How to Flush Marijuana Before Harvest

How to Flush Your Medical Marijuana Plants Before Harvest

You’re approaching that 8-12th week finish line (depending on strains), a sight so anticipated it renders you impatient to leap forward toward completion. But patience in gardening is a quality well rewarded in all stages of the process. The key lies in the ability to read your plants and their cues, and the harmonious interplay of seasoned experience and perfect timing.  Preparing for that final dash can begin as early as week 6 for some growers, or as late as 3 days right before chop for others.  In this week’s post we focus on how to flush marijuana before harvest and will explore the what, why, when, and how’s of the process.

What is Flushing?

Flushing is the act of completely depleting nutritional resources for your medical marijuana plant. This is achieved by feeding nothing but filtered water for a desired period of time in an attempt to starve the plant so it uses up stored nutrients– intentionally lessening the chances that these nutrients are consumed during medication.

Why Should I Flush My Marijuana Plants?

We have searched, re-searched, and overtime have become intimate with the nutrients cannabis plants NEED to live. So why is it important to flush these nutrients out when nearing harvest?

Briefly, recall that there are 2 classes of nutrients, mobile and immobile.

Mobile: N, P, K, Mg and S.
Immobile: Ca, Fe, Zn, Mo, B, Cu, and Mn.

While you can easily cut off the supply and quickly deprive the plant of immobile nutrients, it is the more stubborn mobile nutrients that are heavily targeted during flush.

This is why that dipped-in-yellow-Nitrogen-starved-look is a great indicator that a proper flush game is on point.

An example of flush properly executed. Photo and garden courtesy of @Diskoe1
An example of flush properly executed. Photo and garden courtesy of @Diskoe1

Mobile elements can be relocated anywhere within the plant. For example, think about how you start to see the yellowing leaves of older growth when the plant is trying to support newer growth. That’s a perfect example of mobile nutrients relocating.

And while it is almost impossible to completely eliminate ALL nutrient values from the plant because of the constant recycling and relocating that occurs, flushing is a step in the right direction to at least lowering the amount of trace nutrients present in harvested medicine.

When is the Right Time to Flush My Garden?

Depending on your set up and personal preferences you can flush anywhere from 1-2 weeks before harvest to as little as 3 days right before. The general consensus appears to be 1 week for hydroponic systems, 2-4 weeks for those in soil, 3 days if you’re using a flushing agent such as General Hydroponics FloraKleen or Botanicare’s Clearex, and not at all if growing organically.

Personally I would still flush, to a lesser degree, if solely running organics; because so long as there continues being food in the medium for the MicrobeArmy to attack they will continue providing nutrients for the plant to munch on. 

An interesting argument raised against flushing, ever, can be found here:

If you want to base your flush off physical cues from your ladies, here are some indicators to look out for in preparation to flush:

Read your resin glands! When you start seeing that 25% or more of your glands turn from clear to cloudy/amber it’s time to get ready for flush. Side note- some strains display amber resins throughout bloom, others never do. When in doubt, look for cloudy/milky resins.

In this picture many are still clear, but some are beginning to turn cloudy
In this picture many are still clear, but some are beginning to turn cloudy


Mostly amber trichomes developing
Mostly amber trichomes  developing


Remember those calyx hairs (pistils) initially white and vibrant in color?  When you begin to see an obvious change in a good 25% or more of these hairs, such as drying up or falling off, then your ladies are likely reaching the end of their peak bloom phase.

Young calyx hairs (pistils) in early flower. Notice white coloring
Young calyx hairs (pistils) in early flower. Notice white coloring
Mature calyx hairs (pistils)  that are beginning to darken
Mature calyx hairs (pistils) that are beginning to darken

How to Flush Marijuana Before Harvest

You can use clearing agents such as FloraKleen or Clearex, or simply replace ONLY RO for feedings until significant run off is achieved. You’re looking to get at least 80-90% run off, so if you’re flowering in a 5gal pot you’d need about 3-6 gallons to flush till drain.

Growers who practice elaborate flushing before harvest have reported easier combustible flowers and white powdery ash when smoked. These two are clear indicators that most of the nutrients have been flushed out successfully.

Remember, patience, patience, patience because while those intoxicating sweet smells attempt to tempt you toward a premature harvest, remember that jumping the gun isn’t ideal for top quality medicine. Timing is vital to success, and that finish line isn’t moving so make sure the final dash to harvest is executed perfectly. As always, I hope this has helped and happy growing!

4 thoughts on “How to Flush Marijuana Before Harvest

  1. I’m very interested in this minimalist flush tactic! I’m using Aptus with Canna as my base, weekly flush and plants naturally yellowing to about 30-40% by the time I chop. Thoughts?

    1. Hi,
      Weekly flushes sounds like a great idea, you’re reducing chances of build up and pretty much press the reset button on your ppms. The idea behind the typical “2 week” just water flush is to ensure that most nutrients in reserve are used by the soon to be harvested girl and not combusted and ingested by patients when medicating. The fact that you’re getting 30-40% yellowing by harvest time means you’re girls are experiencing nutrient starvation and at some point eating reserved nutrients, yay. Do you still feed nutrients on the week before harvest and still see yellowing? Thanks for stopping by the blog!

    1. Hi Nancy,
      Not necessarily but a magnifying glass does help. Without one I would look for the darkening of pistils from powdery white to amber. With a magnifying glass you can also follow the trichome heads turn from clear to cloudy. Thanks for stopping by the Bounty Blog!

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