<p>Ultraviolet (UV) radiation induces the production of plant secondary metabolites but can also damage plants. The exact balance depends on the UV wavelength, fluence rate, and exposure duration. Here, we explored pre-harvest UV irradiation conditions to minimize growth inhibition while maximizing the biosynthesis of phenolics in kale (<i>Brassica oleracea</i> var. <i>acephala</i>) plants. We provided supplemental UV light from light-emitting diodes (LEDs) for 8 days before harvest, and used UV-A (385&#xa0;nm, A385) at 39.0 kJ m^–2 day^–1 biologically effective UV radiation, UV-B (315&#xa0;nm, B315) at 0.58 and 1.35 kJ m^–2 day^–1, and UV-B (295&#xa0;nm, B295) at 3.56 and 10.7 kJ m^–2 day^–1. Upon pre-harvest UV-B exposure, growth and <i>Fv/Fm</i> were comparable to the control without UV exposure, except for high B295 treatment. UV-A exposure increased growth and chlorophyll content, while <i>Fv/Fm</i> decreased immediately with UV-A exposure. H₂O₂ content increased only under high B295 treatment. UV radiation increased total phenolic contents, especially for A385 and high B295 treatments, and antioxidant capacities increased after 6 days into treatment. Levels of kaempferol glycoside increased under B295 treatment, while hydroxycinnamic acids decreased after 8 days of treatment. Different transcript levels for <i>COP1</i>, <i>HY5</i>, <i>PAL</i>, and <i>CHS</i> supported the production of distinct secondary metabolite profiles. These results confirmed the distinct UV responses depending on wavelength and dose, and suggested that irradiation with UV-B of 295&#xa0;nm at 3.56 kJ m^–2 day^–1 could enhance phenolic contents in kale plants without inhibiting growth or causing photochemical damage.</p>