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...