The grass family is responsible for most of peoples pollen allergies, and the severity of pollen-based asthma and allergies is expected to increase with global climate change. Identifying grass species through standard pollen monitoring techniques have limitations due to challenges in species-specific pollen identification. As a result, these monitoring methods end up grouping all Poaceae species together, even though there are hundreds of grass species in Europe with flowering times that may vary drastically among species. Given this lack of specificity, it is hard to know which grass species are responsible for causing allergies over the pollen season, and how different species are affected by climate change. To address these issues, we obtained phenological data from thousands of herbarium specimens collected across Denmark spanning 190 years and used pollen monitoring data collected over the last four decades to determine the response of flowering time to climate change for 12 allergenic grass species, and identify which species are likely the biggest contributors to grass pollen loads throughout the pollen season. We find that pollen season duration is lasting longer and starting earlier, and the maximum pollen loads are occurring earlier in response to climate warming. Herbarium specimens provide taxonomic resolution and reveal that many grass species are flowering earlier in response to warmer spring temperatures. Seven out of the 12 species studied in Denmark are identified as major contributors to airborne pollen based on their flowering times, relative abundance and overlap with the time of the year when maximum pollen loads are detected. Four species (Poa pratensis, Dactylis glomerata, Festuca rubra, Holcus lanatus) significantly shifted their flowering time in response to warming temperatures and are flagged as of particular concern to allergy sufferers. Using data derived from natural history collections can contribute to the advancement of pollen forecasting for asthma and allergy patients under both current conditions and amidst future global changes.