Grevillea robusta, commonly known as the southern silky oak, silk oak or silky oak, silver oak or Australian silver oak, is a flowering plant in the family Proteaceae. It is a tree, the largest species in its genus but is not closely related to the true oaks, Quercus. It is a native of eastern coastal Australia, growing in riverine, subtropical and dry rainforest environments.
Grevillea robusta is a fast-growing evergreen tree with a single main trunk, growing to 5–40 m (20–100 ft) tall. The bark is dark grey and furrowed. Its leaves are fern-like, 10–34 cm (4–10 in) long, 9–15 cm (4–6 in) wide and divided with between 11 and 31 main lobes. Each lobe is sometimes further divided into as many as four, each one linear to narrow triangular in shape. It loses many of its leaves just before flowering.
The flowers are arranged in one-sided, “toothbrush”-like groups, sometimes branched, 12–15 cm (5–6 in) long. The carpel (the female part) of each flower has a stalk 21–28 mm (0.8–1 in) long. The flowers are glabrous and mostly yellowish orange, or sometimes reddish. Flowering occurs from September to November and the fruit that follows is a glabrous follicle.
Taxonomy and naming
Grevillea robusta was first formally described in 1830 by Robert Brown after an unpublished description by Allan Cunningham. The type specimen was collected by Cunningham on the eastern edge of Moreton Bay in 1827. Brown’s description was published in Supplementum primum Prodromi florae Novae Hollandiae. The specific epithet (robusta) is a Latin word meaning “strong like oak” or “robust”.
Distribution and habitat
Silky oak occurs naturally on the coast and ranges in southern Queensland and in New South Wales as far south as Coffs Harbour where it grows in subtropical rainforest, and wet forests. It is now relatively rare in its natural habitat but has been widely planted, including on Norfolk Island and Lord Howe Island. It has become naturalised in many places, including on the Atherton Tableland in Australia and in South Africa, New Zealand, Hawaii, French Polynesia, Jamaica and Florida. It is regarded as a weed in parts of New South Wales and Victoria, as “invasive” in Hawaii and as an “invader” in South Africa.
Before the advent of aluminium, Grevillea robusta timber was widely used for external window joinery, as it is resistant to wood rot. It has been used in the manufacture of furniture, cabinetry, and fences. Owing to declining G. robusta populations, felling has been restricted.
Recently G. robusta has been used for side and back woods on guitars made by Larrivée and others, because of its tonal and aesthetic qualities.
When young it can be grown as a houseplant where it can tolerate light shade, but prefers full sun as it grows best in warm zones. If planted outside, young trees need protection on frosty nights. Once established it is hardier and tolerates temperatures down to −8 °C (18 °F). It needs occasional water but is otherwise fairly drought-resistant.
Grevillea robusta is often used as stock for grafting difficult-to-grow grevilleas.
Care needs to be taken when planted near bushland as it can be invasive.
The tree has been planted widely throughout the city of Kunming in south-western China forming shady avenues.
In the UK this plant has gained the Royal Horticultural Society‘s Award of Garden Merit.
Grevillea robusta is also grown in plantations in South Africa and can also be grown alongside maize in agroforestry systems.
Toxicity and allergic reactions
The flowers and fruit contain toxic hydrogen cyanide. Tridecylresorcinol in G.robusta is responsible for contact dermatitis.
silky oaks planted in a tea garden
unripe seed pods
leaves and dry seed pods
dry seed pod
seed pod and seeds
branch end with inflorescence
Grevillea robusta at Chikhaldara
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- “Grevillea robusta“. National Arboretum Canberra. Retrieved 18 June 2018.
- “Grevillea robusta“. APNI. Retrieved 20 June 2018.
- Brown, Robert (1830). Supplementum primum Prodromi florae Novae Hollandiae. London. p. 24. Retrieved 19 June 2018.
- Brown, Roland Wilbur (1956). The Composition of Scientific Words. Washington, D.C.: Smithsonian Institution Press. p. 556.
- “Grevillea robusta“. Australian Tropical Rainforest Plants. Retrieved 19 June 2018.
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- Overseas-grown Australian Timber Species Retrieved on 8 December 2008
- Jackson, N. (2000). “Tree pruning as a means of controlling water use in an agroforestry system in Kenya”. Forest Ecology and Management. 126 (2): 133–152. doi:10.1016/S0378-1127(99)00096-1.
- Everist, S.L., Poisonous Plants of Australia, Angus & Robertson, 1974.
- Menz, J., Rossi, R., Taylor, W.C, Wall, L., Contact dermatitis from Grevillea “Robyn Gordon“, Contact Dermatitis, Vol. 15, Iss. 3, pp 126-131, Apr 2006