Summary Plants of two clonal selections of thyme (Thymus vulgaris) were grown under natural light and natural light supplemented by a PPF of 200 µmol m–2 s–1, provided by high pressure sodium lamps at 90%, 70% and 50% substrate water content in a greenhouse. Stomatal and cuticular transpiration, essential oil glands, epicuticular wax, and morphological traits were measured. Both stomatal transpiration and cuticular transpiration decreased under supplemental light and at low substrate water content. The time for which the stomata remained open or partially open was longer for plants developed under supplemental light at low substrate water content compared with variants grown under natural light and higher soil water content. The number of essential oil glands was significantly higher for plants grown under supplemental light (31 for ‘Laval 1’ and 27/2.27 µm2 for ‘Laval 2’) compared with natural light (23 for ‘Laval 1’ and 19/2.27 µm2 for ‘Laval 2’) grown plants. The lower the substrate water content, the higher the epicuticular wax load. The epicuticular wax concentration under supplemental light varied between 171 and 214 mg m–2 for ‘Laval 1’, and 169 and 208 mg m–2 for ‘Laval 2’, while under natural light, this value fluctuated between 128 and 163 mg m–2 for ‘Laval 1’, and 114 and 146 mg m–2 for ‘Laval 2’. Plants grown under supplemental light developed upright erect shoots with increased tillers, branches, thicker and glossy leaves that were aesthetically more appealing, while plants under natural light had a prostrate type of growth with thinner, broader leaves of slightly pale green colour.