The floral diversity contributes to invaluable ecosystem services and habitat values in a rich forested area. The Hirikatu Oya sub-watershed in the southwestern central mountain area of Sri Lanka is also one of the important forested areas with many recent challenges. This study aims to examine current changes in floral species in the Hirikatu Oya sub-watershed from the year 2000 to 2022. For the study, primary data were collected from a field survey of 25 stratified purposive sample sites (20 m × 20 m plots), along with 30 questionnaire surveys and 14 interviews. Primary-level findings were blended with secondary data sources. A mixed-methods strategy was employed to analyze the data. The study area is home to a diverse range of plant species, including woody, medicinal, tree species as well as, endemic and invasive plant species. Notably, the study revealed that 7 invasive plant species are available, while 4 plant species are already extinct and about 14% of tree species and 50% of vines are threatened. The outcome of the study highlights the need for effective management strategies and continuous efforts need to be made to strengthen management techniques, enforce relevant rules, encourage community involvement, and implement targeted initiatives for the sustainability of the forested area in the Hirikatu Oya sub-watershed in Sri Lanka.