UV-A- or UV-B-enriched growth light was given to basil plants at non-stress-inducing intensities. UV-A-enriched growth light gave rise to a sharp rise in the expression of PAL and CHS genes in leaves, an effect that rapidly declined after 1–2 days of exposure. On the other hand, leaves of plants grown in UV-B-enriched light had a more stable and long-lasting increase in the expression of these genes and also showed a stronger increase in leaf epidermal flavonol content. UV supplementation of growth light also led to shorter more compact plants with a stronger UV effect the younger the tissue. The effect was more prominent in plants grown under UV-B-enriched light than in those grown under UV-A. Parameters particularly affected were internode lengths, petiole lengths and stem stiffness. In fact, the bending angle of the 2nd internode was found to increase as much as 67% and 162% for plants grown in the UV-A- and UV-B-enriched treatments, respectively. The decreased stem stiffness was probably caused by both an observed smaller internode diameter and a lower specific stem weight, as well as a possible decline in lignin biosynthesis due to competition for precursors by the increased flavonoid biosynthesis. Overall, at the intensities used, UV-B wavelengths are stronger regulators of morphology, gene expression and flavonoid biosynthesis than UV-A wavelengths. Graphical abstract: [Figure not available: see fulltext.].