To pre-harvest flush, or not to pre-harvest flush?

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Submitted by on Mon, 04/22/2019 - 08:00
To pre-harvest flush, or not to pre-harvest flush?

Flushing your plants before harvest has been a common practice for many growers for a very long time. But what does the science say? Does it make any difference, and if so, what is the best flushing strategy? 

1. What is flushing?

Flushing is when you stop giving the plants nutrients and instead feed them only water. There are 3 different occasions where you might want to do that:

  • Transitions between feeding and growth cycles
  • Nutrient Lockout
  • Pre-harvest

There is a lot of controversial discussions on flushing and the benefits. Not too many would disagree that a flush works well if nutrients build up or if you are switching between growth cycles. But many have differing views on pre-harvesting flushing.

2. What are the benefits of a pre-harvest flush?

Pre-harvest flushing is said to force the plant to use any remaining nutrients it has stored before the final harvest. This is done so no nutrients would be left in the plants on the day you harvest. It is believed that the nutrients left behind could affect the quality of your smoke. In other words, the less nutrients you leave behind, the nicer the smoke. 

So how much of this is actually true?

There are two articles that give a lot of inisight on the subject.

3. So should I avoid pre-harvest flushing?


There's a lot of people that swear by a pre-harvest flush, but there is also a lot of people suggesting it has no real benefit at all. But I think Smokin Moose's summary gives you everything you need to know:

Preharvest flushing puts the plant(s) under serious stress. The plant has to deal with nutrient deficiencies in a very important part of its cycle. Strong changes in the amount of dissolved substances in the root-zone stress the roots, possibly to the point of direct physical damage to them. Many immobile elements are no more available for further metabolic processes. We are loosing the fan leaves and damage will show likely on new growth as well.

The grower should react in an educated way to the plant needs. Excessive, deficient or unbalanced levels should be avoided regardless the nutrient source. Nutrient levels should be gradually adjusted to the lesser needs in later flowering. Stress factors should be limited as far as possible. If that is accomplished throughout the entire life cycle, there shouldn’t be any excessive nutrient compounds in the plants tissue. It doesn’t sound likely to the author that you can correct growing errors (significant lower mobile nutrient compound levels) with preharvest flushing.

Drying and curing (when done right) on the other hand have proved (In many studies) to have a major impact on taste and flavour, by breaking down chlorophylls and converting starches into sugars. Most attributes blamed on unflushed buds may be the result of unbalanced nutrition and/or overfert and unproper drying/curing.

I read this article before I did my first grow, so I have never pre-flushed any of my harvests. I can say they taste great. I would need to do a few flushes before a harvest before I can actually compare the two, but based on the 420magazine article, I don't think I ever will. The article has convinced me to avoid pre-harvest flushing completely and my harvests taste perfectly fine! 

What do you think? Are you going to keep doing pre-harvest flushing? Leave a comment!