Elevated temperatures can intensify weed issues by accelerating weed growth and germination, especially for C4 species, due to their heat tolerance and efficient carbon fixations, making their control more challenging. Temperature also influences herbicide absorption, translocation, and metabolism. Large crabgrass (Digitaria sanguinalis), a highly problematic C4 weed, poses a significant threat to nursery and field-grown ornamentals under these conditions. This study aims to evaluate the efficacy of postemergence herbicides at different rates (0.5X, 1X, 2X) under normal and elevated temperatures for managing large crabgrass in ornamental production. Experiments were conducted in summer and fall 2024 using a growth chamber and an open hoop house setup. Large crabgrass seeds were sown in pots filled with standard substrate, irrigated, and placed in separate growth chambers where temperatures were maintained at normal (22–28°C) and at 2-5°C above the species’ optimum range (28-34°C). Once weeds reached the 3–6 leaf stage, they were transferred to an open hoop house for herbicide application. Postemergence herbicides such as topramezone and glyphosate were applied at 0.5X, 1X, and 2X rates using a CO₂ backpack sprayer. After the restricted entry interval, treated weeds were placed back to their respective growth chambers and maintained there for four weeks. A control treatment without herbicide application was maintained in both growth chambers. Weed control ratings were visually assessed at 1, 2, 3, and 4 weeks after treatment (WAT) using a 0–10 scale, where 0 = no control (green foliage, upright growth, no damage) and 10 = complete plant death (dark brown foliage, no green tissue, collapsed structure)). At 4 WAT, all weeds were harvested, and dry weight of aboveground parts of weed was recorded. The experiment followed a completely randomized design with eight replications per treatment and was repeated twice. Data were analyzed using ANOVA in SAS 9.4, and treatment means were separated using Fisher’s LSD test. The results showed that at 4 WAT glyphosate weed control efficacy at normal temperature was 88% which reduced to 73% under elevated temperature condition. Whereas the high temperature increased topramezone efficacy by 10%. The lowest amount of weed dry weight was observed at 2X rates in high temperature conditions, but no difference was observed among rates at normal temperature. Hence it can be concluded that with an increase in temperature, higher rates of the tested herbicides will be required for effective weed management in ornamentals.
