Angophora costata

Angophora costata, commonly known as Sydney red gum, rusty gum or smooth-barked apple, is a species of tree that is endemic to eastern Australia. Reaching 30 m (100 ft) in height, the species has distinctive smooth bark that is pinkish or orange-brown when new and fades to grey with age. Its lance-shaped leaves are arranged in opposite pairs along the stems, with white or creamy white flowers appearing from October to December. The flower buds are usually arranged in groups of three, followed by ribbed, oval or bell-shaped fruit.

Angophora costata is a tree that typically grows to a height of 30 m (100 ft) and forms a lignotuber. It has smooth pinkish or orange-brown bark that weathers to grey and is shed in small scales. Young plants and coppice regrowth have sessile, elliptical to egg-shaped leaves arranged in opposite pairs with a stem-clasping base, 60–125 mm (2.5–5 in) long and 20–65 mm (0.8–2.6 in) wide. Adult leaves are arranged in opposite pairs, glossy green but paler on the lower surface, lance-shaped or curved, 70–190 mm (2.8–7.5 in) long and 12–35 mm (0.5–1.4 in) wide on a petiole 9–25 mm (0.4–1.0 in) long. New leaf growth is strongly tinted with red. The flower buds are arranged on the ends of branchlets on a branched peduncle 3–25 mm (0.1–1.0 in) long, each branch of the peduncle with usually three buds on pedicels 3–15 mm (0.1–0.6 in) long. Mature buds are oval to globe-shaped, up to 10 mm (0.4 in) long and 11 mm (0.4 in) wide. There are five sepals up to 3 mm (0.1 in) long and the petals are white to creamy white with a green keel, 3–5 mm (0.1–0.2 in) long and 3–6 mm (0.1–0.2 in) wide. Flowering occurs from October to December. The fruit is an oval or bell-shaped capsule up to 20 mm (0.8 in) long and wide. New seedlings have petiolate round cotyledon leaves 1.5 cm wide and long.

Sydney red gum was first formally described in 1788 by German botanist Joseph Gaertner and given the name Metrosideros costata in his book De Fructibus et Seminibus Plantarum, from material collected by surveyor David Burton around Port Jackson. The specific epithet (costata) is a Latin word meaning 'ribbed'. English botanist Richard Anthony Salisbury named it Metrisoderos apocynifolia in his 1796 work Prodromus stirpium in horto ad Chapel Allerton vigentium. Spanish taxonomist Antonio José Cavanilles erected the new genus Angophora in 1797 in his book Icones et Descriptiones Plantarum, giving the species the name Angophora lanceolata. English botanist George Bentham used this last name in his Flora Australiensis in 1867.

English botanist James Britten changed the name to Angophora costata in the Journal of Botany, British and Foreign in 1916.

In 1986, Gregory John Leach described three subspecies in the journal Telopea and two names have been accepted by the Australian Plant Census:

A third subspecies, subsp. leiocarpa was reclassified as a separate species Angophora leiocarpa. A phylogenetic analysis of the genus published in 2021 confirmed its genetic distinctness from the two costata subspecies.

Recent genetic work has been published showing Angophora to be more closely related to Eucalyptus than Corymbia, and the name Eucalyptus apocynifolia has been proposed for this species if it were to be placed in the genus Eucalyptus. This has not been taken up by the botanical community.

Common names include Sydney red gum, rusty gum, smooth-barked apple, and smooth-barked angophora. Aboriginal people from the Sydney region knew the tree as kajimbourra.