Growing cannabis indoors is considerably less green than outdoor cultivation, according to US researchers.

Early analysis by a team from Colorado State University found indoor growing, the most common form of commercial cannabis production in the US, produced considerable greenhouse gas emissions. The researchers claimed changing to outdoor cultivation could reduce emissions by as much as 96%.

Published in the journal Nature Sustainability, the report said while legalisation of cannabis in many US States has caused a substantial increase in commercial production, the magnitude of the industry’s environmental impact has not been fully quantified.

Cannaponics
The team found growing cannabis under lights was considerably less green than cultivating outdoors.

The study analysed the energy and materials required to grow cannabis indoors and quantified the corresponding greenhouse gas (GHG) emissions using life-cycle assessment methodology for a cradle-to-gate system boundary.

The analysis was performed across the US, accounting for geographic variations in meteorological and electrical grid emissions data.

Based on location, the resulting life-cycle GHG emissions ranged from 2,283kg to 5,184kg CO2-equivalent per kg of dried flower.

The researchers attributed the high emissions to electricity production and natural gas consumption from indoor environmental controls, high-intensity grow lights and the supply of carbon dioxide for accelerated plant growth.

They added all of these inputs contribute to greenhouse gas emissions with the Mountain West, Midwest, Alaska and Hawaii the worst offenders, compared to the Pacific and Atlantic coasts where climates are milder.

The researchers urged the industry to explore technological solutions and policy changes to improve the environmental impact of commercial indoor cannabis production.

However, they added: “Growing cannabis outdoors or in greenhouses could be one way to remove the need for lights and environmental controls, [but we] don’t know the greenhouse gas emissions associated with these growth methods either. All these unknowns make it hard to develop polices or best management practices.”