Abstract: Treatment apparatus (10) comprises red-light emitting means (15) and blue-light emitting means (16), in particular diodes, for emitting red light and blue light, respectively, and directing the emitted light into a target zone (14) for irradiation of plant matter (13) when disposed in the zone. The diodes (15, 16) are operable to provide a substantially homogenous blend of the emitted red and blue light in the target zone for at least a major part of the period of emission and to emit the light at wavelengths of 625 to 660 nanometres for the red light and 440 to 470 nanometres for the blue light with intensities in the target zone of up to 500 microEinsteins for the red light and up to 400 microEinsteins for the blue light.