Use of polymer-coated fertilizers (PCFs) is widespread in the nursery and greenhouse industries. Temperature is the main factor affecting nutrient release from PCFs, yet there are few reports that quantify temperature-induced nutrient release. Since container substrate temperatures can be at least 40 °C during the summer, this research quantified the release of fertilizer salts in the diurnal container substrate temperature range of 20 to 40 °C. Three PCFs (Osmocote Plus 15-9-11, Polyon 18-6-12, and Nutricote18-6-8) were placed in water-filled beakers at 40 °C until one-third (Expt.1) or two-thirds (Expt. 2) of Osmocote's N was released. For Expts. 1 and 2, each fertilizer was put into sand-filled columns and leached with distilled water concurrent with column temperature incrementally increasing from 20 to 40 °C and then to 20 °C over a 20-h period. Leachate fractions were collected at every 2 °C increase and analyzed for fertilizer salts. In Expt.1 and in the range of 22 to 30 °C, salt release was highest, lowest, and intermediate for Nutricote, Osmocote, and Polyon, respectively. In the range of 38 to 40 °C, release was highest, lowest, and intermediate for Osmocote, Nutricote, and Polyon, respectively. In Expt. 2, salt release in the range of 22 to 30 °C was the same as in Expt. 1. However, at 38 to 40 °C, release was highest, lowest, and intermediate for Polyon, Nutricote, and Osmocote, respectively. Results show that salt release for PCFs are dependent on the temperature × fertilizer age interaction.