Clethodim is an important herbicide for managing Texas panicum. However, its efficacy is influenced by the weed size and environmental stress during application. Therefore, field and greenhouse studies were conducted in 2023 and 2024 to evaluate clethodim efficacy on various Texas panicum sizes. Clethodim was applied at Texas panicum sizes ranging from 5 cm to 60 cm. A sequential application was applied two weeks after the initial treatment for larger weed sizes (15 to 60 cm). In separate field and greenhouse studies, non-ionic surfactant (NIS), crop oil concentrate (COC), methylated seed oil (MSO), COC + ammonium sulfate (AMS), and MSO + AMS adjuvants were mixed with clethodim to determine efficacy on 10-15 cm and 20-30 cm Texas panicum. In the weed size study, sequential applications of clethodim increased Texas panicum control compared to a single application. At the 10-15 cm growth stage, a single application provided 90% Texas panicum control, whereas the sequential treatment improved control from 76% to 91% at the 15-20 cm growth stage. However, clethodim efficacy declined as Texas panicum size increased across single and sequential treatments. In the adjuvant studies, clethodim plus COC, COC+AMS, or MSO+AMS provided 91%, 93%, and 90% control at the 10-15 cm growth stage, respectively; however, efficacy decreased for 20-30 cm Texas panicum. Texas panicum efficacy was higher for clethodim plus MSO+AMS than clethodim plus MSO; however, AMS did not increase clethodim + COC efficacy. Overall, Texas panicum control with clethodim was most effective when weed height was 15 cm or less. A sequential application of clethodim was required for larger Texas panicum (>15 cm). Clethodim plus COC or MSO+AMS provided the highest control of Texas panicum. This study demonstrated that successful Texas panicum management depends on applying clethodim at the optimum size and selecting the appropriate oil-based adjuvant especially at larger Texas panicum sizes.