A method, and a device for administering the method, of treating a plant seedling to improve long term hardiness and/or crop yield and/or quality characterised by the step of exposing the plant seedling, prior to a subsequent growth phase, to ultraviolet (UV) irradiation with at least one wavelength, only between 280-310 nm. The method further including the step of selecting a plant seedling or seedlings for a subsequent growth phase.