The yellow giant hyssop (Agastache nepetoides) is an impressive herbaceous perennial plant that can reach over 2 metres in height. In the Greater Montréal region (Québec, Canada), it grows on calcareous soils in semi-open habitats. This species is likely to be designated threatened or vulnerable in Québec, and the local population in the Oka National Park appears to be the only one of a viable size. Surveys conducted from 2010 to 2020 identified 6 occurrences (well-delimited groups of individuals). Two of these disappeared over this period and 2 were newly discovered. Our monitoring of marked plants in 2 of the occurrences showed that the species grows fast, flowers, and dies after 3-4 years. The number of individuals fluctuated greatly over time, reflecting the short life cycle of the species and its association with ephemeral ecological conditions. Twenty-year population projections for the 2 monitored occurrences indicate that in the long-term, they are vulnerable. Three threat mitigation measures are proposed to ensure the survival of the species in the park: i) maintain semi-open habitat, ii) maintain at least 200 individuals distributed among at least 3 occurrences, and iii) introduce the species to suitable habitat in the Grande Baie sector.