 The rare plants are mainly associated with the small exposures of dolomitic limestone in the southern part of the site.Purn Hill is one of only five British sites for the nationally rare coastal limestone umbellifer species Honewort ("Trinia glauca").White Rock-rose ("Helianthemum apenninum") is found at the site and hybridises with Common Rock-rose ("Helianthemum nummularium") to form the hybrid "H. x sulphureum".