Provided is a method of treating a post-harvest plant after harvest, the method including: irradiating a harvested plant with light having a peak wavelength in a wavelength range from 270 to 290 nm and/or light having a peak wavelength in a wavelength range from 370 to 400 nm at an irradiance effective to increase an amount of at least one rare cannabinoid compound and/or at least one terpene compound in the harvested plant, wherein an irradiance of light of all wavelengths in a wavelength range from 410 to 700 nm received by the harvested plant during the irradiation is less than 20% of the irradiance of the light having a peak wavelength in the wavelength range from 270 to 290 nm and/or less than 20% of the irradiance of the light having a peak wavelength in the wavelength range from 370 to 400 nm.