Interactions between temperature and fertilizer concentration affect growth of subirrigated petunias

To evaluate effects of fertilizer concentration and temperature on growth o f petunia (Petunia X hybrida Hort. Vilm-Andr.), we grew petunias under thre e different day/night temperature regimes (35/27 degreesC, 25/17 degreesC, and 15/7 degreesC) and with five different concentrations of fertilizers [e lectrical conductivity (EC) of 0.15, 1.0, 2.0, 3.0, and 4.0 dS m(-1)]. Ther e 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 th e 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 temperatur e. At the highest temperature (35/27 degreesC), newer diameter also decreas ed with increasing EC of the fertilizer solution. The EC of the growing med ium increased with increasing EC of the fertilizer solution and with increa sing temperature. Effects on shoot nutrient concentrations were inconsisten t among the three temperature treatments. At 25/17 degreesC, a fertilizer E C of 2 dS m(-1) resulted in the highest shoot N concentrations, while shoot N was not affected by fertilizer EC at 35/27 degreesC or 15/7 degreesC. Sh oot P concentration increased with increasing fertilizer EC at a temperatur e of 15/7 degreesC, but not at higher temperatures. These results indicate that fertilization guidelines for greenhouse growers should be based on mai ntaining the EC of the growing medium within an optimal range, instead of t he more traditional recommendations based on the concentration of the ferti lizer solution.