Intensive agriculture needs new sustainable tools for weed management. Allelopathy offered a valid alternative. Sesquiterpene lactones (STLs) are the most important cultivated cardoon (Cynara cardunculus L. var. altilis DC.) allelochemicals involved in weed toxicity. The present study aims to investigate the effect of field light stress (by 60% of plant shading) on the qualitative and quantitative composition of STLs in cultivated cardoon leaf extracts harvested in January and April. The phytotoxicity of leaf extracts was evaluated on the wheat coleoptile elongation, seed germination, root and shoot length of the weeds Amaranthus retroflexus L. and Portulaca oleracea L. Light stress significantly increased the concentrations of total and single STLs, and this increase was more marked if combined with harvest time (+338, 102 and 94 of cynaratriol, desacylcynaropicrin and 11,13-dihydro-deacylcynaropicrin, respectively) in April. The concentration of STLs and the phytotoxic activity were correlated (light stressed extracts belonging from April reduced by 91, 86 and 93% the wheat coleoptile elongation and the root and shoot length of A. retroflexus, respectively, at 800 ppm). Therefore, the induction of light stress could be used to increase the concentrations of STLs in cultivated cardoon leaves for industrial applications and to improve their phytotoxic potential.