The mechanical properties of leaves are important in many aspects of plant science. Because of their delicate structure, leaves are sensitive to different, potentially harmful, environmental factors. The mechanical properties of leaves are important factors affecting leaf quality, longevity, susceptibility to damage, and decomposition. Two growth stimulants were applied to investigate selected mechanical properties of Solomon’s seal Variegatum [Polygonatum multiflorum (L.) All.] leaves grown under field cultivation and in an unheated polytunnel. The mechanical properties of leaves were assessed by measuring Young’s modulus. The agents used in the treatment were Actisil Hydro Plus at a concentration of 0.4% in the first series and Pentakeep V at a concentration of 0.04% in the second series. Foliar treatment with the stimulants was conducted six times at weekly intervals. The control plots were sprayed with distilled water. The research was carried out between 2012 and 2014. In comparison to the test series, the respective effects of Actisil Hydro Plus and Pentakeep V on the change in Young’s modulus was more notable in plant leaves obtained from the tunnel than those from field cultivation. Specifically, growth stimulants had more impact on the stiffness of leaves obtained from plants grown in the tunnel. Generally, plant leaves from field cultivation were stiffer than those from the tunnel.