Antioxidant, antimicrobial and phytochemical analysis of three endemic Rhododendron spp. of Sikkim Himalaya

•Phytochemicals screening of R. dalhousiae, R. ciliatum and R. maddenii was performed.•Total flavonoids and phenols content were found maximum in methanol extracts of Rhododendron spp.•HPLC analysis identified the presence of various phenolic compounds in Rhododendron spp.•Among three species, high anti-oxidative potential was recorded for R. dalhousiae.•Rhododendron spp. exhibited antibacterial potential against pathogenic bacteria.