Grevillea robusta A. Cunn.
Proteaceae -- Protea family
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.
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 (3,16). 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 (18). It is common as an ornamental in Hawaii, Florida, California, and Puerto Rico (5). Because of its prolific reproduction, it has been classed a noxious weed on ranchland in Hawaii (9). In the tropical highlands of India, where it has also been extensively planted, it is often an undesirable escapee from cultivation (13).
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 (9). 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 (16). 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 (2).
Silk-oak is tolerant of a wide range of soils if they are well drained (16). It will grow on neutral to strongly acid soils but does best on those that are slightly acid (2,12). 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.
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).
Although it is most commonly grown in nurseries and planted, silk-oak regenerates naturally at the edges of plantations, in openings within plantations, and under open stands of other species. It does not regenerate directly underneath itself either in closed stands or under open-grown trees. Natural regeneration is reported from India, Tanzania, and Queensland, Australia (at the edges of plantations), as well as from Hawaii (6,9,11,12,14,16,18).
Flowering and Fruiting- In Hawaii, silk-oak flowers from March through October, with the peak of flowering usually in June. The perfect yellowish orange, showy flowers are borne on 8- to 18-cm (3 to 7-in) long racemes that occur in panicles of one to several branches (3). Trees usually begin to flower at about 10 years. The fruit, a podlike follicle, 20 mm (0.8 in) in diameter, is slightly flattened and has a long-curved style. The hard dark-brown to black follicle splits open in late fall to release the one or two seeds it contains but remains on the tree up to 1 year after opening. Trees near San Jose in California have been observed to flower, fruit, and seed at times similar to those in Hawaii.
Seed Production and Dissemination- Silk-oak is a prolific seeder. Seeds are about 10 mm (0.4 in) long, flattened, and surrounded by a membranous wing. There are reported to be 64,000 to 154,000 seeds per kilogram (29,000 to 70,000/lb). Because of their relatively large wing, the lightweight seeds are widely disseminated by wind. Possibly because seedfall coincides with the onset of winter rains in dry leeward rangeland in Hawaii, regeneration is most prolific on these sites.
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).
Seedling Development- Germination is epigeal. Seedlings are grown in flats or containers in nurseries. Methods vary among the countries where silkoak is grown. In some countries 4- to 6-week-old wildings are lifted and potted and later replanted (2). Elsewhere plants are grown to 45-cm (18-in) heights in large baskets so that they can compete when outplanted (12). In Hawaii, seedlings in individual containers can be grown to a plantable size of 20 cm (8 in) height and 4 mm (0. 16 in) caliper in 12 to 14 weeks.
Vegetative Reproduction- Silk-oak coppices when cut. After being damaged by fire, a 5-year-old stand in Karnataka State, India, was cut. One year later, 93 percent of the stumps had coppiced. After 2 years 72 percent of the stumps still retained the coppice shoots, which by then averaged 4 m (13 ft) in height (1). As far as is known, vegetative propagation has not been practiced with the species.
Growth and Yield- In Hawaii, the tree usually produces a straight, erect stem even when open-grown (15). Where subjected to drought stress sufficiently severe to cause dieback, it forms forks and multiple leaders. On good sites (500 m; 1,600 ft altitude; 2030 mm.; 80 in annual rainfall), dominant trees planted at spacing of 3 by 3 m (10 by 10 ft) can be expected to be 8 to 9 m (25 to 30 ft) tall in 5 years, 15 m (48 ft) in 10 years, and 20 m (65 ft) or more in 20 years (11). Mean annual increment of dominants on 21 different sites in Uganda, for trees 2 to 20 years in age, ranged from 1.3 to 3.3 cm (0.5 to 1.3 in) in diameter and from 0.5 to 3.4 m (1.7 to 11.2 ft) in height (4). This indicates that the tree is fast growing as a sapling and pole.
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).
Rooting Habit- Silk-oak does not develop a strong taproot and roots shallowly on sites that lack moisture stress (16). On droughty sites it roots throughout the soil profile to depths of about 2 in (6 ft).
Reaction to Competition- Silk-oak is classed as very intolerant of shade. In Australia, seedlings do not survive beneath closed pure stands of the species because of some substance toxic to them that is produced by or associated with roots of the trees (18). This substance is specific to silk-oak seedlings, causing rapid chlorosis, blackening, and death of seedlings soon after they emerge and begin to grow. Consequently, the tree is nongregarious in its natural habitat. The toxic substance has not been investigated in Hawaii, but it has been observed that reproduction is lacking within dense stands or directly beneath individual trees.
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).
Damaging Agents- The oleander pit scale, Asterolecanium pustulans Cockerell, was so damaging in Puerto Rico that further planting of the species was discouraged (7). Amphichaeta grevilleae is a serious leaf spot and defoliating disease in India where it kills young plants (14). Also in India, a serious dieback is caused by a fungus, Corticium salmonicolor (8). No serious primary insects or diseases of the species have been noted in Hawaii, although severe dieback, believed caused by drought, is common on most droughty sites.
Grevillea robusta is a popular ornamental because of its fernlike foliage even in areas where it does not flower abundantly, such as California and Florida north of Miami. In more tropical climates its showy flowers cause it to be widely used.
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).
No studies of the genetics of the species have been reported (2). A test of 11 different genera in Brazil showed 1-year-old silk-oak seedlings to be the most uniform in height growth (Silva and Reichmann 1975 cited in 2).