<p>The integration of medicinal and aromatic plants (MAP) in conventional cultivation is hindered by a limited understanding of appropriate cultivation practices and nutritional requirements, particularly for under-evaluated MAP such as the <i>Sideritis cypria</i> Post. species. Furthermore, the development of comprehensive nutrient management is essential, with potassium and iron to have a fundamental role in plant growth and mitigate potential adverse effects of heavy metal toxicity. The current study assessed the effect of nutrient solutions (NS) with different potassium (K; 150, 350, 550 mg L^− 1) and copper (Cu; 5, 100 µM) levels, in combination with foliar iron (Fe) applications, in the soilless cultivation of <i>S. cypria</i> plants. The lowest K application (150 mg L^− 1) was associated with increased fresh biomass production, while the highest K application (550 mg L^− 1) revealed improved chlorophyll and carotenoid contents. Foliar Fe modulated these responses, and was associated with growth inhibition, oxidative stress and Fe accumulation in leaves. It also increased the accumulation of N and Cu. However, copper accumulation was well below the maximum residue levels of Cu in herbal infusions. <i>S. cypria</i> antioxidant activity and stress response to excess Cu was altered under the different K levels and foliar Fe application, highlighting the interplay between heavy metal toxicity and K levels. Moreover, treatments with intermediate K levels produced plants with high antioxidant activity and moderate antibacterial activity. Low to moderate K levels in the NS can be deployed when in excess Cu, without compromising <i>S. cypria</i> production and safety. Moreover, foliar Fe applications may be applied for biofortification, upon consideration of potential toxicity risks.</p>