Irradiated carrageenan (IC) elicits an array of plant defense responses and biological activities in plants. An experiment was carried out in the naturally illuminated conditions of net house in order to assess the effects of foliar spray of IC on agricultural performance of fennel (Foeniculum vulgare Mill.), which is a high-value essential oil bearing medicinal crop used in pharmaceutical, food and cosmetic industries. There were applied four IC concentrations (40, 60, 80 and 100 mg L⁻¹) as foliar sprays. Application of IC significantly improved the growth attributes, physiological and biochemical parameters, essential oil yield and the contents of main components of essential oil of fennel. IC applied at 80 mg L⁻¹ enhanced these parameters maximally. Unirradiated carrageenan and deionized water had no effect on the attributes studied. Moreover, GLC analysis revealed a significant increase in the components of essential oil, viz. fenchone (4.48–7.82%) and anethole (78.38–86.08%) compared to the control.