Outdoor Cannabis Farming Has a Much Smaller Carbon Footprint, But Fertilizer and Potting Media Still Matter

A 2024 study found that growing cannabis outdoors in Quebec produced substantially fewer greenhouse-gas emissions than comparable indoor cultivation, but it also showed that outdoor production is not automatically low-impact. Fertilizer use, transportation, potting media and fuel-intensive processing remained important sources of environmental damage.

Published in ACS Agricultural Science & Technology, the study was conducted by researchers from McGill University and the University of Michigan. It combined field trials with a life-cycle assessment—the standard method for estimating environmental impacts across a product’s supply chain.

The researchers grew the autoflowering “Candy Cane” cannabis variety near Thetford Mines, Quebec, during three growing seasons from 2020 through 2022. The plants were cultivated outdoors in pots and subjected to six fertilizer treatments that varied the amount of nitrogen and potassium supplied.

The analysis examined global-warming potential, fossil-fuel depletion, metal-resource depletion, terrestrial acidification, and marine and freshwater eutrophication. Unlike earlier cannabis studies that generally measured impacts per kilogram of dried flower, the researchers also assessed impacts per 100 grams of THC, recognizing that cannabinoid content affects the product’s functional value.

Outdoor production used far less energy

Outdoor cultivation was estimated to produce between 61.8 and 110.7 kilograms of carbon-dioxide equivalent per kilogram of dried flower. That was up to 50 times lower than estimates for indoor cultivation, whose lighting, heating, ventilation and air-conditioning systems can consume large amounts of electricity and fuel.

The comparison should not be interpreted as a direct, controlled trial of identical indoor and outdoor farms. The researchers compared their outdoor results with figures from previous indoor-cultivation studies, which used different locations, production systems and assumptions. Even so, the findings reinforce a consistent conclusion in the cannabis-environment literature: eliminating energy-intensive climate control and artificial lighting can greatly reduce the carbon footprint of production.

Outdoor farming did not eliminate other environmental impacts. Nutrient losses can contribute to water pollution and eutrophication, while fertilizer production and application can increase acidification and greenhouse-gas emissions. Outdoor cultivation can also create land-use, erosion and habitat concerns that are less prominent in indoor systems.

Lower fertilizer use did not automatically produce the best result

The field trial was designed to explore whether growers could reduce fertilizer inputs without sacrificing flower yield or THC production. Previous research has found that excessive nitrogen can increase plant growth while diluting cannabinoid concentrations.

However, the results showed that the relationship between fertilizer use, yield, THC content and environmental performance was more complicated than simply “less fertilizer is better.” The treatment with the lowest modeled carbon footprint used a higher nitrogen input than the alternative low-input treatment, because its stronger yield performance helped offset the environmental burden associated with producing and applying more fertilizer.

This result highlights why agricultural assessments should consider both the amount of product harvested and its potency. A fertilizer program that reduces inputs but produces substantially less flower or THC may not have the lowest impact per unit of usable product.

Potting media was a major environmental hotspot

Peat-based potting media, including peat moss and perlite, made a significant contribution to the study’s environmental estimates. Producing and transporting fresh media added emissions, while media handling and sterilization also created additional burdens.

The researchers modeled a scenario in which potting media was reused on-site and sterilized with a diesel-powered steam system instead of being replaced every growing cycle. Reuse could reduce some impacts, particularly those associated with obtaining and transporting new media, but the benefits depended on the energy and materials required for sterilization. Perlite production and diesel combustion remained notable contributors.

These findings suggest that growers seeking to reduce emissions should look beyond the choice between indoor and outdoor cultivation. Reusing growing media where agronomically and legally appropriate, reducing unnecessary transport, improving fertilizer efficiency and replacing fossil-fuel-powered equipment could all affect the final footprint.

A useful benchmark, not a universal answer

The study provides one of the first detailed life-cycle assessments of outdoor cannabis production and offers a valuable baseline for producers and policymakers. Its results are most directly applicable to the Quebec production system examined, including its climate, crop variety, potting method, transportation assumptions and fertilizer treatments.

Outdoor cultivation appears to offer a major carbon advantage over energy-intensive indoor production, but that advantage does not make every outdoor operation environmentally sustainable. Future assessments will need better data from different climates and production systems, along with closer examination of soil erosion, habitat loss, water quality, drying, processing and postharvest operations.

For an overview of the broader environmental issues associated with cannabis production, readers can consult the American Chemical Society’s review of cannabis and the environment. The central lesson from the Quebec study is straightforward: moving cannabis production outdoors can sharply reduce emissions, but responsible fertilizer management and careful handling of growing media remain essential.