicely. It is worth putting in the extra effort so you can be sure your buds are nutrient-free.

Most cannabis nutrients that are not organic are chemically synthesised. That doesn't necessarily mean they are harmful to smoke, but your weed will have a more enjoyable flavour without them. Feeding cannabis plants right up until the day you harvest means higher quantities of those minerals are still present in the plant when you harvest. We want to help the plant remove them so we (or others) have a delicious product to smoke at the end.

Flushing can also be useful when your plants receive too many nutrients. Overloading your plants with fertilizer can cause nutrient lockout, which means the concentration of minerals around the roots is too high for them to function properly. As a result, they are unable to uptake efficiently. Heavily fed plants may also experience nutrient burn.

With commercial nutrients being so heavily concentrated, nutrient lockout can be a serious problem.

In the event of a nutrient burn or lockout, flushing your medium with pH corrected water can help to remove the build up so you can continue with the feeding schedule. This is why some growers get into the habit of flushing once or twice throughout the grow cycle, regardless of whether salt build up or nutrient lockout is an issue. 

Tip: The curing process can also help to 'flush' out some out the impurities left in the bud and is a key stage in making your weed taste much nicer.

How To Flush Cannabis Plants

Trichomes On Cannabis Bud

As we mentioned earlier, the flushing cycle should start around 2 weeks before you plan to harvest. It is really up to you when to begin, but if you have used a very light feeding schedule it could be a good idea to start flushing a little later.

On the other hand, if you start flushing your plants too early they might not yield as much, so make sure your plants are showing signs of ripening beforehand. Check the trichomes to see if they are changing colour or going cloudy to gauge when the right time to start flushing might be. Once around 10 - 20% of the trichomes have gone milky a cannabis plant will be getting close to harvest day, meaning you can consider starting the final flush.

Flushing in Soil and Hydroponics

Flushing cannabis plants is an easy process and can be completed in 4 simple steps. The idea is to give an initial heavy flush and then provide water for the rest of the cycle.

  1. Flood the container or grow medium with pH stable water (at room temperature) until it drains from the bottom. Most tap water is fine but you may want to adjust the pH just in case.
  2. Wait 10 - 15 minutes.
  3. Repeat step 1. You can use an EC/PPM meter to check the runoff until the reading more or less matches the number of dissolved solids in the water (before you use it to flush).
  4. Feed plants only water until harvest, waiting for soil to nearly dry between each watering.

Medium

Flush Time

pH

Potting Soil

2 weeks

6 - 7

Hydroponics

3 - 7 Days

5.5 - 6.5

Coco Coir

1 week

5.5 - 6.5

Compost/Super Soils

0 Days

6 - 7

Some growers skip the first step and just jump straight to feeding water. It depends on how early you start flushing in relation to the amount of nutrients they have been given. There is a risk of overwatering your plants at this stage so make sure you have a clear idea of how much they are drinking first and do not overdo it. We want to be certain that we are not inviting mold into the garden by keeping the environment too wet, especially at this stage.

Flushing in hydroponics requires you to simply add fresh water (ph 5.5-6.5) to your reservoir for the last few days of flowering. Again, it depends on how heavy your feeding schedule was. In hydroponics, there is no medium for salts to build up in so when we flush, the plants are cut off instantly from any source of nutrients other than what remains in storage. It takes time for minerals to be fully flushed out of the soil.

Cannabis Flushing Tips

Overfed Cannabis Plants Can Display Green Foliage

Growth Stage - It can be a good idea to flush whenever there is a drastic change in nutrients. For example, the switch between the vegetative stage and the flowering stage is a moment where the feeding requirements likely shift. Flushing before introducing the new routine can give plants a 'fresh' start. Plus, you'll have a clearer idea of the nutrient levels in the soil and you help to keep a stable pH.

Enzymes - The minerals and salts are compounds that can become rock solid, making them difficult to move. After heavy feeding schedules, enzyme supplements can be beneficial to help dislodge large nutrient salts from the soil by breaking them down into smaller molecules. These can be very useful at the end of the flowering period if the leaves still look overly green (normally dark green from too much feeding). They prevent the roots from absorbing more nutrients by thoroughly cleaning the soil. 

Fat Banana (Royal Queen Seeds) flush by Leonweed10 from GrowDiaries.

Conclusion

Flushing becomes a standard practice once you have grown a few times. If you're not convinced, try growing the same plant with and without flushing. You'll likely notice the difference and be glad you decided to give it a try. And you'll save nutrients for the next grow!

How did flushing work for you? Did your weed taste better with a proper flush? Let us know your opinion on flushing down in the comments section below!

External References

Heavy Metal Soil Contamination and Bioremediation. - Muzafar, Malik & Goyal, Pooja & Gupta, AJAI & Gupta, Suphla. (2020).

This article was updated October 2020.






