Roger G. Skolmen
Silk-oak (Grevillea robusta), also often called silver-oak, is a medium to large tree commonly planted as an ornamental in many warm-temperate and semitropical climates. It has been established as a forest tree in some countries and shows promise as a fast-growing timber tree.
Derivation of specific name
Regularity: Regularly occurring
Native of East Australia; now common in tropics and subtropics
State - Kerala, District/s: All Districts"
In racemes on leafless stem; yellow to orange. Flowering from November-December.
Boat-shaped follicles, woody; seeds 2, winged. Fruiting December onwards.
Bark grey, rough, irregular and deeply fissured.
Soils and Topography
Associated Forest Cover
Diseases and Parasites
Reaction to Competition
In Hawaii, silk-oak has been planted in mixture with numerous other species. Two of the species it dominates when in mixture are melaleuca (Melaleuca quinquenervia) and horsetail casuarina (Casuarina equisetifolia). Three that grow well in mixture with it are Australian toon (Toona ciliata var. australis), tropical ash, and koa. Three that dominate silk-oak are Norfolk-Island-pine (Araucaria heterophylla), saligna eucalyptus (Eucalyptus saligna), and robusta eucalyptus (E. robusta).
In Brazil, several spacing studies indicated that at 2 years, a spacing of 1 by 3 in (3 by 10 ft) resulted in the best height growth, but at 6 years, 2 by 2 in (6 by 6 ft) was best, with thinning planned at age 10 or 15 (Viega 1958 as cited in 2). In Brazil, an attempt is made to maintain a basal area of 49 to 61 m²/ha (213 to 265 ft²/acre) throughout the life of the stand. In Hawaii, silk-oak has always been planted at a spacing of 3 by 3 in (10 by 10 ft) and left untended. In Uganda experiments, a number of thinnings were made at various ages, but with little apparent effect on mean annual diameter increment (4).
Life History and Behavior
Seed Production and Dissemination
The seeds, if kept at 10 percent or less moisture content, can be stored for as long as 2 years at -7° to 3° C (20° to 38° F) with little loss in germinability. Germination of fresh, unstratified seeds requires about 20 days. Stratification at 3° C (38° F) for 30 days, or a 48-hour water soak, substantially increases germinative capacity of seeds that have been stored (19).
Flowering and Fruiting
Growth and Yield
Many plots have been measured in 32- to 48-yearold silk-oak plantations in Hawaii (11). All the plantations had been planted at 3 by 3 m (10 by 10 ft) and left untended since planting. Average d.b.h. of dominant and codominant trees at 44 years in four of the plots was 46 cm (18 in), and the average total height was 32 m (105 ft). The most outstanding stand, at 36 years, yielded a mean annual increment of 17.5 m/ha (1,250 fbm/acre) (11). Typically, merchantable trees in these untended stands were 36 to 46 cm (14 to 18 in) d.b.h. with 9 to 11 m (30 to 36 ft) of branch-free stem.
In India, trees reach 50 cm (20 in) diameter in 30 years when grown at an initial spacing of 3 by 4 m (10 by 13 ft) and thinned once at about 5 years, and again later if needed to maintain growth rate. Such stands yield about 140 m³/ha (2,000 ft³/acre) with another 70 m³/ha (1,000 ft³/acre) from thinnings (13).
One 14-year-old plantation had a mean diameter of 27 cm (11 in) and height of 19 in (61 ft) and yielded 217 m³/ha (3,100 ft³/acre) (13). Another author in India suggests that silk-oak at 10 to 15 years and 1,000 stems per hectare (370/acre) yields 10 to 12 m³/ha (143 to 172 ft³/acre) (10). In the western Himalayas, 6-yearold silk-oak had outgrown 45 other species, including such fast growers as Eucalyptus globulus, Populus x euroamericana, and Albizia lebbek (17).
Molecular Biology and Genetics
Statistics of barcoding coverage: Grevillea robusta
Public Records: 0
Specimens with Barcodes: 16
Species With Barcodes: 1
National NatureServe Conservation Status
Rounded National Status Rank: NNA - Not Applicable
Relevance to Humans and Ecosystems
The timber is of medium strength and is used for furniture, packing cases, flooring, paneling, plywood and the manufacture of small wooden items such as pencils."
Planted as a shade tree in coffee and tea plantations.
It has been planted extensively in India and Sri Lanka as shade for tea, and in Hawaii, India, and Brazil to some extent as shade for coffee (2,12,14,16). It is frequently used as a windbreak, although opinions differ as to its wind firmness and branch-shedding tendencies (2). Silk-oak is an important honey tree in India where it is also regarded as a good fuelwood producer (13).
The tree produces an attractively figured, easily worked wood, which was once a leading face veneer in world trade, where it was marketed as "lacewood." The wood contains an allergen that causes dermatitis for many people (15).
Grevillea robusta, commonly known as the southern silky oak or silky oak, or Australian silver oak, is the largest species in the genus Grevillea of the family Proteaceae. It is not closely related to the true oaks, Quercus. It is a native of eastern coastal Australia, in riverine, subtropical and dry rainforest environments receiving more than 1,000 mm per year of average rainfall.
It is a fast-growing evergreen tree, between 18–35 m (59–115 ft) tall, with dark green delicately dented bipinnatifid leaves reminiscent of a fern frond. It is the largest plant in the Grevillea genus, reaching trunk diameters in excess of 1 m (3 ft). The leaves are generally 15–30 cm (6–12 in) long with greyish white or rusty undersides.
Its flowers are golden-orange bottlebrush-like blooms, between 8–15 cm (3–6 in) long, in the spring, on a 2–3 cm long stem and are used for honey production. Like others of its genus, the flowers have no petals, instead they have a long calyx that splits into 4 lobes. The seeds mature in late winter to early spring, fruiting on dark brown leathery dehiscent follicles, about 2 cm long, with one or two flat, winged seeds.
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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.
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 weedy.
The tree has been planted widely throughout the city of Kunming in south-western China forming shady avenues.
Toxicity and allergic reactions
- RHS A-Z encyclopedia of garden plants. United Kingdom: Dorling Kindersley. 2008. p. 1136. ISBN 1405332964.
- "Garden Guides: Silkoak".
- "Grevillea robusta AGM". RHS Plant Finder. Royal Horticultural Society. Retrieved 27 July 2013.
- 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
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