When & How to Flush Cannabis Plants? Do You Really Have to?

Because of its long history of illegality, there has been little scientific research conducted on the cannabis plant. This leads to a lot of "Lore" and "Bro Science" spreading throughout the growing community.

Luckily, with legalization in 2014, more and more studies have been conducted to try and understand this incredibly complex plant! There's one university in particular, Utah State, that is heavily focused on cannabis growing research. One thing they have uncovered is the truth behind flushing cannabis. Let's get into it!

What is Flushing?

Flushing, in the traditional sense, is the practice of rinsing the root zone with only water to remove any excess nutrients out of the root zone and grow medium.

But when to flush cannabis? Traditonally, it's conducted at the end of the plant's life cycle with the goals of improving flavor and aroma, reducing harshness of smoke, and enhancing terpene profile. All great benefits.

However, recent studies have found that if excess fertilizer isn't used to begin with, there is no added benefit, or even any value in flushing the root zone. In fact, traditional-style flushing can pose several risks.

Bud rot, already a challenge to manage even in near-perfect conditions, becomes more likely when the plant cells get flooded with water, keeping the interior of the buds too moist. Additionally, removing calcium for two weeks creates somewhat of a hidden calcium deficiency, which weakens the immune response and is likely an invitation for bud rot (botrytis) to set in.

As flushing cannabis also removes ALL nutrients from the plant, it can have a negative impact on the final yield. Overall, the intentions and benefits of flushing are pure, but the path of execution is a high-risk affair.

How to Properly Flush Cannabis Plants: The Precision Flush!

Removing Nitrogen is Key

The intention of traditional-style flushing is right—you want to remove excess nutrients from the medium, roots, or the plant itself. But if possible, the focus should be on one specific nutrient: Nitrogen (N)!

Nitrogen is a key mineral/nutrient found in all plants with many key functions, and chlorophyll production is one of them. Chlorophyll is a necessary compound needed to keep the plant green, and help carry out photosynthesis, so it's very important! However, too much chlorophyll can affect the flavor and smoking experience—leading to harsher, hotter burns and darker, firmer ash.

The strategy of removing only nitrogen during the final weeks of bloom ensures there are no hidden deficiencies that might negatively impact yield. This approach can increase the plant's trichome, terpene, and THC production, while allowing pigments like anthocyanin to shine, which brings out the colors in the plant!

Insights from Recent Studies

As shown in this graph, 2 studies were conducted. The 1st study, represented by the light blue line, tested the traditional flushing method of rinsing the root zone with only water for the final 2 weeks. The results show minimal to no increase in CBD or THC content.

The 2nd study, represented by the dark green line, focused on removing only N during the last 2 weeks. This approach resulted in a 15% increase in THC, and a roughly 10% increase in CBD!

A quote from the Utah State study: "After evaluating all of the data, it was conclusive that removing N during the last weeks of flower reliably increased terpene and total cannabinoid production without increasing the risk of deficiencies and bud rot. There is even some evidence of a slight increase in yield."

How to Perform the Precision Flush Technique

This prompts the question: "Well, how do I remove nitrogen from my nutrients?" Removing N can be very difficult, unless your nutrient line allows for it. Most nutrient products contain N in almost all formulations. If you wish to try the Precison Flush technique, you will have to find a nutrient line that does not have any nitrogen, or nitrates in the flowering portion of the nutrient line.

Fortunately, some companies have taken note of recent studies, and started producing end-of-life flower additives and supplements which eliminate nitrogen, making a Precision Flush technique possible.

When to Flush Cannabis?

When flushing traditionally, you start removing nutrients 1-2 weeks before harvest. This allows enough time for all the nutrients to be eliminated from medium, roots, and partially from the plant tissue. However, make sure not to overwater the cannabis plant. Many growers will over-flush the medium, flooding the plant with water and increasing the risk of bud rot! You will also need to ensure the growing environment is as cool and dry as possible, so no bud rot or mold starts to grow.

When using the Precison Flush technique, you'll start to eliminate Nitrogen 2 weeks before the end of life, while maintaining your normal watering and feeding practices.

If I'm Growing Cannabis Organically, Should I Still Flush My Plant?

No, organic soils don't need to be flushed out. The plant relies on a different process to access nutrients from organic soils, so they won’t build up in the plant, roots, or medium. Organic nutrients are slow-releasing so flushing isn’t necessary.

Since organic soils and nutrients are slow-releasing, you cannot pinpoint or remove certain nutrients from the plant. Therefore, the Precision Flush technique cannot and doesn't need to be used!

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