Comments

NorthernFrostCannabis
NorthernFrostCannabis

Flushing is Bro Science... https://www.rxgreentechnologies.com/rxgt_trials/flushing-trial/

https://cannabis-science-consulting.com/flushing-cannabis/?fbclid=IwAR2234VFGBbzVlWwn1Q85Ij6rdrr1ssRXDnvKXeaff2BzPaRXjUXHmGJqYM

https://atrium.lib.uoguelph.ca/xmlui/bitstream/handle/10214/12125/Stemeroff_Jonathan_201712_Msc_with_erratum.pdf?sequence=8&isAllowed=y
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JHGD420
JHGD420

@NorthernFrostCannabis, Flushing is indeed a tricky subject and has been interpreted in all sorts of ways. It is more about cleaning the soil than 'flushing' nutrients directly out of the plants. We feel it has its benefits when used at the right time. Thank you for sharing that valuable information, very good links. Happy growing :)

SEMRON
SEMRON

Flushing is bullshit and has been proven to do absolutely nothing beneficial, even on the contrary.

NorthernFrostCannabis
NorthernFrostCannabis

Flushing...
I will list some points that not only challenge the absurd impracticality and illogicality of this myth, but point out how the pseudoscience behind it is fundamentally flawed (as is all pseudoscience) and can be countered by what is known about basic plant biology.
1. Robbing plants of essential nutrients at any stage of their life cycle is NOT beneficial for growth. I challenge anyone to provide a single peer reviewed paper from a reputable journal that provides evidence suggesting otherwise.
2. If this was practical, wouldn’t you expect all big agricultural hydroponic growers adopt the same practice?
3. Plants take minerals into their tissues, from their roots via the treachery elements; i.e. xylem. Once these minerals are in the plant, they are there to stay, the plant does not expel them, unless it’s through senescence-driven abscission of leaf petioles. From the treachery elements nutrients are translocated into the phloem - the plant’s ‘blood supply’ - after being integrated into various biomolecules, or are used for various metabolic functions. Where is the logic in thinking the plant ‘uses’ these up in that last week of flushing, in order to avoid smoking them? All the N P K Fe Mg Ca etc. is still there.
4. For arguments sake say we counter the last point by suggesting these minerals in their ‘raw form’ will taste ‘hasher’ or ‘nastier’ in the form of pyrolytic breakdown products (formed when weed is burned) than artifacts of larger biomolecules of which these minerals/macro nutrients are now a part of, for example phosphorylated PO43-. Even if this was the case it still doesn’t correlate with the myth, as the transports steam in the treachery elements is measured in minutes not a week. i.e. a PO43- molecule does not wait around in these vessels for a week before subsequent translocation and modification.
5. If there was any truth to this myth, then plants grown in soil would always taste worse than plants grown in hydro. Why? Because obviously soil is not an inert medium you can flush for a week. And a plant CANNOT distinguish between a PO43- molecule that comes from soil from that of a PO43- molecule that comes from hydro solution (which also debunks another myth, but we’ll leave that one).
6. Are there studies that have conducted double blind trials to investigate if flushed weed tastes any ‘sweeter’ than unflushed weed. Again, need peer reviewed papers. And doesn’t have to be weed, can be strawberries or any other type of fruit.
7. What is the proposed mechanism to support this myth, and how is it consistent with fundamental plant biology.
8. How does starving the plant of food in the last week increase thc production in the trichome? Papers?
9. Given, under certain conditions stressed plants upregulate certain defence compounds, but they will almost certainly produce less inflorescence weight per watt of light. Growth is always retarded under stress - not promoted. Nutrient starvation is a form of stress. Looking for peer reviewed papers that suggest otherwise.
Those of you set in your ways, each to their own and best of luck to you. Those who are willing to change their views in light of new evidence, or lack thereof, be ready for increased yields by feeding those hungry ladies right up until the second you chop.

Slurpy_Terpy
Slurpy_Terpy

@NorthernFrostCannabis,
Thanks for taking the time to share details of how the nutrients stay in the plants. Indeed there is a lack of research in plants thc/terpene production, hopefully in a few years more experiment results become available.

Slurpy_Terpy
Slurpy_Terpy

NOTE BEFORE READING: I have never grown full organic, nearly always hydro and with chemical nutrients (smoke is clean AF though)
It's annoying there are incomplete studies about flushing, showing that it basically doesn't make a difference. As mentioned in this article, in soil when using chemical nutrients if you gave enough nutrients for your plants to not have deficiencies you will want to flush - at least from personal experience the 10 batches I didn't flush were not smooth. Growing DNA Genetics OG#18, I did several batches in soil and hydro flushing between 7 and 24 days. The 24 days flush was done in hydro, the growth was slow during these 24 days and the plants were ripe only after 80 days flower (its a 9-10 week strain), also the harvest did not weigh much (180g/m2, the area was stuffed 2/3 full so could have gotten a bit more). However!!! The QUALITY! Not exactly bro science here because I'm not going into bro-theory, but just trying it out was a world of difference... The 24 day flushed og18 tasted tremendous in a bong - day and night compared to the 2nd best bud Ive smoked... These days I progressively lower the nutrient dosage in hydro and flush only 5 days. I have a better setup and can monitor how much my plants drink with more precision so I aim for the lower bound with the nutrient dosage (giving the minimum so they just don't complain too much, apart from during flower stretch then they get fed as much as they will take!).
Its been 8 years and I haven't come across any smoke as smooth/tasty/powerful as that 24 day flushed og18, and yes I sampled more than 2000 different batches/strains (many of them of the same strains but different grow) since then ...

