Legal cultivation of hemp—a cousin of marijuana used to make everything from fabrics to pharmaceuticals—is closer to becoming reality as a result of a provision tucked into the farm bill that passed the House last week.

Federal law effectively prohibits the cultivation of hemp, making no distinction between that plant and marijuana, which is much more potent. However, nine states have passed laws legalizing industrial hemp farming, including California, Kentucky and Vermont, according to Vote Hemp, a not-for-profit advocacy...