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.
Habitat
Native Range:-
Silk-oak is native to coastal eastern Australia from the Clarence River, New South Wales, to Maryborough, Queensland, and is now naturalized in Hawaii and southern Florida. It was introduced into Hawaii about 1880 and is found on all islands where it reproduces prolifically in certain leeward grassland locations. Although a nongregarious tree in its native habitat, it grows well in pure plantations in Hawaii. It is common as an ornamental in Hawaii, Florida, California, and Puerto Rico. Because of its prolific reproduction, it has been classed a noxious weed on ranchland in Hawaii. In the tropical highlands of India, where it has also been extensively planted, it is often an undesirable escapee from cultivation.
Climate
In Hawaii, silk-oak has been planted extensively in both wet and dry locations on all islands from near sea level to more than 900 in (3,000 ft) elevation. The mean temperature ranges from 10° to 26° C (50° to 78° F) within this elevational range, with extremes of 4° and 35° C (40° and 95° F). Silk-oak, for many years, was thought to be best suited for planting in and areas because of its success as a seedling and sapling in such areas. Later it became apparent that frequent severe moisture stress in the dry areas (less than 760 mm [30 in] annual rainfall) caused disease susceptibility resulting in dieback as the trees became older. Natural reproduction, however, was sometimes excellent in these dry locations. The largest silk-oak trees in Hawaii grow in 3050 min (120 in) winter maximum or evenly distributed annual rainfall at 610 m (2,000 ft) elevation, but the most prolific natural reproduction coupled with excellent growth occurs in 1780 to 2400 min (70 to 95 in) evenly distributed annual rainfall at 460 to 670 in (1,500 to 2,000 ft) elevation. Elsewhere than Hawaii, silk-oak is reported to be capable of withstanding occasional light frosts but must be considered frost-tender. It is also reported elsewhere to be fairly hardy to drought but tends to die back on droughty sites at 15 to 20 years of age.
Soils and Topography
Silk-oak is tolerant of a wide range of soils if they are well drained. It will grow on neutral to strongly acid soils but does best on those that are slightly acid. In Hawaii, good growth is achieved on soils of a wide range of orders. Silk-oak grows well on Histosols, Inceptisols, and Ultisols. The majority of the best stands are on Dystrandepts and Tropofolists developed on gentle to moderate slopes of basalt lava rock or ash.
Associated Forest Cover
Where planted in pure stands in Hawaii, silk-oak maintains its purity with little woody competition. In naturalized stands, it grows in association with many other tree species including the native koa (Acacia koa), ‘ohi ‘a (Metrosideros collina), and introduced species such as tropical ash (Fraxinus uhdei), jacaranda (Jacaranda mimosifolia), molucca albizzia (Albizia falcataria), black-wattle (Acacia decurrens), Christmas-berry (Schinus terebinthifolius), and guava (Psidium guajava).