robusta A. Cunn. Silk-

Proteaceae family Roger G. Skolmen

Silk-oak (Grevillea robusta), also often called sil- cellent growth occurs in 1780 to 2400 mm (70 to 95 ver-oak, is a medium to large tree commonly planted in) evenly distributed annual rainfall at 460 to 670 as an ornamental in many warm-temperate and m (1,500 to 2,000 ft) elevation. Elsewhere than semitropical climates. It has been established as a , silk-oak is reported to be capable of forest tree in some countries and shows promise as withstanding occasional light frosts but must be con- a fast-growing timber tree. sidered frost-tender (16). It is also reported else- where to be fairly hardy to drought but tends to die Habitat back on droughty sites at 15 to 20 years of age (2). Soils and Topography Native qange

Silk-oak is native to coastal eastern Australia from Silk-oak is tolerant of a wide range of soils if they the Clarence River, , to are well drained (16). It will grow on neutral to Maryborough, , and is now naturalized in strongly acid soils but does best on those that are Hawaii and southern Florida (3,16). It was intro- slightly acid (2,121. In Hawaii, good growth is duced into Hawaii about 1880 and is found on all achieved on soils of a wide range of orders. Silk-oak islands where it reproduces prolifically in certain grows well on Histosols, Inceptisols, and Ultisols. The majority of the best stands are on Dystrandepts leeward grassland locations. Although a non- and Tropofolists developed on gentle to moderate gregarious tree in its native habitat, it grows well in slopes of basalt lava rock or ash. pure plantations in Hawaii (18). It is common as an ornamental in Hawaii, Florida, California, and Puer- to Rico (5). Because of its prolific reproduction, it has Associated Forest Cover been classed a noxious weed on ranchland in Hawaii (9). In the tropical highlands of India, where it has Where planted in pure stands in Hawaii, silk-oak also been extensively planted, it is often an un- maintains its purity with little woody competition. In desirable escapee from cultivation (13). naturalized stands, it grows in association with many other tree species including the native koa Climate (Acacia koa), ‘ohi ‘a (Metrosideros collina), and intro- duced species such as tropical ash (Fraxinus uhdei), In Hawaii, silk-oak has been planted extensively jacaranda (Jacaranda mimosifolia), molucca albizzia in both wet and dry locations on all islands from near (Albizia falcataria), black-wattle (Acacia decurrens), sea level to more than 900 m (3,000 ft) elevation (9). Christmas-berry (Schinus terebinthifolius), and The mean temperature ranges from 10” to 26” C (50” guava (Psidium guajava). to 78” F) within this elevational range, with extremes of 4” and 35” C (40” and 95” F). Silk-oak, for many Life History years, was thought to be best suited for planting in arid areas because of its success as a seedling and Reproduction and Early Growth sapling in such areas. Later it became apparent that frequent severe moisture stress in the dry areas (less Although it is most commonly grown in nurseries than 760 mm 130 in] annual rainfall) caused disease and planted, silk-oak regenerates naturally at the susceptibility resulting in dieback as the trees be- edges of plantations, in openings within plantations, came older. Natural reproduction, however, was and under open stands of other species. It does not sometimes excellent in these dry locations. The regenerate directly underneath itself either in closed largest silk-oak trees in Hawaii grow in 3050 mm stands or under open-grown trees. Natural regenera- (120 in) winter maximum or evenly distributed an- tion is reported from India, Tanzania, and nual rainfall at 610 m (2,000 ft) elevation, but the Queensland, Australia (at the edges of plantations), most prolific natural reproduction coupled with ex- as well as from Hawaii (6,9,11,12,14,16,18).

Flowering and Fruiting-In Hawaii, silk-oak The author is Principal Silviculturist (retired), Pacific Southwest flowers from March through October, with the peak Forest and Range Experiment Station, Berkeley, CA. of flowering usually in June. The perfect yellowish-

370 Grevillea robusta

series. Methods vary among the countries where silk- oak is grown. In some countries 4- to 6-week-old wildings are lifted and potted and later replanted (2). Elsewhere are grown to 45cm (18-in) heights in large baskets so that they can compete when out- planted (12). In Hawaii, seedlings in individual con- tainers 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 R) in height (1). As far as is known, vegetative propaga- tion has not been practiced with the species.

