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Flushing 101: For Best Results, Just add Water

Posted on March 18 2019

By Aimee Accinno and Eric Robichaud.

If you are new to the world of cannabis cultivation, the term “flushing” may be foreign to you. However, if your goal is to harvest top shelf buds rather than mediocre herbs, you’ll want to read on to familiarize yourself with this important pre-harvest step.

If you do your own research on this topic, you’ll find overwhelming support of flushing based on the firsthand experience of countless seasoned growers. However, as with most things in life, there are some who hold a contrasting view of this practice and believe it’s unnecessary - we disagree.

Whether you decide to flush your plants or not is up to you. However, after reading the information below, you’ll be armed with enough basic information needed to make an educated decision.

What Is Flushing?

Pretty much as the name sounds, Flushing is the act of running a larger volume of water through your soil to flush nutrients, salts and minerals out of the soil, and feeding your cannabis plants pure, nutrient-free water several days before harvest. The goal is to reduce the concentration of chemicals and dissolved minerals that accumulate in the plant and its root zone and is an essential step in producing a higher quality, better tasting product.

Why Is Flushing Important?

Cannabis plants are fed nutrients such as nitrogen, phosphorus and potassium as well as a variety of salts and minerals, in order to boost their growth potential. These compounds will build up in the plant over time, resulting in buds that produce a harsh smoke and leave a bitter taste in your mouth. Many growers find that unflushed cannabis also tends to burn unevenly and produces a black ash, whereas properly flushed cannabis has a smoother finish and more desirable flavor when smoked.

Ever get that really harsh throat hit? Most people just write it off as “this herb is dried out” or slap it with some vague “low quality” / “mids” label, but more often than not this is a result of not flushing.  Especially when you know beforehand that it’s not really dried out! That’s because you’re getting latent salts and minerals and junk in your smoke, not just pure cannabinoids and terpenes. When properly flushed, the hit is smooth -- just like buttah!

Timing Is Critical

Flushing is actually a simple process, but timing can be difficult to predict for the new grower. For those growing in “The Armoire,” our simplified protocol includes a week of flushing as the final step before harvesting, so we walk you through it.  However, if you’re working off a tent or some other apparatus, flowering times may vary based on several different factors, so you’ll need to have a general idea of when your plants will achieve peak maturity so that the flushing process can be timed accordingly.

As the peak harvest date draws near, you’ll want to carefully inspect your plants’ trichomes using a magnifying glass, pocket microscope or jewelers loupe. If the trichomes are still clear and upright, they aren’t yet ready for flushing. Continue checking them daily until they start to appear cloudy or amber in color. Once this occurs, you have the green light to start flushing in preparation for harvest. Many folks look for 80% or more brown pistils as another metric instead of looking at trichome color.

The Art of Flushing

Once you’ve determined the right time to start, the next step is to begin the actual flushing process. We strongly suggest using Purified (reverse osmosis or “RO” water) or Distilled water, NOT tap water - because the point is to remove nutrients from the soil, not introduce salts, minerals and fluorides from your local water system.  Although it’s not absolutely necessary, we do prefer to purchase an organic flushing agent such as “Flawless Finish” from Advanced Nutrients. This flushing agent will bond with nutrients in the soil creating microscopic “clumps” (think about clumping cat litter) of molecules that are too large for the roots to absorb - so even if you haven’t flushed the nutrients out of the soil entirely, they’re still clumped too large for the roots to absorb anyway.

Some expert growers test the water’s pH before flushing to ensure levels are not too high or low as this can hinder the process. Ideally, the water you use should have a pH level around 6.0-6.8 for soil grown plants or 5.5-6.5 for coco coir or hydroponically grown plants.

If you are growing in soil or coco, start the flushing process by fully saturating your plants, giving them as much water as the medium can store. Wait several minutes to allow the excess nutrients to be picked up and then flood the soil again until the water begins to pour out of the drain holes in your pots. The water runoff will initially appear dark and murky.  For hydroponically grown plants, you will follow a similar process of adding RO water. Be sure to drain the reservoir daily and replace it with clean water. Otherwise, you will continue to recycle nutrient runoff through your plants, which defeats the purpose.

For users of The Armoire we typically suggest running 2 gallons of Purified water through the soil, emptying the saucer before returning the plant to the grow chamber.  We do this 3 times during the final week before harvest.

Flushing soil-grown plants usually takes place about one week before harvest. Hydro-grown plants will typically only need to be flushed for a handful of days (or less) prior to harvest.

While it’s not a big deal for the single-plant casual home grower, those with larger gardens and more invested time may deem it worthwhile to invest in a TDS (total dissolved solids) meter to test the drained water.  You will want to continue flushing until the meter reading matches the level of the water you are using to flush. The goal is to wash away as much of the stored nutrients as possible getting as close to 0 ppm as you can.

Finally, keep in mind that once you start flushing, the cannabis leaves may change in color and appear to be dying off. This happens because the plant is pulling the nutrients out of the leaves to finish ripening, and produce the best possible product - that’s completely normal “end of life” behavior. 

Stated another way: during veg growth if your leaves are yellowing it can indicate there’s a nutrient deficiency that needs to be corrected.  But in the last few days before harvest when you’re flushing out the nutrients on purpose, of course you’re seeing nutrient deficiency -- it’s by design!

The Great Debate - Common Myths Dispelled

There are some who are strongly opposed to flushing and believe it is a myth. These opponents claim that once nutrients are absorbed into the plant tissues, they cannot be expelled by denying additional nutrients. However, growers who have encountered “nutrient burn” know that the way to alleviate toxicity buildup is to flush plants. This helps them use up the excess nutrients that caused the burn effect in the first place.

The second myth is that robbing plants of nutrients at any point in the grow cycle is counterproductive and stresses the plants, which impedes their growth. First, it is important to note that at this stage in the growth cycle, the goal is not to encourage substantial growth - we’re in the last few days before harvest at this point.  And furthermore, flushing allows the plant to use up excess nutrients, salts, minerals and other compounds that accumulated during growth and have the potential to negatively impact the taste, smell and overall smokability of your plants. Finally, any stress that the plants endure during this process results in the formation of defense compounds, which are actually desirable components that increase the value of the final product.


To Flush or Not to Flush?

At the end of the day, what you do with your plants is a personal decision. You may want to run your own trials and compare the two methods to determine if you notice a significant difference.

However, if you want follow the time tested methods of seasoned cannabis cultivators (High Times Magazine has written extensively on the topic and recommends flushing too) and avoid risking a bad harvest, then you will want to properly flush your plants using the guidelines outlined above. You’ll be amazed at the difference this small effort makes and you will know your efforts were worth it when you take that first, extra smooth inhale.

If you are interested in growing your own cannabis at home in a discreet and easy manner, check out "The Armoire” from Green Goddess Supply. This revolutionary new home grow chamber is a complete kit that offers an easy, fast and fun way to grow your own high quality organic flowers in just 60-90 days!  If you can water a house plant, you can grow top shelf herbs.

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