AbstractUnderstanding how a warmer and more variable climate will affect the productivity and stability of perennial forages in the establishment year is essential for determining forage management strategies resilient to the changing climate. We conducted an experiment from 2020–2021 and repeated it in 2021–2022 to determine how warmer and more variable temperatures affect yield of an alfalfa (Medicago sativa L.)–orchardgrass (Dactylis glomerata L.) mixture, as well as on the abundance of a winter annual weed, chickweed (Stellaria media), during the establishment year. We utilized open‐top chambers (OTCs) to manipulate air temperature in situ to impose three treatments: (1) constant warming (OTC present year‐round), (2) fluctuating warming (OTC present year‐round but temporarily removed prior to abrupt cold snaps), and (3) control (no OTC, ambient air temperature). OTCs increased air temperature within the alfalfa–orchardgrass canopy by an average of 0.45 and 0.72°C in Year 1 and Year 2, respectively; however, the effects of OTCs on air temperature varied temporally within and between years. Constant warming decreased alfalfa frost injury compared with the control, while the control and fluctuating warming resulted in similar levels of frost injury. Warmer temperature within the OTCs neither affected chickweed biomass nor the biomass of all other weed species. Both constant and fluctuating warming increased forage dry matter yield at the first harvest timepoint in the spring of both years, the period when preceding ambient air temperature tended to be cooler than other harvest timepoints. We found that the constant warming increased total forage yield (summed over the entire year) in Year 1, when precipitation was well above the 30‐year normal for our region, but decreased total forage yield in Year 2, when summer precipitation was substantially below the 30‐year normal. As global temperature continues to rise, yield loss of cool‐season perennial forages, such as alfalfa and orchardgrass, may be exacerbated in years without adequate precipitation. Therefore, exploring heat‐ and drought‐tolerant alternative forages, as well as management strategies that improve resilience to weather variability, will be increasingly important.