Figure l-Silk-oak flowers and leaves. Sapling and Pole Stages to Maturity orange, showy flowers are borne on 8- to l&cm (3- Growth and Yield-In Hawaii, the tree usually to 7-in) long racemes that occur in panicles of one to produces a straight, erect stem even when open- several branches (3). Trees usually begin to flower at grown (15). Where subjected to drought stress suffi- about 10 years (fig. 1). The fruit, a podlike follicle, ciently severe to cause dieback, it forms forks and 20 mm (0.8 in) in diameter, is slightly flattened and multiple leaders. On good sites (500 m; 1,600 ft al- has a long-curved style. The hard dark-brown to titude; 2030 mm; 80 in annual rainfall), dominant black follicle splits open in late fall to release the one trees planted at spacing of 3 by 3 m (10 by 10 ft) can or two seeds it contains but remains on the tree up be expected to be 8 to 9 m (25 to 30 ft) tall in 5 years, to 1 year after opening. Trees near San Jose in 15 m (48 ft) in 10 years, and 20 m (65 ft) or more in California have been observed to flower, fruit, and 20 years (11). Mean annual increment of dominants seed at times similar to those in Hawaii. 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) Seed Production and Dissemination-Silk-oak in diameter and fi-om 0.5 to 3.4 m (1.7 to 11.2 ft) in is a prolific seeder. Seeds are about 10 mm (0.4 in) height (4). This indicates that the tree is fast growing long, flattened, and surrounded by a membranous as a sapling and pole. wing. There are reported to be 64,000 to 154,000 Many plots have been measured in 32- to 48-year- seeds per kilogram (29,000 to 70,00O/lb). Because of old silk-oak plantations in Hawaii (11). All the plan- their relatively large wing, the lightweight seeds are tations had been planted at 3 by 3 m (10 by 10 ft) widely disseminated by Cind. Possibly because seed- and left untended since planting. Average d.b.h. of fall coincides with the onset of winter rains in dry dominant and codominant trees at 44 years in four leeward rangeland in Hawaii, regeneration is most of the plots was 46 cm (18 in), and the average total prolific on these sites. height was 32 m (105 ft). The most outstanding The seeds, if kept at 10 percent or less moisture stand, at 36 years, yielded a mean annual increment content, can be stored for as long as 2 years at -7” of 17.5 m3/ha (1,250 fbm/acre) (11) (fig. 2). Typically, to 3” C (20” to 38” F) with little loss in germinability. merchantable trees in these untended stands were 36 Germination of fresh, unstratified seeds requires to 46 cm (14 to 18 in) d.b.h. with 9 to 11 m (30 to 36 about 20 days. Stratification at 3” C (38” F) for 30 ft) of branch-free stem. days, or a 48-hour water soak, substantially in- In India, trees reach 50 cm (20 in) diameter in 30 creases germinative capacity of seeds that have been years when grown at an initial spacing of 3 by 4 m stored (19). (10 by 13 ft) and thinned once at about 5 years, and again later if needed to maintain growth rate. Such Seedling Development-Germination is epigeal. stands yield about 140 m3/ha (2,000 ft3/acre) with Seedlings are grown in flats or containers in nur- another 70 m3/ha (1,000 R3/acre) from thinnings (13).

371 Grevillea robusta

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 m (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, caus- ing rapid chlorosis, blackening, and death of seed- lings soon after they emerge and begin to grow. Con- sequently, the tree is nongregarious in its natural habitat. The toxic substance has not been inves- tigated in Hawaii, but it has been observed that reproduction is lacking within dense stands or direct- ly 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 salig- na), and robusta eucalyptus (E. robusta). In Brazil, several spacing studies indicated that at 2 years, a spacing of 1 by 3 m (3 by 10 ft) resulted in the best height growth, but at 6 years, 2 by 2 m (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 m%a (213 to 265 ftYacre) throughout the life of the stand. In Hawaii, silk-oak has always been planted at a spacing of 3 by 3 m (10 by 10 ft) and left untended. In Uganda experiments, a number of thinnings were made at various ages, but with little apparent effect Figure 2-Silk-oak stand at Kapapala Ranch, Island of Hawaii, on mean annual diameter increment (4). which yielded 1470 m3/ha (105,000 fbmlacre) when harvested at age 36. Damaging Agents-The oleander pit scale, Asterolecanium pustulans Cockerell, was so damag- One 14-year-old plantation had a mean diameter of ing in Puerto Rico that further planting of the 27 cm (11 in) and height of 19 m (61 ft) and yielded species was discouraged (7). Amphichaeta grevilleae 217 m3/ha (3,100 ft3/acre) (13). Another author in India is a serious leaf spot and defoliating disease in India suggests that silk-oak at 10 to 15 years and 1,000 where it kills young plants (14). Also in India, a stems per hectare (370/acre) yields 10 to 12 m3/ha (143 serious dieback is caused by a fungus, Corticium to 172 ft3/acre) (10). In the western Himalayas, 6-year- salmonicolor (8). No serious primary insects or dis- old silk-oak had outgrown 45 other species, including eases of the species have been noted in Hawaii, al- such fast growers as Eucalyptus globulus, Populus x though severe dieback, believed caused by drought, euroamericana, and Albizia lebbek (17). is common on most droughty sites.

