Growing cannabis does have an environmental footprint. Indoor grows burn huge amounts of electricity (one study puts it at 2.2 to 5.1 tonnes of CO2 per kilo of dried flower), while outdoor grows can drain local water supplies, with a mature plant drinking up to 22.7 liters (6 gallons) a day, and pollute streams with fertilizer and pesticide runoff. Hemp, on the other hand, can capture carbon and clean up contaminated soil. The cannabis plant has been grown around the world since ancient times and used for thousands of years for medicine and recreation. It's still taboo or banned in many places, but in Argentina, Congress passed a law regulating the medical cannabis and industrial hemp industry, and the country is now taking its first steps in producing, selling and exporting the plant. Which raises the big question: what does all that growing do to the environment?

First things first: the two ways to grow cannabis

Indoor growing

Indoor growing (including greenhouses) gives you full control over everything your plants experience, like light and temperature, but it's held back by higher costs, heavy energy demand and the environmental baggage that comes with it. Shrinking agriculture's overall environmental footprint is essential if we want it to be sustainable.

Outdoor growing

Growing outdoors is the traditional, original way to grow cannabis. It's cheap, but it's at the mercy of the weather and natural resources. Poor management of soil, water and pests can cause serious environmental problems.

How each method can harm the environment

• Each approach affects the environment differently. According to a study published in Nature Sustainability, the emissions from growing one kilo of indoor cannabis are roughly the same as burning up to 2,000 liters (about 530 gallons) of gasoline. New Scientist, reporting on large-scale indoor production, put the carbon footprint at between 2.2 and 5.1 tonnes of carbon dioxide per kilo of dried flower.

• Outdoor growing, on the other hand, can use up to 140 times less electricity, according to a study by the Cannabis Environmental Best Practices Task Force in Oregon. But the same group flagged concerns about water use: a mature cannabis plant can drink up to 22.7 liters (6 gallons) of water a day over a 150-day growing season, almost twice as much as a wine grape vine.

Still, there's no systematic framework for sustainable cannabis growing yet, because its environmental impact still isn't fully understood. With cannabis being legalized in more and more countries, a full environmental assessment is badly needed.

How hemp can help the environment

The risks are real, but cannabis can also have a positive environmental impact. A University of Cambridge researcher has said that hemp can capture atmospheric carbon "twice as effectively as forests," while also supplying carbon-negative biomaterials for architects and designers.

Industrial hemp, which doesn't get you high because it's so low in THC, can also help the soil: it's been shown to grow in, and help clean up, heavily contaminated land where other crops wouldn't stand a chance.

Hemp's ability to offset some of farming's damage also supports more sustainable development in other industries: it can replace a range of raw materials, since its fibers can be turned into biodegradable fabric, paper and plastic.

Cannabis and water: three big environmental issues

How much water cannabis uses

Cannabis is a water- and nutrient-hungry crop, needing twice as much water per season as corn, soybeans or wheat. On average, a cannabis plant is estimated to use 22.7 liters (6 gallons) of water a day during the growing season. Daily water use varies from site to site depending on geography, soil, climate and growing method, but it's widely accepted that cannabis is a thirsty plant. Between population growth and climate change, farming's heavy water demand is very likely to make water shortages worse in the years ahead.

That high demand puts pressure on water sources. Water diversion, pulling or transferring water from one watershed to another to meet irrigation needs, is common practice. It eases the water shortage for cannabis growers, but it creates new problems of its own.

Water pollution

Cannabis growing, especially illegal grows, can degrade water quality. Pesticides used for routine pest and disease control (including herbicides, insecticides, fungicides, nematicides and rodenticides) end up in the water unchecked and pose serious risks to aquatic life. Where fertilizers and pesticides end up varies.

Nitrogen and pesticides, for example, can wash off as runoff or leach into groundwater after rain or overwatering. If that contaminated water keeps getting used, it adds even more pollutants to the soil, surface water and groundwater. In the end, those chemicals can threaten people and crops all the way up the food chain.

Aquatic ecosystems

Water diversion and pollution both take a toll on aquatic ecosystems. Heavy water use by cannabis grows in watersheds hits wildlife like fish and amphibians hard, since grows are often spread across watersheds whose downstream waters are home to sensitive aquatic species.

It's vital that every grower gets on the same page about water, because changing stream flows is one of the biggest threats to aquatic biodiversity. The cannabis industry is turning into one of the worst offenders when it comes to water diversion.

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