Spotted gum

Scientific name

Corymbia citriodora subsp. variegata, C. citriodora subsp. citriodora, C. maculata, C. henryi. Family: Myrtaceae.

Local names

Spotted gum, lemon-scented gum (C. citriodora subsp. citriodora only), spotted irongum.

Description and natural occurrence

On favourable sites, these species grow to 45 m in height and 1.3 m in stem diameter, but attain only half these dimensions on poorer sites. They have straight, slender trunks with smooth bark. The bark is shed in patches, giving the species its characteristic spotted appearance. Colour tones range from pink to grey-blue.

Corymbia citriodora subsp. variegata occurs mainly in the coastal areas of northern New South Wales and southern Queensland, but also in western areas of southern Queensland. C. citriodora subsp. citriodora grows from the mid-north coast of NSW to the Windsor Tableland, North Queensland. C. maculata occurs from Bega (NSW) to the mid-north NSW coast, and also a disjunct occurrence in eastern Victoria. C. henryi grows in northern New South Wales and southern Queensland.

Sawn timber from these species is generally available, and spotted gum is currently the highest volume native hardwood harvested in Queensland. Future supplies of plantation-grown spotted gum should be available from most regions in central and southern Queensland on suitable soils and where the mean annual rainfall exceeds 600 mm.

Wood appearance

Colour. The heartwood ranges from light brown through to dark red-brown. Sapwood is usually white and up to 50 mm wide.

Grain. Moderately coarse textured and variable. Gum veins common. The presence of wavy grain can produce an attractive fiddleback figure.

Properties of mature, natural grown timber

Density. 1010 kg/m3 at 12% moisture content; approximately 1.0 m3 of seasoned sawn timber per tonne. The density of plantation-grown timber of C. citriodora subsp. variegata aged 11 and 41 years: 87% and 108% mature timber density. C. citriodora subsp. citriodora aged 3 years: 71% mature timber density.

Strength groups. C. citriodora and C. henryi (S2); C. maculata S2 unseasoned. C. citriodora and C. henryi (SD2); C. maculata SD2 seasoned (brackets indicate provisional value).

Stress grades. F11, F14, F17, F22 (unseasoned), F17, F22, F27, F34 (seasoned), when visually stress graded in accordance with AS 2082:2000, Timber - hardwood - visually stress-graded for structural purposes.

Joint groups. J1 unseasoned; JD1 seasoned.

Shrinkage to 12% MC. (The shrinkage values apply to C. citriodora subsp. variegata). Tangential 6.1%, radial 4.3%. Plantation-grown (41 years):tangential 5.8%, radial 3.4%.

Unit shrinkage. Tangential 0.4%, radial 0.3% (natural and plantation grown).

Durability above-ground. Class 1 - life expectancy over 40 years.

Durability in-ground. Class 2 - life expectancy 15 to 25 years.

Lyctine susceptibility. Untreated wood susceptible to lyctid borer attack.

Termite resistance. Resistant.

Preservation. Sapwood readily accepts preservative impregnation but penetration of heartwood is negligible, using currently available commercial processes.

Seasoning. Can be satisfactorily dried using conventional air and kiln seasoning methods.

Hardness. Very hard (rated 1 on a 6 class scale) in relation to resistance to indentation and ease of working with hand tools.

Machining. Machines well due to its natural greasiness.

Fixing. No difficulty has been experienced with the use of standard fittings and fastenings.

Gluing. As with most high-density species, machining and surface preparation should be done immediately before gluing.

Finishing. Will readily accept paint, stain and polish. Has lower tannin content than most other eucalypts, therefore staining of paintwork, brickwork etc., as a result of water running over unpainted timber surfaces, is less likely to occur.

Uses

Engineering. As sawn or round timber in wharf and bridge construction, railway sleepers, cross-arms, poles, piles and mining timbers.

Construction. As unseasoned timber in general house framing and as seasoned dressed timber in cladding, internal and external flooring, linings and joinery. Also in fencing, landscaping, retaining walls and as structural plywood and hardboard.

Decorative. Internal fine furniture, outdoor furniture, turnery, joinery, parquetry.

Others. Tool handles, boat building (keel and framing components, planking, decking), coach, vehicle and carriage building, agricultural machinery, sporting goods (baseball bats, croquet mallets, spring and diving boards, parallel bars) and bent work. It has been used for butcher´s blocks, meat skewers, mallet heads, ladder rungs, wheel spokes, wine casks and broom handles. Spotted gum is the main Australian species for tool handles which are subjected to high impact forces, such as axe handles.

Identification features

General characteristics

Sapwood. White in colour and distinct from heartwood.

Heartwood. Colour variable from light brown to dark red-brown often with lighter shades.

Texture. Open, often with interlocked grain; greasy to touch.

Wood structure

Growth rings. Absent.

Vessels. Small to moderately large, generally arranged in short radial multiples with few solitary. Vessel lines very prominent on dressed longitudinal surfaces. Tyloses abundant.

Parenchyma. Abundant, paratracheal (surrounding pores) and diffuse with a tendency to zonate arrangement.

Rays. Fine, visible in tangential section.

Other features

Burning splinter test. Splinter burns to a complete white ash.

Further reading

Boland, DJ, Brooker, MIH, Chippendale, GM, Hall, N, Hyland, BPM, Johnston, RD, Kleinig, DA and Turner, JD 2006, 'Forest trees of Australia', 5th edn, CSIRO Publishing, Collingwood Australia.

Bootle, K 2005, 'Wood in Australia: types, properties and uses', 2nd edn, McGraw-Hill, Sydney.

Hopewell, G (ed.) 2006, 'Construction timbers in Queensland: properties and specifications for satisfactory performance of construction timbers in Queensland, Class 1 and Class 10 buildings', books 1 and 2, Department of Primary Industries and Fisheries, Brisbane.

Ilic, J 1991, 'CSIRO atlas of hardwoods', Crawford House Press, Bathurst, Australia.

Standards Australia, 2000, 'AS 2082-2000: Timber - hardwood - visually stress-graded for structural purposes', Standards Australia.

Last updated 03 August 2010