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How to Flush Weed Plants Effectively

How and when to flush cannabis

You want to know how to flush weed plants? We have you covered.

So, here you are. Finally, after months of effort – pouring your love and energy into growing some truly impressive, juicy as the name implies, premium cultivars – the time to chop is upon you. You’ve taken meticulous notes, followed all of the grow dairies and their contained wisdom, and now comes the time to harvest.

Hold up though, there’s one step that many novice growers overlook – flushing.


  • What is flushing exactly?
  • How does it help the final product?
  • And why do you sometimes need to flush your crop weeks or months before that harvest date is at hand?

Let’s jump on in and find out.

What Is Flushing?

Before we go any further, we should mention that there are actually three separate situations that call for a flush.

The Final Flush

First up, and the most commonly talked about, is the final flush. In the last week or two before you cut down your crop, it’s generally accepted that it’s a good idea to flush the plants to remove any lingering nutrients and minerals that could add harshness to the final product.

How to flush cannabis plants

The timing and intensity of the flush are dependent on the cultivation style, but we will get into this in more detail a little later.

Flushing To Deal With Nutrient Issues

If you’re new to the game, then welcome. You’re in for a great time, but just as any experienced motorbike rider can attest to the fact that “if you ride bikes for long enough, you’re going to hit the deck at some point” – experienced cultivators will agree with us saying that if you grow ganga, you are going to run into nutrient issues, and probably sooner rather than later.

The remedy for such issues is to, you guessed it, flush. The idea here is that by washing away the nutrient buildup, we can give the plants the best chance at coming through to the other side of the nutrient issue and offer a substrate that is something closer to optimal growing conditions.

The Cyclical Flush Method

While this method of flushing has been around for decades, we have seen a resurgence in popularity in the past couple of years. The cyclical flush is used to help manage nutrient and mineral levels throughout the entire growth cycle. By allowing the medium to ‘clear out’ all of the already used-up nutrients, you can boost the chance of maintaining a healthy environment in the root zone.

Flushing weed plants in the vegetative stage
Flushing cannabis plants in the vegetative stage can help solve nutrient issues.

When it comes to certain top-shelf strain options the cyclical flush can be especially important as these plants often have higher demands in terms of nutrition specificity. What this means is that they will require more frequent flushing than a lower-maintenance strain, in order to keep their levels on point. It can also just help you dial things in to increase the chances of building that yield size every single crop run.

This method is used only with hydroponic mediums (coco-coir included). If you are growing in a super soil or cannabis-specific organic soil blend, you will be watering with fresh water either way, with a top-up of organic fertilizer from time to time.

How Flushing Your Cannabis Crop Helps The Final Product…

Ok, so let’s take a deeper look at flushing pre harvest.

The idea here is that by replacing whatever nutrient solution you have been feeding to the plants with fresh, plain water (that has obviously been pH balanced) you can help remove any accumulation of nutrients that may have built up in the root zone. This sudden drop-off causes the flowering plant to draw up whatever remaining nutrients haven’t been washed away, and also start chewing through the remaining internal nutrients.

There are two main benefits to this process.

  • First up – if done properly, this reduces the harshness of the final product. By removing all of those last-minute nutrient accumulations, you’re decreasing the risk of a harsher smoke or vapor, which also allows the terpenes to shine on through.
  • Then there’s the boost to the yield – Look, this one still is still up for debate and needs more peer-reviewed research, but many cultivators agree that forcing the plant to use up all of the last reserve nutes causes a form of shock that compels the plant to push out all of the remaining dankness. This not only adds to the density and weight of the buds but also increases trichome coverage.

How To Perform A Final Flush

Right, let’s get into the actual process of flushing weed. The method differs depending on if you are growing in soil, coco-coir, or hydro – but it’s all pretty simple and easy.

Flushing Before Harvest For Soil or Coco-Coir Grown Cannabis Crops

We can lump soil and coco-coir together as the process is almost identical, and also because they are so often used in tandem.

To ensure you get a nice, thorough flush you are going to want to use enough of that clear, clean water to see a 25% runoff out of the bottom of the pots. You should already know how many liters or gallons are needed due to your experience watering the crop, but an easy way to calculate an estimate is to take the size of the pot and divide it by 2 for soil, or 3 for coco.

So as we said, super simple, but let’s take a quick look at a few examples either way.


For the best results, you are going to want to hit it with the correct amount of water once, then wait for a good 30 to 60 minutes before repeating the process.

For soil-based mediums, you should have the water sitting in the 6 to 7 range, although this may not even be necessary with full organic grows. If you have a pH meter and some pH up or down then go right ahead, but don’t stress if you are growing in soil and don’t have these items.

