Flushing is the art of washing your growing medium with plain water or an enzyme solution.
The goal behind this is to push the plants over the finishing line and produce a superior, smooth-tasting bud full of flavor and aroma. I will explain the science behind flushing, the different ways to flush, and how to flush various growing mediums. Then, I’ll round it all off with my appraisal of the never-ending debate on the topic of whether it is even required.
TL/DR
- Flushing removes excess nutrients in the form of minerals and salts from the growing medium.
- The pH level of the water used to flush in hydro should be between 5.5 and 6.0.
- The water temperature should be 65 – 77 degrees Fahrenheit. (18 – 25 degrees Celsius)
- Reverse osmosis water is commonly used but can lower the pH level.
- The flushing period usually lasts over the course of two weeks.
- Enzymes are an excellent way to break down undissolved salts.
- EM1 (effective microbes) will also accelerate the flushing process in soil-based mediums.
What Does Flushing Your Cannabis Plants Mean?
Flushing cannabis simply means replacing your nutrients with plain water at the right pH level. In theory, when water passes through your growing medium, it will wash away any remaining undissolved salts that may have accumulated throughout the grow.
What is the Goal of Flushing Your Weed?
Flushing aims to reduce the amount of nutrients available in the root zone so that the cannabis plant metabolizes what is already in the plant tissue, thus eventually reducing the amount of nutrients in the smokeable buds.
After harvesting, you’re left with dried flowers, which, when smoked, will have a smooth and delicious taste, easily burning down to the filter tip and showcasing a cloudy white ash. When did this practice of flushing plants start? Honestly, I could not tell you who the first grower was to wake up one day and think, I am going to cut off all nutrients during the pinnacle stage!
The Different Ways to Flush Weed Plants
Flushing is usually done over the course of two weeks by either cutting off the nutrients or replacing them with pH-adjusted water. Some growers prefer a different approach that involves flooding the pots with a large volume of water.
Enzymes are an excellent way to go one step further in any growing medium, accelerating the breakdown of undissolved excess salts and nutrient availability. EM1 (effective microbes) are a lifesaver and will also increase the decomposition rate. When combined, you will see how effective cannabis flushing with these two truly is!
What is the science behind flushing cannabis plants?
Flushing reduces nutrient concentration levels around the root zone. This allows the plants to metabolize and use the remaining nutrients in the plant tissue. When you flush plants, the water used should be at room temperature and slightly acidic. Ranging between pH levels of 6.0 – 6.-3
Nitrogen is the most water-soluble nutrient, so it’s typically the first nutrient to be fully removed. Resulting in yellowing of the fan leaves. Research has shown that nitrogen’s effects as a fertilizer can reduce the production of total flavonoids and phenolics due to the chlorophyll content.
The EC / TDS levels of the runoff water should be low, close to 50 ppm. This indicates that the substrate is clean, the plants are close to harvest, and the roots are fully flushed.
For those who would like to learn more about the science of flushing, I encourage you to take the time to watch Professor Debacco’s video on flushing.
Matthew DeBacco is a Professor of Agronomy (MS) as well as Plant Pathology and Horticulture (BS), who teaches several courses on the Horticulture of Cannabis at the Department of Plant Science and Landscape Architecture at the University of Connecticut.
Be sure to like and subscribe to his channel: https://www.youtube.com/@DeBaccoUniversity
How and When to Flush Your Cannabis Plants
Not everyone will have the same approach when it comes to flushing cannabis plants. During my time as a grower, I have experimented with numerous methods to remove excess nutrients during the final two weeks of the flowering cycle.
I’ve tried everything from bringing my plants into the bathtub and flooding them with the shower head once a week to cutting off the nutrients and using only water. I’ve also tried using an enzyme solution, enzymes and EM1 probiotic microbe tonic, bottled water, reverse osmosis water, and standard tap water. Below is my take on flushing your cannabis plants for hydro and soil-based mixes so you can learn from my experiments.
