New Experiments in Flushing: Let Your Cultivation Goals Dictate Your Plans

Flushing is a common practice in the cannabis industry that involves restricting fertilizer application and leaching nutrients from the growing substrate during the final weeks of flowering. The industry hypothesis is that smoking quality improves when nutrient levels in the buds are reduced, according to the article “White Ash vs. Black Ash” published in June 2019 in Cannabis Business Times. No scientific studies have evaluated this hypothesis, so the evidence is anecdotal. Limited studies conducted in 2017 at the University of Guelph and in 2019 by cannabis nutrient supplier Rx Green Technologies have explored how flushing affects yield of dried flower buds as well as the elemental and cannabinoid concentrations of flower buds.

2017 University of Guelph research led by master’s student Jonathan Stemeroff found that flushing for two weeks or less before harvest had no effect on the elemental or cannabinoid concentration of dried flower buds. (Elemental concentration refers to all forms of a particular element, like nitrogen or phosphorus, rather than the concentration of specific nutrients containing that element, like ammonium or phosphate.) Also, there was no consistent change to flower yield (dry mass) after flushing. These results suggest flower buds are generally insensitive to changes in the growing substrate during the final two weeks before harvest, so flushing to reduce nutrient levels may be futile. Stemeroff noted that flushing may be effective for reducing fertilizer costs because the flower buds were unaffected; if growth is the same regardless, then why waste the fertilizer? According to a 2021 Cannabis Business Times study, about half of cannabis growers spend 10% to 29% of their annual production budget on fertilizer. Therefore, terminating fertilizer application one to two weeks before harvest could result in substantial savings.

Bennett, K., Jent, J., Samarakoon, U., Schnabel, G., & Faust, J.E. 2020. Reduction of Botrytis cinerea Infection on Petunia Flowers following Calcium Spray Applications. American Society for Horticultural Science. (https://doi.org/10.21273/HORTSCI14208-19)

Alden will present exclusive findings on flushing, as well as studies testing other common cannabis cultivation and crop steering practices. Find details here: cannabisconference.com