The effects of suspensions of cerium oxide nanoparticles sprayed onto marigold seedlings (Calendula officinalis L.) at concentrations between 50 and 3200 mg L−1 were studied. At concentrations of 50 and 100 mg L−1, the nanoparticles had a slight positive effect on some growth parameters, but at higher concentrations they caused severe oxidative damage. (302 characters)