Flushing Cannabis: When, Why, How


We have mentioned cannabis flushing in several situations: flush when you face nutrient burn/lockout; flush before harvest. In this article, we will discuss cannabis flushing systematically. Flushing is easy and every grower should know when, why, and how to flush cannabis.

What is flushing?

Flushing is the process of removing the leftover nutrients and minerals from the growing medium by using pH-balanced water.

flushing cannabis

When and why should I flush cannabis?

Flushing is usually done before harvest to improve the flavor and quality of cannabis. When plants can’t absorb nutrients from the soil they will use up the nutrients stored in their body. Just like people, we eat a lot of food and some of those nutrients turn into fat and the stored fat will be converted into energy in an extreme situation, like when we are going hungry. Plants do the same, in their own way.

It is best to flush plants 1-2 weeks before harvest (but this depends on the growing medium). If the flowering period is 8 weeks, we can flush our plants at 6 weeks after the flowering stage has begun. Checking the trichomes can help you decide if your plants are ready for harvest—once trichomes start to turn from transparent into milky white you should prepare to flush your plants since milky white trichomes will turn an amber color after about two weeks, which will indicate higher CBD levels.

Flushing before harvest is crucial. If stored nutrients are not used up they will change the flavor of cannabis—you don’t want to leave an unpleasant taste in your mouth after smoking so try to flush out those nutrients and encourage your plants to use what they have stored up.

Flushing can also be applied when the pH level of the growing medium is imbalanced when plants are facing nutrient burn or lockout. It is the buildup of nutrients and salts that contribute to the incorrect pH levels so as long as the excess nutrients are flushed away, plants can absorb nutrients from the soil again.

Growers also flush their plants when they are preparing to change the nutrients. Flushing the old nutrients allow growers to adjust the new nutrients based on the plants’ different stages of growth.

Flushing is important during the whole cycle of cannabis, however, flushing too often or too early will harm your plants, affecting your harvest. Flushing too early will stunt growth because the buds haven’t gotten enough development. THC will be at its peak when trichomes are milky white.

How do I flush cannabis?

If you have decided to flush your plants, you can follow our steps.

Step 1: Test the pH level of the water. Make sure the pH level of water is at the appropriate range for cannabis. For soils, the pH level should be between 6.0-6.8; for hydroponics, the pH level should be between 5.5-6.5.

Step 2: Use three times as much water as there is soil. If there is one gallon of soil in the pot then use three gallons of water to flush it. When you flush, allow the water to sit for a few minutes so it has time to pick up nutrients.

Step 3: Don’t provide any nutrients while flushing. Plants will continue to grow when they are flushed, so there is no need to add extra nutrients.

Step 4: Flush again until the soil is completely dry. We should avoid rot as best we can.

Step 5: Use a TDS reader to measure water runoff. If the runoff draining out of the pot is clear and the TDS levels are close to clean water, then the flushing is done! 

How far ahead of harvest should I flush cannabis?

1-2 weeks for soil grows

It takes the longest time to flush if you grow plants in the soil since there will always be some leftover nutrients in the soil. You can’t remove all the nutrients at once.

1 week for coco-coir grows

Coco-coir doesn’t store many nutrients. One flushing will wash away most of the nutrients.

Several days for hydroponic grow

Once you change the reservoir to pH-neutral water, plants can’t absorb any nutrients from their growing medium because there isn’t any. Thus hydroponic growers usually flush their plants several days before harvest. Flushing for too long or too early will lead to nutrient deficiency, reducing yields.

Keep observing your plants while flushing. It’s normal if the leaves are becoming a bit yellow while or after you flush.

Is flushing really necessary?

As a matter of fact, there is no scientific data to support that cannabis should be flushed before harvest.

Most experienced growers flush before harvest, but some argue that there is no difference between flushing and not. Conversely, if you didn’t flush at the correct time, it can cause a lot of problems, some so bad that they will ruin your harvest. So whether you flush or not is up to you—experience is the mother of wisdom. Try dividing your plants into two, flush one half before harvest and don’t flush the other, and then compare them with each other and see which way works best for you.

After flushing, you’ll be near the end of your growing journey. The next step you need to do is to dry and store your buds.

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