Grevillea robusta

Common name: Silky oak

Other common names: Fernleaf, Golden pine, Silk oak, Silver oak, Southern silky oak

Names in non-English languages: Spanish

Description

Silky oak is a flowering, timber and agroforestry tree originating in Australia, its natural range limited to pockets of subtropical forest on the east coast of the continent.

It is a fast-growing tree, to heights of up to 35 m (114 ft) in its natural habitat, though elsewhere it is more typically 15 to 25 m (50 to 80 ft) tall and develops a straight, round trunk supporting a pyramidal or rounded crown of ascending branches. The bark is grey-brown and fissured.

The leaves are fern-like, green on top, silvery beneath and cast a moderately dense shade. They stay on the tree in areas with a short dry season but fall where the dry season is long, leaving the branches mostly bare for a short period, just before flowering.

The flowers are thin, erect, orange-yellow tubes in long clusters that somewhat resemble a brush with bristles. They bloom in perfusion from spring to early summer, with the most striking displays seen on near leafless trees. 

The fruit are small, flat oval seedpods which, when mature, are dark brown to black with either one or two small winged seed inside.

Use

It is widely cultivated in urban parks in Australia for its spectacular flowering, ferny foliage and the shade it gives.

Long introduced into India, Sri Lanka and Kenya, it is cultivated there as a shade tree, to shade crops such as tea and coffee, on account of its fast growth and good adaptation to tropical highland climates. It is also cultivated for the production of fuelwood, for firewood and charcoal making, as well as for protective windbreaks and honey production.

The flowers produce good quantities of nectar and it is reported as a honey tree in Australia and in countries where it is introduced, including India, Tanzania, Guatemala and the Dominican Republic. The honey is dense, dark amber with a pronounced flavour and is quick to granulate. The nectar also helps to nourish and sustain nectar eating birds in its native range, making it an important wildlife.



The wood is in the 540 to 660 kgs per cubic meter (34 to 41 lbs per cubic ft) range, has attractively figured pinkish-brown heartwood and is moderately resistant to rot and decay, making it fit for indoor and some outdoor applications. 

Well-formed logs are sawn into planks used mostly for fine furniture, cabinets, interior flooring, interior house trim, doors, window-frames and the like. Selected logs are sliced for decorative veneer and the branchwood, apart from fuelwood, is used in turnery.

Climate

It flowers reliably and develops a good form in sub-humid to humid subtropical and tropical mid- to high-elevation climates, generally areas with annual lows of 9 to 19 °C, annual highs of 20 to 32 °C, annual rainfall of 700 to 2400 mm and a dry season of 7 months or less. In Hawaii, the best-developed trees are to be found in mid-elevation areas with annual rainfall of 1500 to 2000 mm.

Growing

New plants are mostly raised from seed, which remain viable for up to a year under cold, dry airtight storage and usually germinate within thirty days of sowing. It performs best on free-draining clay and loam soils of an acid to neutral nature, generally with a pH of 4.5 to 7.5 and on sites with full to partial sun exposure.

Problem features

The winged seeds are carried by the wind over long distances and germinate readily. It has naturalised in many areas where it is introduced and in some areas has become an undesirable weed. In South Africa, it is recorded as a serious invader of the natural ecosystem and it is assessed to be a high weed risk for Hawaii, by the Hawaii Pacific Weed Risk Assessment project (HPWRA). 

The branches are brittle and are prone to breakage in strong winds, potentially causing personal injury or damage to property. The flowers, fruit and sap may cause dermatitis in some and the fallen leaves create litter.

Where it grows

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