Spotted gum

Scientific name

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

Other names

Lemon-scented gum (C. citriodora subsp. citriodora only); spotted iron gum

Description

  • Grows to 45m high and 1.3m stem diameter on favourable sites otherwise to only half.
  • Straight, slender trunks with smooth bark.
  • Bark is shed in patches, giving its typical spotted appearance.
  • Bark is pink to grey-blue.

Occurrence

  • 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 separately in eastern Victoria.
  • C. henryi grows in northern New South Wales and southern Queensland.
  • Sawn timber 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 600mm.

Appearance

Colour

  • Heartwood is light brown to dark red-brown.
  • Sapwood is usually white and up to a 50mm wide band.

Grain

  • Texture is moderately coarse and varies.
  • Gum veins are common.
  • Wavy grain, when present, can produce an attractive fiddleback figure (pattern).

Uses

Engineering

  • Sawn or round timber to construct wharves and bridges, railway sleepers, cross-arms, poles, piles and mining timbers.

Construction

  • Unseasoned timber in general house framing
  • Seasoned dressed timber in cladding, internal and external flooring, linings and joinery.
  • Fencing, landscaping, retaining walls and 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.
  • Used, in the past, 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.

Properties

  • Density: 1010kg/m3 at 12% moisture content, about 1.0m3 of seasoned sawn timber per tonne; plantation-grown mature timber of C. citriodora subsp. variegata—87% (11 years), 108% (41 years); C. citriodora subsp. Citriodora—71% (3 years).
  • Strength groups: C. citriodora and C. henryi—(S2), C. maculata—S2 unseasoned; C. citriodora and C. henryi—(SD2), C. maculate—SD2 seasoned (brackets indicate provisional value).
  • Stress grades: F11, F14, F17, F22 (unseasoned); F17, F22, F27, F34 (seasoned) when visually stress-graded according to AS 2082—2000: Timber— Hardwood—Visually stress-graded for structural purposes.
  • Joint groups: J1 unseasoned, JD1 seasoned.
  • Shrinkage to 12% MC: C. citriodora subsp. variegate—6.1% (tangential), 4.3% (radial); plantation-grown (41 years)—5.8% (tangential), 3.4% (radial).
  • Unit shrinkage: 0.4% (tangential), 0.3% (ra dial)—natural and plantation grown.
  • Durability above-ground: Class 1 (life expectancy more than 40 years).
  • Durability in-ground: Class 2 (life expectancy 15–25 years).
  • Lyctine susceptibility: untreated wood susceptible to lyctid borer attack.
  • Termite resistance: resistant.
  • Preservation: sapwood readily impregnates with preservative, but penetration of heartwood is negligible using available commercial processes.
  • Seasoning: satisfactorily dries using conventional air and kiln seasoning.
  • Hardness: very hard (rated 1 on a 6-class scale) to indent and work with hand tools.
  • Machining: machines well due to its natural greasiness.
  • Fixing: no difficulty using standard fittings and fastenings.

Identification features

General characteristics

  • Sapwood: white and distinct from heartwood.
  • Heartwood: 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 a few solitary; vessel lines are very prominent on dressed longitudinal surfaces; abundant tyloses.
  • Parenchyma (soft tissue):abundant; paratracheal (surrounding pores) and diffuse, often zonate arrangements.
  • Rays: fine, visible in tangential section.

Other features

  • Burning splinter test: splinter burns to a complete white ash.

Research and resources

  • Boland, DJ, Brooker, MIH, Chippendale, GM, Hall, N, Hyland, BPM, Johnston, RD, Kleinig, DA and Turner, JD 2006, Forest trees of Australia, 5th ed., CSIRO Publishing, Collingwood, Australia.
  • Bootle, K 2005, Wood in Australia: Types, properties and uses, 2nd ed., McGraw-Hill, Sydney.
  • Ilic, J 1991, CSIRO atlas of hardwoods, Crawford House Press, Bathurst, Australia.
  • Queensland Government, DAF 2018, Construction timbers in Queensland: Properties and specifications for satisfactory performance of construction timbers in Queensland. Class 1 and Class 10 buildings, Books 1 & 2, Department of Agriculture and Fisheries, Brisbane.
  • Standards Australia, 2000, AS 2082–2000: Timber—Hardwood—Visually stress-graded for structural purposes, Standards Australia International, Strathfield, NSW.