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.
The origin of flushing is unknown, but flushing may have been devised to mitigate symptoms of excessive fertilization, e.g., stunted growth, tissue necrosis and wilting. High substrate electrical conductivity (EC) occurs when the fertilizer concentration exceeds plant demand, which results in salt accumulation in the growing substrate. Excessive application of fertilizer is common during cannabis cultivation, particularly phosphorus, according to 2022 research from Utah State University’s Bruce Bugbee and Mitchell Westmoreland. Thus, the flushing technique may have evolved to avoid damage to the flower buds in the last weeks of production rather than stimulating improved bud quality.
