The percentage of THC in cannabis flowers can vary significantly, but generally, a high THC in cannabis flowers is around 20%. These types of flowers are considered marijuana, which remains federally illegal but legal in select states.When it comes to hemp plants, the legal threshold for delta-9 THC content is 0.3%—but even within hemp plants, the THC concentration can vary.When it comes to THC flowers derived from hemp, a high percentage of 0.3% of the total dry weight. However, there are some hemp flowers with low THC (less than 0.1%) and some with virtually no THC at all (but this is due to an extra processing step to strip away all THC cannabinoids from the plant).