Lodging can be a limiting factor of hard red spring wheat (HRSW) production. The main objective of this study was to determine the optimum timing and rate of trinexapac-ethyl (TE) to improve straw strength, resistance to lodging, and related agronomic responses of HRSW. Field experiments arranged in randomized complete blocks were conducted from 2004 to 2006 in Crookston, MN. Five TE rates (0, 62.5, 93.75, 125.0, and 250.0 g a.i. ha-1) and one ethephon rate (280.2 g a.i. ha-1) were applied at Zadoks growth stage (GS) 30, 32, or 37. Measurements included crop injury, plant height, lodging, straw strength, acid detergent lignin (ADL) content, plant maturity, plant density, and yield. Increasing TE rates linearly decreased plant height and increased lodging resistance, straw strength, and ADL content. Lodging resistance was negatively correlated with plant height and positively associated with straw strength and ADL content. The TE rate of 125 g a.i. ha-1 decreased plant height by approximately 6%, and increased plant erectness by 9% and straw strength by 13%, without causing crop injury, delaying maturity, or affecting yield. Applications of TE at GS 37 resulted in less crop injury, shorter stand, and more erect plants than those at GS 30 or 32. These data suggest that the optimum application rate and timing of TE may be 125.0 g a.i. ha-1 at GS 37 for HRSW.