Using Enzymes to Flush Your Cannabis Plants
I love to use 5 ml per liter of enzymes with every nutrient solution for the duration of the entire grow. During the flushing stage, the enzymes are increased to 20 ml per liter and watered every 48 hours. I find this produces the cleanest cannabis buds around!
Diluting Your Nutrients as a Strategy
An advanced option to consider when flushing is using ¼-strength nutrients during the final two weeks. I also know well-experienced cannabis growers who will feed a mild solution of 1-2 ml per liter of cal mag nutrients during the flush. These are just some food for thought – there is no set way of flushing your cannabis plants. However, as a beginner, I strongly recommend cutting off all nutrients.
Flush Your Cannabis Plants Growing in Hydro
With hydroponics/aquaponics, you only need to flush for a week. So that’s an extra week for those commercial-scale growers to crank up the nutrients! The reason is that the growing substrate is inert (contains no nutrients). So the plants will use up the remaining nutrients very quickly.
- The pH level of the water used during the flush should be between 5.5 and 6.0.
- The water temperature should be 65 – 77 degrees Fahrenheit. (18 – 25 degrees Celsius)
- Reverse osmosis water is commonly used but can lower the pH level.
Weed Growing in Soil-Based Mediums
Soil-based growing mediums will be full of beneficial microorganisms, and recycling the growing medium is always a good way to save costs.
Flush cannabis plants grown in soil-based mediums two weeks before the harvest date. You will need plain water at room temperature or make an enzyme solution. You can test the run-off using a digital E.C. pen until your E.C. levels are around 50 ppm.
- The pH level of the water used during the flush should be between 6.0 and 6.8.
- Cutting off nutrients and using enzymes for 14 days will give plants a final push.
- Follow the same watering schedule and do not increase the volume of water.
Flushing Outdoors Cannabis
Keep in mind that big containers may require a large amount of water to thoroughly wash the roots and growing medium. However, be careful not to overwater. Especially if you plan to simply give your plants two to three times the amount of water you would typically provide.
I have seen many outdoor growers with giant plants cause their crops to lose vigor and essence due to excessively over-watering during the final two weeks. Working with a perlite-rich growing medium is one simple and effective way of preventing an oversaturated growing medium after heavy amounts of watering.
Do You Need to Flush Cannabis Plants?
As Shakespeare once said, “ To flush…or not to flush?…that is the question. I often wonder how the cannabis industry has been impacted by specific parameters and terms such as 18/6, 12/12, and flushing for two weeks. I know many growers who do not abide by these rules or flush their crops at all.
That being said, millions of cannabis growers would testify that if you flush marijuana plants before harvest, it will significantly improve the overall taste, smell, and smoking experience.
Recent Studies Show Flushing Might Not Be Beneficial!
Just when we all thought the universal flushing debate was put to bed once and for all, RX Green Technologies decided to provide new evidence based on trials using flushed and non-flushed cannabis buds.
Overall, the results are interesting, to say the least. They prove that it makes no difference to a judge’s panel based on smoking, ash quality, and other factors. I strongly recommend that you read the RX Green Technologies study to learn more.
Reasons for not flushing cannabis plants
- Soil growers who recycle their crops may take the time to add organic amendments.
- Commercial-scale growers will only increase yields and produce the biggest buds.
- Many believe through experience and trial and error that it is not beneficial.
- You may be feeding only water using soil-based mixes from seed to harvest.
- The final two weeks may be when plants need the nutrients the most.
Final Thoughts
I honestly think flushing your crop is a must regardless of whether you grow in soil, rockwool, or coco coir. The most practical way to flush your plants without overwatering them is to feed plain pH-balanced water every 48 days. And use the same volume as before.
So, if you have always given your plants 3 liters of nutrient solution for each feed. Simply flush with 3 liters of water. Connoisseurs will celebrate well-flushed cannabis buds and possibly enter them into cups. Good luck learning the ways to flush and get your crop dialed in!
