Climate change in grassland ecosystems may lead to divergent shifts in the abundance and distribution of C₃ and C₄ grasses. Many studies relate mean climate conditions over relatively long time periods to plant cover, but there is still much uncertainty about how the balance of C₃and C₄ species will be affected by climate at a finer temporal scale than season (individual events to months). We monitored cover at five grassland sites with co-dominant C₃ and C₄ grass species or only dominant C₃ grass species for 6 yr in national parks across the Colorado Plateau region to assess the influence of specific months of climate and water balance on changes in grass cover. C₄ grass cover increased and decreased to a larger degree than C₃ grass cover with extremely dry and wet consecutive years, but this response varied by ecological site. Climate and water balance explained 10–49% of the inter-annual variability of cover of C₃ and C₄ grasses at all sites. High precipitation in the spring and in previous year monsoon storms influenced changes in cover of C₄ grasses, with measures of water balance in the same months explaining additional variability. C₃ grasses in grasslands where they were dominant were influenced primarily by longer periods of climate, while C₃ grasses in grasslands where they were co-dominant with C₄ grasses were influenced little by climate anomalies at either short or long periods of time. Our results suggest that future changes in spring and summer climate and water balance are likely to affect cover of both C₃ and C₄ grasses, but cover of C₄ grasses may be affected more strongly, and the degree of change will depend on soils and topography where they are growing and the timing of the growing season.