The method involves avoiding irradiation of plants (4) from a natural spectrum with electromagnetic waves of wavelengths within an interval between 600 nano meters to 800 nano meters. A heat medium is removed for formation or photosynthesis of carbohydrates of plants. The plants are irradiated with electromagnetic waves from a spectrum of wavelengths within another interval between 350 nano meters to 500 nano meters, where carbon di-oxide is supplied as gas. Herbal substance is converted into combustible gas such as biogas, e.g. methane. An independent claim is also included for a device for cutting plants before irradiation with electromagnetic waves of the wavelengths of an interval.
