Recent debates about marijuana legalization have focused on the potential social, health and economic impacts, with little attention paid to environmental issues. A new study in the journal BioScience, co-authored by Woods Senior Fellow Rosamond Naylor (Earth System Science) and spearheaded by the California chapter of The Nature Conservancy, brings environmental concerns to the forefront of the policy discussion. Between 60 and 70 percent of the marijuana consumed in the United States comes from California.

Water and Wildlife

Marijuana plants require nearly twice as much water as do grapes or tomatoes, and the last five years have brought a 50 to 100 percent increase in the amount of northern California watershed lands used for marijuana production – figures that are causing growing concern among conservationists in the midst of a severe statewide drought.

The majority of California agriculture is subject to heavy water use regulations. Farmers of most irrigated crops help their plants through the dry summer months by filling water tanks in the winter, when streams and springs are full.

By contrast, many marijuana growers draw surface water during the plant’s summer growing season, when drought conditions are worst.

“Taking water directly from rivers and streams in the summer not only reduces the water available for agriculture but also threatens wildlife species, especially birds and fish, that depend on these wetland ecosystems for survival,” said Naylor.

Illegal marijuana plantations in California are associated with a wide range of other environmental impacts, including pollution, poaching, and pesticides that poison wildlife. Even legal outdoor cultivation can cause deforestation and soil erosion.

Policy Options

The research team identified several opportunities to reduce the environmental impacts of marijuana cultivation in California. For example, states can:

  • Offer incentives for growers to protect natural resources
  • Enforce new or existing environmental laws,
  • Use sales tax revenues to fund restoration projects
  • Implement certification or labeling programs to encourage consumers to buy sustainably grown products.

“Regardless of the legal status of marijuana, the way we are currently managing its impacts on water and wildlife in California just doesn’t work,” said Naylor. “Bringing these impacts into future policy discussions about marijuana is critical for protecting California’s environmental resources given the high value and demand for the crop.”

Naylor is William Wrigley Professor of Earth Science at Stanford, director of the Center on Food Security and the Environment, and a senior fellow at the Freeman Spogli Institute for International Studies. She serves as a trustee of The Nature Conservancy California Chapter.