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Pre-Harvest Flushing

ByBC

The term “flushing” refers to the process of using an excessive amount of water to reduce the concentration of fertilizers in the growing media. In this tutorial, we provide guidance on when and how to flush cannabis plants in coco. There are certain situations during the growth cycle where flushing may be necessary, such as when the electrical conductivity (EC) becomes unmanageable or if a lockout problem occurs. Additionally, many growers practice a final flush before harvest, although this is a topic of debate within the cannabis cultivation community.

It’s important to note that properly balanced nutrients should not require regular flushing with non-nutrient water. If a nutrient line recommends frequent flushes, it may raise concerns about the quality of their nutrient blend.

Flushing to Lower EC

When flushing to lower the EC, the run-off produced during regular fertigation events can serve as a small flush. However, there are instances where this small amount may not be sufficient to maintain the EC within the desired range. If the run-off EC consistently remains more than 400 points higher than the inflow EC, even with frequent fertigation and 20% run-off, it may be necessary to consider a flushing procedure. Never flush with straight water instead use a properly balanced nutrient solution. If you flush prior to the preharvest flush with straight water, you will strip out the calcium (Ca+2), magnesium (Mg+2) that the coco stores in its fibers. As a result, with subsequent feedings the calcium (Ca+2), magnesium (Mg+2) in the balanced nutrient solution will be pulled out and stored in the coco instead of being available for the plants. For more information on this topic see our article titled, “The Importance of using CalMag in Coco“.

Process:

  • Use a balanced nutrient solution that is at half your normal Inflow EC
  • Apply twice the normal quantity of solution.
  • Wait ten minutes following the application
  • Add additional dilute nutrient solution to produce new run-off for measure
  • Test the EC of this new run-off, it should be at or below your normal target inflow EC
  • If the EC is still high, repeat this process at the next opportunity to fertigate

Flushing to Release Lock-out

Flushing is an effective method for addressing nutrient lock-out, which can occur due to pH imbalances or an unbalanced Nutrient Element Ratio (NER). When lock-out happens, you may observe signs of nutrient deficiencies despite regular fertigation with appropriate nutrients.

Process to treat lock-out:

  • Use a balanced nutrient solution that is at half your normal Inflow EC
  • Apply twice the normal quantity of solution.
  • Wait ten minutes following the application
  • Add additional dilute nutrient solution to produce new run-off for measure
  • Test the EC of this new run-off, it should be at or below your normal target inflow EC
  • If the EC is still high, repeat this process

The Final Pre-Harvest Flush

The Final Flush is a subject of ongoing debate in the cannabis growing community. While there is limited scientific evidence to support its effectiveness, many growers continue to practice it and believe in its benefits. The concept behind the final flush is to withhold nutrients from the plant’s roots, encouraging it to rely on stored nutrients. Advocates argue that this can enhance the taste and overall quality of the final harvest. In coco, the use of a flushing agent can expedite the process, reducing the potential drawbacks associated with flushing.

When plants are within days of harvest, I begin their final flush. My Process:

  • I use General Hydroponics “FloraKleen” at 5ml/gallon in plain water
  • No pH adjustment. The purpose of pH adjustment is to make nutrients in the media accessible to the plant. During the final flush we no longer want those nutrients to be absorbed by the plant!
  • Hand apply the first few flush treatments at heavy quantities
  • Just twice per day is adequate (at lights on and just before lights off for photoperiod plants)
  • During first 2-3 days aim to reduce run-off EC to 200
  • Maintain run-off EC less than 200 for at least 48 hours before harvest

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