3015

11: Auxiliary plants

Grevillea robusta A. Cunn. ex R. Br.

Suppl. prodr. fl. Nov. Holl.: 24 (1830).

PROTEACEAE

2n = 20

Silky oak, silver oak (En). Silkoak (Am). Indonesia: salamandar (Sundanese). Thailand: son-india (central). Vietnam: ng[aa]n hoa (northern), tr[ax]i ban (southern).

Grevillea robusta occurs naturally in Australia, over the latitudinal range of 26—30°S in southern Queensland and northern New South Wales from the coast up to 160 km inland. It has been introduced into warm temperate, subtropical and tropical highland regions around the world and is widely planted in India, Sri Lanka and many countries in Africa. It performs poorly in lowland tropical environments and is not grown very commonly in Malesia.

Grevillea robusta is used to provide high shade for tea and coffee plantations in southern Asia and Africa. It is now very popular in agroforestry systems in the highlands of east and central Africa, being planted in rows along farm boundaries and between fields, and scattered among crops. It is regarded as more compatible with crops on small farms than most other tree species. In Burma (Myanmar), it is planted on a limited scale as a shade tree in coffee plantations.
The wood is used for firewood, poles and sawn timber. The leaves are applied as mulch. Some farmers in Kenya use fresh leaves as a dry-season fodder supplement for cattle. Grevillea robusta has some value for honey production. It is planted in many countries, e.g. in Thailand, for shade and ornamental purposes because of its attractive fern-like foliage and brilliant orange floral display. The cut leaves are used in flower arrangements and young plants are grown as indoor pot plants in Europe.
Properties Leaves contain rutin, a quercetin glucoside (about 0.6 g per 100 g dry matter). Through contact with the leaves sensitive persons may develop contact dermatitis due to tridecylresorcinol, a chemical compound related to the allergen of Toxicodendron spp. (Anacardiaceae). Gum from Grevillea robusta exudates contains small amounts of hydroxyproline, a free amino acid, in addition to galactose and arabinose. The flower buds, fruits and seeds are cyanogenic. Seed extracts exhibit antifungal activity. The weight of 1000 seeds is 10—20 g.
The air-dry density of the heartwood is 550—600 kg/m³, that of sapwood and branches is lower. The sapwood is cream-coloured, heartwood pale pink or red-brown after drying. Broad rays give the wood an attractive appearance on both the quartersawn and backsawn faces. The sawn timber is of medium strength and is used for furniture, flooring, packing cases, and the manufacture of small wooden items. The wood is used for firewood in African countries, and trials indicate it is suitable for pulping. Mean fibre length is about 1.5 mm and width about 26 µm. Some fair-skinned people are allergic to the sawdust.

An erect, single-stemmed tree up to 25(—40) m tall with a stem diameter of 50 cm or more and with a strong taproot. Crown conical and symmetrical with major branches spaced at intervals of about 1 m, projecting upwards at a 45° angle. Bark dark greyish-brown, rugged, furrowed. Branchlets angular and ridged, subsericeous to tomentose, becoming glabrous on older growth. Leaves pinnate with (4—)10—20 pinnatifid segments, fern-like; petiole 1.5—6.5 cm long; blade in outline 10—34 cm x 9—15 cm, secondary lobes or segments entire or again lobed, lanceolate or rarely linear, terminal one mostly longer than 2.5 cm, margins recurved, upper surface glabrous, green, lower surface subsericeous, silvery. Inflorescence a raceme, 7—12 cm long, simple to 4-branched from near the base, borne on very short, leafless, tomentulose branches on the older wood, many-flowered and all flowers pointing one way; rachis slender to stoutish, glabrous; pedicel 1.5 cm long, glabrous; flowers borne in pairs, 2 cm long, bright orange to golden-yellow or golden-brown; tepals 4, glabrous inside and outside, tube 0.6—1 cm long, rolled back under the 3 mm long ovate limb, with concave apex of each tepal holding a small anther 0.1 cm long; receptacle slightly oblique with prominent disc; ovary glabrous, stipitate; style filiform, 1.5 cm long, protruding from a slit on the lower side of the perianth tube before the apex is free from the limb, accrescent during and after anthesis, ultimately straight and erect, bearing the small stigma at its apex. Fruit a 2-seeded follicle, broad, very oblique, boat-shaped, pointed, 1.5—2 cm long with a slender persistent style. Seed flat-ovoid, 13—19 mm x 8—10 mm x 0.8—0.9 mm, brown, with papery wing all around.