You are definitely going to want to pH regulate the flush water if you are growing coco-coir. as long as you have it sitting somewhere between 5.5 and 6.5 then you should be all gravy. If you are growing in coco-coir then you should already have a pH pen, but if not click right here to see our favorite option.

In terms of timing

  • For soil – Its recommended to flush for between 7 to 10 days
  • For coco-coir – A 5 to 7-day flush should be totally fine

Flushing Before Harvest For Hydroponic Cannabis Crops

Flushing a hydro-grown crop is pretty similar to coco or soil, but instead of using a large amount of water with a 25% runoff rate, you are going to want to replace the nutrient solution in the reservoir with fresh, clean pH-adjusted water  – but not all at once. Most growers agree that with hydro crops, it’s better to slowly reduce the nutrient levels in the reservoir over a period of 3 to 5 days. At this point, a final clean water flush can be run for 3 to 5 days.

You will want to use the same pH levels as coco-coir (5.5 to 6.5), and if you already have a pH pen then all you need to do is slowly add in the new water over the course of a few days while using your EC or TDS tester to check that nutrient levels are dropping in a slow consistent rate.

Or you can just go whole hog from the start, cut out the nutes completely, and go straight to the clear pH-regulated stuff. Unless you have a super picky strain the crop should be totally fine.

How To Flush A Plant Showing Signs Of Nutrient Issues

Now that we have a solid flushing foundation, the rest is a piece of cake. 

If you think your plants are showing signs of nutrient issues, then you can use the same flush procedure as normal but cut down a few days. For example, if your crop is growing in soil then go for a 5 to 7-day flush instead of 7 to 10. For coco-coir, a 3 day flush should be fine, and the same goes for hydro setups.

These numbers are pure estimates though. If you feel the crop looks like it needs a slighter longer or shorter flush then you do you.

How To Implement Cyclical Flushing

Finally, let’s talk about cyclical flushing. If you are running an organic grow that uses compost teas or liquid fertilizers, then it’s recommended to perform a flush every week. Just use the same calculations as above for the pre-harvest flush and run that amount of water through the substrate twice, with a 30 to 60-minute break between the two waterings.

If you are running coco-coir or hydro, then you can flush once every third day if you like, but once a week should be totally fine also.

Using Reverse Osmosis Water

The use of reverse osmosis water is something that some experienced cultivators swear by, whereas others don’t think it’s that important. The truth is, flushing with RO water will remove many of the added salts and minerals in the water that can build up over time, making it easier for your plants to absorb what they need during the flush.

What is reverse osmosis water?

Reverse osmosis water is water that has been filtered through a special process to remove any impurities, such as minerals and salts. This type of water can be more beneficial to cannabis plants during the flushing stage, due to its ability to remove excess nutrients from the soil or medium thanks to the fact that it contains close to 0% of any minerals or nutrients.

If you have access to clean RO water then by all means go ahead with it for your flushing routine – but, if not, then don’t worry too much, as plain tap water should do the job just fine.

Be Careful Not To Over Water Your Crop

One of the most common mistakes that growers just starting out make is over-watering. And it’s no huge surprise – It really does seem like common sense to give more water than less, but this is actually the opposite way to think about it.

To ensure the best possible rate of growth, you want to get in the habit of creating a wet/dry cycle. This goes for soil or coco-coir based cultivation methods, but not so much for hydroponics.

This wet/dry split offers a couple of marked advantages

  • The plant’s root structure doesn’t stay wet for too long, reducing the chances of root rot as well as mold and mildew growth on the substrate. A constantly wet or moist substrate also promotes the chances of certain pest infestations, such as fungus gnats.
  • It encourages the roots to actively search for water by increasing their surface area and promoting deeper rooting. When the root system has a constant stream of water on offer, there’s no incentive to send out branching roots in search of a better environment. By cycling a wet and dry routine, those vital roots will continually seek new sources of water which will massively increase the size of the root ball

The Bottom Line

Flushing your plants before harvest is an important step in ensuring that they have ample time to rid themselves of any excess nutrients or salts that may have built up over the growing period. Its less important for pure organic crops, and even though there is a chunk of the cultivation community who don’t bother with flushing what-so-ever, we still think the best way to get the most out of your bud is by clearing them out at least one week before harvest.

So, there you go, our full guide on everything there is to know about flushing. If you have any lingering questions, hit us up in the comment section below!

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Sam North

Sam North is a content writer with a passion for everything cannabis. After working multiple seasons on weed farms absorbing the ins and outs of cannabis cultivation and culture, he decided to transition into a role that would allow him to work from anywhere, anytime. Sam now writes for multiple weed publications. He has extensive experience with a wide range of canna-agriculture styles, from smaller artisanal farms to large-scale commercial operations, and is here to share his knowledge to give you all the best chance of cultivation success. About this Author

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