Researchers at the University of Georgia dried freshly-harvested, greenhouse-grown leaves of rosemary (Rosmarinus of- ficinalis), motherwort (Leonuris cardiaca), and peppermint (Mentha piperita) in one of two ways.
These leaves were dried in one of two ways – solar exposure in a greenhouse or a forced-air dryer at 104F or in the oven at 158F. All times were 4 days of drying.
Greenhouse dried leaves tended to be the most potent with the highest heat oven the lowest.
Researchers are thinking that the lower heats encourage the development of post-harvest anti-oxidants while higher heats discourage them.
Bottom line – dry your herbs with the lowest heat source (hung up in the window) you can for the best healthy effects.