To evaluate effects of fertilizer concentration and temperature on growth of petunia (Petunia hybrida Hort. Vilm-Andr.), we grew petunias under three different day/night temperature regimes (35/27°C, 25/17°C, and 15/7°C) and with five different concentrations of fertilizers [electrical conductivity (EC) of 0.15, 1.0, 2.0, 3.0, and 4.0 dS m^-1]. There was an interactive effect of temperature and fertilizer EC on the growth of the petunias. Optimal fertilizer EC decreased as temperature increased. Growth was better correlated with the EC of the growing medium than with the EC of the fertilizer solution. Irrespective of growing temperature, plant growth was best in treatments with a final growing medium EC of 3-4 dS m^-1. The time to flowering was decreased, but flowers senesced more rapidly at higher temperature. Flower diameter decreased with increasing temperature. At the highest temperature (35/27°C), flower diameter also decreased with increasing EC of the fertilizer solution. The EC of the growing medium increased with increasing EC of the fertilizer solution and with increasing temperature. Effects on shoot nutrient concentrations were inconsistent among the three temperature treatments. At 25/17°C, a fertilizer EC of 2 dS m^-1 resulted in the highest shoot N concentrations, while shoot N was not affected by fertilizer EC at 35/27°C or 15/7°C. Shoot P concentration increased with increasing fertilizer EC at a temperature of 15/7°C, but not at higher temperatures. These results indicate that fertilization guidelines for greenhouse growers should be based on maintaining the EC of the growing medium within an optimal range, instead of the more traditional recommendations based on the concentration of the fertilizer solution.