372 Grevillea robusta

5. Little, Elbert L., Jr. 1978. Important forest trees of the Special Uses United States. U.S. Department of Agriculture, Agriculture Grevillea robusta is a popular ornamental because Handbook 519. Washington, DC. 70 p. of its fernlike foliage even in areas where it does not 6. Mallikajunaiah, T. S. 1965. A note on the natural of Myforest, Forest flower abundantly, such as California and Florida regeneration Grevillea robusta. north of Miami. In more tropical climates its showy Department Mysore 1(4):31-33. 7. Marrero, J. 1950. Results of forest planting in the insular flowers cause it to be widely used. forests of Puerto Rico. Caribbean Forester 11(3):107-135. It has been planted extensively in India and Sri 8. Nayar, R. 1987. Die back in Grevillea robusta A. Cunn. Lanka as shade for tea, and in Hawaii, India, and Myforest 23(2):89-93. Brazil to some extent as shade for coffee (2,12,14,16). 9. Nelson, R. E. 1960. Silk-oak in Hawaii...pest or potential It is frequently used as a windbreak, although timber? USDA Forest Service, Miscellaneous Paper 47. opinions differ as to its wind firmness and branch- Pacific Southwest Forest and Range Experiment Station, shedding tendencies (2). Silk-oak is an important Berkeley, CA. 5 p. honey tree in India where it is also regarded as a 10. Pandey, D. 1987. Yield models of plantations in the tropics. good fuelwood producer (13). Unasylva 39(3-4):74-75. The tree produces an attractively figured, easily 11. Pickford, G. D. 1962. Opportunities for timber production in worked wood, which was once a leading face veneer Hawaii. USDA Forest Service, Miscellaneous Paper 67. in world trade, where it was marketed as “lacewood.” Pacific Southwest Forest and Range Experiment Station, The wood contains an allergen that causes dermatitis Berkeley, CA. 11 p. for many people (15). 12. Rao, Y. R. A. 1961. Shade trees for coffee. II. Grevillea robusta A. Cunn. Indian Coffee, Bangalore 25(11):329-332. 13. Sagwal, S. S. 1984. Silver oak: a tree of many uses. Indian Genetics Farming 34(3):29-32. No studies of the genetics of the species have been 14. Sharma, Y. M. L. 1966 Silver oak (Grevillea robusta A. Cunn.). Myforest, Forest Department Mysore 3(1):35-42. reported (2). A test of 11 different genera in Brazil 15. Skolmen, Roger G. 1974. Some woods of Hawaii - properties showed l-year-old silk-oak seedlings to be the most and uses of 16 commercial species. USDA Forest Service, uniform in height growth (Silva and Reichmann 1975 General Technical Report PSW-8. Pacific Southwest Forest cited in 2). and Range Experiment Station, Berkeley, CA. 30 p. 16. Streets, R. J. 1962. Exotic forest trees in the British Literature Cited Commonwealth. Clarendon Press, Oxford. 765 p. 17. Toky, 0. P., and P. K. Khosla. 1984. Comparative growth of Basappa, B. 1986. Coppicing in silver oak (Grevillea robusta agroforestry trees (indigenous vs. exotic) in subtropical Cunn). Myforest 22( l):l-2. Fenton, R., R. E. Roper, and G. R. Watt. 1977. Lowland western Himalaya. Journal of Tree Sciences 3(1/2):93-98. tropical hardwoods. An annotated bibliography of selected 18. Webb, L. J., J. G. Tracey, and K. P. Haydock. 1967. A factor species with plantation potential. Ministry of Foreign Affairs, toxic to seedlings of the same species associated with living Wellington, . unpaged. roots of the nongregarious subtropical rainforest tree Francis, W. D. 1951. Australian rainforest trees. Forestry and Grevillea robusta. Journal of Applied Ecology 4( 1):13-25. Timber Bureau, Canberra. 469 p. 19. Wong, Wesley H. C., Jr. 1974. Grevillea robusta A. Cunn. Kriek, W. 1967. Report on species and provenance trials on Silk-oak. In Seeds of woody plants in the United States. p. montane forest sites in Kigezi District. Uganda Forest 437-438. C. S. Schopmeyer, tech. coord. U.S. Department of Department, Technical Note 140. Kampala. 67 p. Agriculture, Agriculture Handbook 450. Washington, DC.

373