If the level creeps above 0.3 percent, it reaches the legal threshold that classifies the crop as marijuana, which is illegal.

Measuring the THC level in growing plants is a delicate, high-stakes task. USDA is under pressure to overwrite a patchwork of state regulations on measuring THC by setting a national testing standard. The department has yet to produce federal guidelines that will shape how the new commodity is grown and sold, though USDA has said it plans to do so ahead of the 2020 growing season. A proposed rule is still pending at the Office of Management and Budget.

THC amount depends on a range of environmental factors, such as elevation, rain, temperature and even at what time in the season the plant is tested.

Source: Politico