Final note:
In the Nethelands in coffeeshops when it states 'organic grown' or 'bio' or anything similar, well basically the smoke is sooo bad that I just can't... Also goes straight to a headache no joke the commercial grows are horrible here, and usually worse when they mention organic or bio. So if they are not lying then it's very well possible to get black ash and stinky burning weed growing organic... My take on this is that as always it depends on the details and bombing them with nutrients whatever the type can lead to degradation of quality.

TheBelgian
TheBelgian

Flushing is great, especially when trying to balance the Nitrogen dosage right around when flowering is going/kicking in. Regarding the flushing to cleanse the plant before consumption, there is a public university study paper which proofs this to be actually fiction, no significant (<- significant as in academia significant) traces were found. I read the entire paper once and it seems to be legit/sound, however as it was full of errata and post-addendums from the honest update of the researcher, I would like to see additional research first, to confirm the paper statements again.
When I flush, it's either to reduce nutrients or to conserve (as most resources will still be present in the soil or the available solar/auxiliary leaves.

TheBelgian
TheBelgian

@NorthernFrostCannabis, I hope that you interpret my response on this article that as: I too believe that flushing has no proven beneficial effect (yet) besides reducing excess of nutrients in case of overfeeding or to save on resources/money for the final week as the plant has to "find/use up it's own resources". And untill proven otherwise, I will only be flushing for those two reasons only.

NorthernFrostCannabis
NorthernFrostCannabis

@TheBelgian, Flushing...
I will list some points that not only challenge the absurd impracticality and illogicality of this myth, but point out how the pseudoscience behind it is fundamentally flawed (as is all pseudoscience) and can be countered by what is known about basic plant biology.
1. Robbing plants of essential nutrients at any stage of their life cycle is NOT beneficial for growth. I challenge anyone to provide a single peer reviewed paper from a reputable journal that provides evidence suggesting otherwise.
2. If this was practical, wouldn’t you expect all big agricultural hydroponic growers adopt the same practice?
3. Plants take minerals into their tissues, from their roots via the treachery elements; i.e. xylem. Once these minerals are in the plant, they are there to stay, the plant does not expel them, unless it’s through senescence-driven abscission of leaf petioles. From the treachery elements nutrients are translocated into the phloem - the plant’s ‘blood supply’ - after being integrated into various biomolecules, or are used for various metabolic functions. Where is the logic in thinking the plant ‘uses’ these up in that last week of flushing, in order to avoid smoking them? All the N P K Fe Mg Ca etc. is still there.
4. For arguments sake say we counter the last point by suggesting these minerals in their ‘raw form’ will taste ‘hasher’ or ‘nastier’ in the form of pyrolytic breakdown products (formed when weed is burned) than artifacts of larger biomolecules of which these minerals/macro nutrients are now a part of, for example phosphorylated PO43-. Even if this was the case it still doesn’t correlate with the myth, as the transports steam in the treachery elements is measured in minutes not a week. i.e. a PO43- molecule does not wait around in these vessels for a week before subsequent translocation and modification.
5. If there was any truth to this myth, then plants grown in soil would always taste worse than plants grown in hydro. Why? Because obviously soil is not an inert medium you can flush for a week. And a plant CANNOT distinguish between a PO43- molecule that comes from soil from that of a PO43- molecule that comes from hydro solution (which also debunks another myth, but we’ll leave that one).
6. Are there studies that have conducted double blind trials to investigate if flushed weed tastes any ‘sweeter’ than unflushed weed. Again, need peer reviewed papers. And doesn’t have to be weed, can be strawberries or any other type of fruit.
7. What is the proposed mechanism to support this myth, and how is it consistent with fundamental plant biology.
8. How does starving the plant of food in the last week increase thc production in the trichome? Papers?
9. Given, under certain conditions stressed plants upregulate certain defence compounds, but they will almost certainly produce less inflorescence weight per watt of light. Growth is always retarded under stress - not promoted. Nutrient starvation is a form of stress. Looking for peer reviewed papers that suggest otherwise.
Those of you set in your ways, each to their own and best of luck to you. Those who are willing to change their views in light of new evidence, or lack thereof, be ready for increased yields by feeding those hungry ladies right up until the second you chop.

NorthernFrostCannabis
NorthernFrostCannabis

I was flushing for 25+ years... I stopped flushing about 3 year ago. My flowers are bigger, tastier In turn my yeilds are about 30% more so the little nutrients used the last few weeks is no loss, I lose money by flushing. Flushing is the biggest and worst myth IMO ... This articular is just Bro Science.