Growing practices and the use of potentially harmful chemical additives from a web survey of mainly small-scale cannabis growers in 18 countries.

Background: There is emerging recognition of the risks of harmful chemical pesticides, fertilizers and 'nutrients' by cannabis growers. One group of chemicals, Plant Growth Regulators (PGRs), many of which have been banned from food crops for decades, have been found unlisted in a number of fertilizers and supplements marketed at cannabis growers.

Methods: This paper predominately uses data from a 2020-21 convenience web survey of mainly small-scale, recent (last 5yrs) cannabis growers from 18 countries (n = 11,479). We describe their growing practices and use of chemicals and employ logistic regression to explore predictors of chemical use. We also compare chemical use in our 2020-21 sample with that from our 2012-13 data in the 3 countries (Australia, Denmark, UK) where respondents were asked about their use of growing chemicals in both surveys.

Results: In 2020-21, 26 % of recent cannabis growers reported use of chemicals. Growers who were at highest odds of using chemicals were male, older, living in urban/cities, not growing for environmental reasons, growing in order to sell, growing where they believed cannabis was legal, and growing under artificial light in soil or non-soil media. We found significant reductions in the proportions of our samples who reported using chemical fertilizers in the 3 countries where we collected data in both waves.

Conclusion: Growers using soil and artificial light comprised over half of all the chemical users in the sample. Efforts at informing and educating growers about the problems of chemical fertilizer, nutrient and pesticide use should include all growers including those who grow in soil under artificial light. Possible explanations for the apparent decrease in chemical use from our 2012-13 to 2020-21 samples are discussed. Stricter regulation of the legal cannabis fertilizer market is required to empower growers to reduce the toxicity of cannabis they grow, distribute and consume.