Persoonia glaucescens, commonly known as the Mittagong geebung, is a species of flowering plant in the family Proteaceae and is endemic to New South Wales. It is an erect shrub with smooth bark, hairy young branchlets, lance-shaped leaves with the narrower end towards the base, and yellow flowers. It is the only persoonia in eastern Australia with strongly glaucous leaves. Description Persoonia glaucescens is an erect shrub that typically grows to a height of , sometimes to , with smooth bark, brownish red branches and branchlets that are covered with greyish hairs when young. The leaves are narrow spatula-shaped to lance-shaped with the narrower end towards the base, strongly glaucous, especially when young, long, wide and twisted through 90° at the base. The flowers are arranged in groups of up to thirty along a rachis up to long, each flower on a hairy pedicel long. The tepals are yellow, long and sparsely to moderately hairy on the outside. Flowering occurs from January to May and the fruit is a fleshy green drupe with purple markings, and contain a single seed. This species is the only persoonia in eastern Australia to have strongly glaucous leaves. Taxonomy Persoonia glaucescens was first formally described in 1827 by Kurt Polycarp Joachim Sprengel in the 17th edition of Systema Vegetabilium from an unpublished description by Franz Sieber. In 1848, Stephan Endlicher reduced the species to a variety as Persoonia lanceolata var. glaucescens. Lawrie Johnson originally thought that P. lanceolata and the present species grew in separate areas, and gave it the name Persoonia subsp. B, but more recent fieldword by Peter Weston showed that they do sometimes grow together without appearing to interbreed. As a result, Weston reinstated the name P. glaucescens, publishing the change in the journal Telopea. Within the genus, P. glaucescens is classified in the Lanceolata group, a group of 58 closely related species with similar flowers but very different foliage. These species will often interbreed with each other where two members of the group occur. Distribution and habitat Mittagong geebung occurs in small scattered patches in an area bordered by Buxton to the north and Berrima to the south, having vanished from the vicinities of Thirlmere Lakes and Fitzroy Falls. It grows on clay and gravel soils over laterite on upper slopes and ridges, at altitudes between . It is found in dry sclerophyll eucalypt forest with such trees as red