Seed germinates readily in a moist environment. The optimum temperature for germination is about 25°C. Seedling growth is quickest during the summer months at temperate latitudes, and during the wet season in tropical highlands. In its natural range Grevillea robusta is semi-deciduous, shedding most of its leaves in the dry spring. Substantial leaf fall is also noted in the dry season in tropical environments. When climate and soil are suitable and weed competition not severe, annual height and diameter increments of at least 2 m and 2 cm respectively are usually achieved for the first few years in row plantings on farms. Annual height increments of 3 m have been observed at the most favourable sites.
The tree first flowers when about 6 years old. In the region of natural occurrence, flowering occurs over a few weeks in October—November, but when planted in equatorial latitudes, flowering is sporadic throughout the year, or absent as in Jakarta. During flower development the style protrudes through a slit in the lower side of the perianth tube before the perianth apex opens, giving the flower a 'looped' appearance. At anthesis, the apex of the perianth opens and the perianth falls away, depositing pollen on the stigma. Studies in Australia show that at the time of pollen deposition the receptive cells of the stigma are still covered by protective cells. The stigma does not become sticky and receptive until several days after anthesis, by which time the pollen has usually fallen off. Birds attracted by the nectar produced by the scales around the gynophore are believed to be the principal pollinating agents. The period from fertilization to fruit maturity is about 2 months. Fruits open during hot, dry weather, releasing the seeds, which can be carried considerable distances by wind. Seeds exhibit no dormancy, and remain viable for at least two years if dried to below 8% moisture content and stored dry and cool (20°C or less).
Proteoid roots develop seasonally near the growing tips of the young roots; these regions with a cylindrical mass of finely-divided root hairs are believed to increase the plant's ability to take up water and nutrients under unfavourable conditions.

Grevillea robusta occurs naturally in two distinct habitats. It grows in riverine rain forest of the Castanospermum australe A. Cunn. & C. Fraser ex Hook. association, usually within a few dozen metres of the water's edge on soils of fairly high fertility and good moisture availability. It also occurs away from the rain forest along creeks and rivers in association with Casuarina cunninghamiana Miquel. The second major habitat is the vine forest dominated by Araucaria cunninghamii Aiton ex D. Don, which covers extensive areas including steep upper-valley slopes. This type of forest is found on basalt-derived soil. Grevillea robusta occurs at very low densities in these forests. Climatic studies in the areas of natural and planted occurrence indicate the following ecological requirements for satisfactory growth: mean maximum temperature of hottest month 25—31°C, mean minimum temperature of coldest month 2—12°C, mean annual temperature 14—23°C, mean annual rainfall 700—1700 mm, dry season 0—6 months (0—4 months on shallow soils or towards the hotter extreme of the acceptable temperature range). Rainfall distribution has a summer maximum in the region of natural occurrence, but Grevillea robusta also grows well in climates with a winter maximum or a bimodal rainfall distribution. Growth is best over the altitudinal range of 130—2300 m at equatorial latitudes, the preferred altitudinal range decreasing to 0—1000 m at 30° latitude. In temperate areas Grevillea robusta can survive moderate winter frosts. It is not resistant to persistent strong winds.
Soils should be of moderate to high fertility with pH of 4.5—8; heavy clays and waterlogging are not tolerated. On acid soils, symptoms of boron deficiency and manganese toxicity have been observed.

Grevillea robusta can be propagated by seed and by cuttings. No pretreatment of seed is required for germination. Seeds are usually germinated on loamy soil with a thin covering of sand. Seedlings are pricked out when their second leaf pair starts to develop and are put into tubes containing a fertile loamy potting mix. Seedlings are grown in the nursery until planting out during the rainy season when a height of 20—40 cm is attained. Farmers also obtain planting stock by digging up wildlings. Cuttings can easily be established using shoots from seedlings or saplings, which can also be air layered. A plant density of 800—1200 trees per ha is recommended for plantations.

Some control of competing vegetation is required for the first 1—2 years after planting. This is normally achieved by manual weeding. Fertilizer is seldom applied; 50 g per tree of an NPK fertilizer (12:12:12) applied shortly after planting would be appropriate for infertile soils. If symptoms of boron deficiency such as dieback of main leader, bronze leaf colour and loss of leaves become apparent, an application of about 100 g of borax per tree at planting or preferably the equivalent amount of the less soluble ulexite is recommended.
Grevillea robusta regrows well after complete defoliation following pruning and pollarding, which can be carried out repeatedly to yield wood and to regulate shading and competition with adjacent crops.

