To Flush or Not to Flush

Written by Derek Johnson

Flushing is a widely practiced technique used by cannabis growers to improve the overall end product of their harvests. Many swear by it as do many buyers looking for flower. However, there’s reason to doubt the effectiveness of flushing, or at least be suspicious of it.

Flushing a plant occurs just before harvest. It involves removing excess nutrient and salt buildup at the roots of the plant using plain water. This sediment of different chemicals and compounds, from salts and other minerals, is considered “contamination” that hinders the plant from producing its best material. Proponents of the method argue that flushing the medium and the roots of nutrients forces the plant to essentially cannibalize itself, thus consuming any leftover nutrients and other chemicals used during the growth stage.

Most of the evidence for the effectiveness of flushing is anecdotal. Studies and research of flushing is very limited. Ed Rosenthal notes that flushing likely robs plants of needed nutrients and stunts growth. One aspect of the myth (debunked) is that flushed cannabis burns white due to greater purity. Another is that unflushed cannabis carries a harsh chemical taste. Until more scientifically guided investigations are performed, anecdotal proof will reign. However, there are some completed investigations into flushing.

Rx Green Technologies, a designer and supplier of commercial growing spaces, performed an in-house evaluation of cannabis flushing. Researchers took a sample of Cherry Diesel cannabis grown at Rx Green Technologies. After steady feeding during both vegetative and flower stages, the plants were flushed at different intervals as follows: 14 days, 10 days, and 7 days before harvest, or not at all.

The harvested buds were subsequently given to participants at a tasting panel made up of cannabis industry professionals. In a blind taste test, the experts rated the samples for ash color, harshness of the smoke consumed, and the flavor of the flowers. The samples were also tested for tetrahydrocannabinol (THC), terpene content, and mineral content.

The results of the Rx study clearly show that there was no significant benefit at any stage conferred by flushing the plants. In fact, results from the tasting panel showed that flowers that weren’t flushed were preferred over ones that were. Flushing caused yellowing and necrosis of leaves; the 14-day flush caused an (insignificant) increase in zinc and iron.

The Rx study is only one case. More study is needed to fully understand what, if any, benefits (or harms) flushing, a popular technique used throughout the cannabis industry, confers.


Image Source: Mark, flickr, CC BY 2.0

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Derek Johnson

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