Phenology and its interannual variability are altered through anthropogenic climate change. Feedbacks of plant phenology to the regional climate system affect fluxes of energy, water, CO2, biogenic volatile organic compounds as well as canopy conductance, surface roughness length, and are influencing the seasonality of albedo. We performed simulations with the regional climate model COSMO-CLM (CCLM) with 3km horizontal resolution over Germany covering the period 1999 to 2015 to study the sensitivity of grass phenology to different environmental conditions by implementing a new phenology module. We provide new evidence that the standard annually-recurring phenology of CCLM is improved by the new calculation of leaf area index (LAI) dependent upon surface temperature, day length, and water availability. Results with the new phenology implemented in the model showed a significantly higher correlation with observations than simulations with the standard phenology. The interannual variability of LAI, the representation of years with extremely warm spring or extremely dry summer, and the start of the growing season also improved with the new phenology module. The number of hot days with maximum temperature exceeding the 90th percentile and heavy precipitation events (> 20mm) with the new phenology are in very good agreement with the observations. We also show that lower LAI values in summer lead to a decrease of latent heat flux in the model due to less evapotranspiration. The CCLM simulation with improved representation of the phenology should be used in future applications with an extension on more plant functional types.