In the lowland humid tropics and other very humid regions Grevillea robusta is vulnerable to attack by fungal diseases such as Corticium salmonicolor. Fungi such as Amphichaeta grevilleae, Cercospora sp. and Phyllostica sp. have been observed to cause considerable damage to leaves and stems of young plants in Sri Lanka, particularly if they are overwatered in the nursery. Under lowland conditions in the Caribbean Grevillea robusta is severely attacked by the scale insect Asterolecanium pustulans. Attack by termites can be a problem when it is planted on dry sites in Africa. In Peninsular Malaysia the big white ant Termes gestroi destroyed experimental trees.

Farmers in the East African highlands commonly harvest branches of Grevillea robusta by high pruning and pollarding every 3—4 years from 4—6 years after planting onwards, using the leaves for mulching or sometimes as cattle fodder. The main trunk of the tree may be harvested as a sawlog from the age of 15—25 years.

When grown in monoculture in woodlots or plantations, annual wood increments of 10—15 m³/ha of Grevillea robusta as measured over bark with stem diameter down to 10 cm have been recorded in rotations of 10—20 years in Uganda, Kenya and Hawaii.

Firewood of Grevillea robusta dries quickly, within a few days of cutting, except in wet weather. Logs are commonly pit-sawn green in rural areas. Timber for external use has to be treated chemically to improve its durability. It is susceptible to attack by Lyctus borers.

Isozyme studies have demonstrated that the genetic base of Grevillea robusta in a number of African countries is very narrow. In recent years the Australian Tree Seed Centre of the Commonwealth Scientific and Industrial Research Organization (CSIRO) and the Queensland Forest Service have distributed seed collections from natural provenances, covering the altitudinal and geographical range of sites where it occurs, for evaluation in other countries.

Studies of reproductive biology and genetic improvement programmes have commenced in several African countries, coordinated by the International Centre for Research in Agroforestry (ICRAF). Several provenance and progeny trials have been established in Africa and India, and selection of preferred genotypes has commenced in Kenya.

Because of the importance of Grevillea robusta in the tropical highlands of Africa, considerable research effort has started in the fields of genetic improvement and silviculture for agroforestry. Grevillea robusta is little used in South-East Asia, but could be of substantial value as an agroforestry tree for use on small farms in the highlands of countries such as Burma (Myanmar), Indonesia, Laos, the Philippines and Vietnam.

Brough, P., 1933. The life history of Grevillea robusta Cunn. Proceedings of the Linnean Society of New South Wales 58: 33-73.
Harwood, C.E., 1989. Grevillea robusta: an annotated bibliography. International Council for Research in Agroforestry (ICRAF), Nairobi, Kenya. 123 pp.
Harwood, C.E. (Editor), 1992. Grevillea robusta in agroforestry and forestry: proceedings of an international workshop. International Council for Research in Agroforestry (ICRAF), Nairobi, Kenya. 190 pp.
Kalinganire, A., Harwood, C.E., Simons, A.J., Moncur, M.W. & Slee, M.U., 1996. Reproductive ecology of Grevillea robusta in western Kenya. In: Dieters, M.J., Matheson, A.C., Nikles, D.G., Harwood, C.E. & Walker, S.M. (Editors): Tree improvement for sustainable tropical forestry. Proceedings of the QFRI-IUFRO Conference, Caloundra, Queensland 27 October-1 November 1996. pp. 238-243.
McGillivray, D.M. & Malinson, R.O., 1993. Grevillea, Proteaceae, a taxonomic revision. Melbourne University Press, Melbourne, Australia. 465 pp.
Skeney, K.R., Sprent, J.L. & Ong, C.K., 1996. Cluster roots of Grevillea robusta - foragers or scavengers? Agroforestry Today 8: 11-12.
Sleumer, H., 1955. Proteaceae. In: van Steenis, C.G.G.J. (Editor): Flora Malesiana, Series 1, Vol. 5. Noordhoff-Kolff, Djakarta, Indonesia. pp. 154-157.

C.E. Harwood

Harwood, C.E., 1997. Grevillea robusta A. Cunn. ex R. Br.. In: Faridah Hanum, I & van der Maesen, L.J.G. (Editors): Plant Resources of South-East Asia No 11: Auxiliary plants. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea