Marijuana growers, many believed to be affiliated with Mexican drug cartels, are aggressively expanding their illegal farming operations in the U.S., clearing land to plant pot in dozens of national forests from coast to coast.

Illicit cannabis farms on public land first sprang up in California more than a decade ago and remain a serious problem in that state. But in the past two years, the U.S. Forest Service has documented a rapid expansion of the practice.

Authorities have discovered pot farms in 61 national forests across 16 states this year, up from 49 forests in 10 states last year. New territories include public land in Colorado, Wisconsin, Michigan, Alabama and Virginia.

"They're moving across the country," said David Ferrell, director of law enforcement for the U.S. Department of Agriculture, which includes the Forest Service.

With the expansion comes an increased risk to campers and hikers -- a particular concern this Labor Day weekend, as families converge on public land just as many cannabis crops are ready for harvest.