Hoary alyssum [Berteroa incana (L.) DC.] is a nonnative invasive forb that is noxious in California, Idaho, Michigan, Montana, Oregon, Washington, Wyoming, Alberta, British Columbia, and Saskatchewan. Managing B. incana is difficult, because it has an extended flowering period, during which plants simultaneously flower and produce seeds. Consequently, poorly timed herbicide applications may kill B. incana flowers but not prevent viable seed production. We examined how different herbicide management practices used by invasive plant managers affected B. incana seed production and viability the year of application as well as population density 1 yr after application. Professional invasive plant managers sprayed B. incana with various herbicides as part of their current management practices at six sites in southwestern Montana in summer 2016. We collected B. incana plants at 4 wk postapplication for seed biology analyses. Across the six sites, nonsprayed B. incana produced 5 to 1,855 seeds plant−1 and averaged 429 seeds plant−1. Seed production was reduced by 64% to 99% with 7 of the 11 herbicide applications. Berteroa incana seed viability in nonsprayed areas averaged 53% and ranged from 36% to 73% across the sites. Nine of the 10 herbicide applications used by invasive plant managers reduced seed viability 49% to 100%. Few of the herbicide management practices reduced B. incana’s population density the following growing season, suggesting that managers should expect reoccurring infestations at least 1 yr after application. Our results show that invasive plant managers can reduce B. incana viable seed production even when spraying plants that have flowered and formed seed pods. However, sites may need to be monitored for additional years to treat reoccurring infestations.