A hot topic among cannabis cultivators today is the theory of flushing – the act of cleaning out any residual nutrients from the soil prior to harvest to increase the buds' overall quality. The idea is if you stop giving a plant its nutrient feed pre-harvest, and run large quantities of pure water through the grow medium, excess nutrients will leach out of the soil and the plant will use up any nutrients that it has internally stored up. An overabundance of stored nutrients in the plant matter can lead to buds that have a bad taste, harsh smoke, and charred, black ashes after being smoked. Hearing this, why wouldn’t you flush your plants?
It is a common practice in the cannabis industry to flush plants pre-harvest, though there is no widely accepted standard practice to do it, and reliable scientific research is lacking. Flushing methods vary from grower to grower and are usually based on their own best judgment or personal preference. Some growers use chemical or organic agents to help flush, some do not.
Time periods for a flush can range from more than two weeks to one day before harvest. The amount of water used to flush the medium can be anywhere from 2x to 5x or more the size of the container the plant is growing in. Currently, these variables are determined by each individual grower or cultivation facility. This means there is no consistent flushing method used between the cannabis flower and other products you can purchase at a dispensary.
Common Flushing Practices
In a typical flush, you begin by watering with a flushing agent, or product designed to help keep nutrients and salts soluble. It's also good to adjust the pH to 5.8 - 6.0, as this is the range that the most relevant nutrients become soluble in water. The grower then proceeds by running enough treated water through the plant’s medium to get the water draining and running out of the container.
It's common practice to collect some of this run-off water, test it with an EC/ppm meter, and record the results. Let the medium dry back for one to two days, then flush again with plain water, testing and recording the run-off again. The EC/ppm numbers should begin to fall as you continuously repeat this process.
As the nutrients are leached from the soil, the plant uses up any stored nutrients it had been reserving. The plant’s leaves start to change color at this point, as mobile nutrients are diverted to keep the buds healthy and alive. It's a big misconception that you are flushing nutrients out of the plant, and it's important to understand that this is not happening, once nutrients are in the plant, they remain there. Flushing only removes the nutrients from the soil.
Ask the average cannabis grower, and they’ll more than likely tell you that cannabis should always be flushed for as long as possible. It's commonly believed that cannabis flowers that burn to white ash have been flushed properly, while cannabis that burns to a black charcoal consistency still has nutrients in it and produces a much harsher smoke.
Also, it's believed cannabis that crackles and pops while burning is the result of excess nutrients stored in the plant matter. While no studies have actually confirmed any of this, it does seem to be the case and is widely accepted. Soil flushing rids the flowers of unwanted taste, and produces a smoother smoking experience and better overall product.
Do I Need to Flush My Plants?
Some growers question if flushing is necessary or not. It should really just be thought of in terms of how much to flush, and it could be none. Depending on the medium the plant is in and how the nutrients are being used, only some soil flushing might be necessary. For instance, a coco medium that has been fed to the point of achieving 10% - 20% run-off with each water will have less built-up nutrients and won’t require a long flushing period.
Plants that haven’t been draining correctly and retaining nutrients throughout their life will need a longer time flushing. If your plants are having nutrient deficiency issues two weeks before harvest, flushing will make the problem worse and could possibly affect the quality and color of the finished flowers. A good grower can size up the situation and determine how much flushing will be necessary to get the best final product.
In 2020, RX Green Technologies held a “flush trial” event, a blind taste test of cannabis flower with different flushing periods. Samples either had a 14-day flush, 10-day, 7-day, or no flush at all. They held a private event and participants judged each sample based on qualities like aroma, flavor, how harsh the smoke, and potency. Turns out, no one could tell the difference between the samples. They reported that the flower with no flush at all was actually preferred by the event participants.
Keep in mind though, this study wasn’t peer-reviewed but privately funded and none of the methods or data used to come up with these results have been released to the public. Since no one could tell the difference between samples, it's highly probable that if the experiment were run again there would be different results as the preferred flower. Also, this study doesn’t take into account any other nutrients or growing methods, only the ones used to grow the samples.
All this begs the fact that we need proper research done on the subject. Until then, intuition seems to point to the need to keep flushing.
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Frequently Asked Questions
How Much Water Should I Flush my Cannabis Plant With?
A general rule is to flush with at least 2 times the amount of container size, each time you water over a period of up to 2 weeks. If you have a 2 gallon pot, flush with 4 gallons of water, when the soil dries flush with 4 gallons again. Repeat until harvest. Be sure and check the runoff water with an EC/ppm meter each watering, the EC/ppm should be dropping with each flush.
Do I Have to Flush my Plants?
It's up to the grower to determine if it is necessary. Depending on the type of nutrients, the medium the plants are growing in, and the amount the plants are being fed, it might be a good idea. It is highly recommended but not required.
How Long Do You Flush Autoflowers?
The same amount of time for non-autoflowers. No more than 2 weeks is recommended.
Do You pH Water When Flushing Cannabis?
At a pH range of 5.8 to 6.0 all nutrient salts become soluble, so this is the best range for a nutrient flush. Though, for mediums with a pH higher than this, it is recommended to use a flushing agent in conjunction with water pH consistent with your nutrient feed to avoid leaf curl and pH stress in the plant.
Is One Week Flush Enough?
Depends on the medium and the amount of nutrients fed over the plant's lifetime. A 7-day flush period would be adequate to clean up any unwanted nutrient taste for plants that have been fed a standard nutrient amount, in coco or peat moss. Plants that have been pushed to the max with nutrients may require a longer flush period.
Do Buds Fatten Up During Flush?
During a pre-harvest flush, the plant is in its last phase of flower growth and is really packing on the weight. Depending on how much nutrients the plant has stored, the buds can still grow vigorously during a flush.