Silky Oak scientifically known as Grevillea robusta is an agroforestry, timber, flowering and honey tree in the Proteaceae (Protea family). It has many names in many languages; the ones given above are its Urdu names. In English it is also known as River Oak, Silk Oak, Silver Oak and Southern Silky Oak. It gets the oak name because the wood from it looks like oak wood. It also has Latin synonyms: "Grevillea umbratica" and "Grevillea pectinata". It was names by Allan Cunningham after Charles F. Greville (1749-1809) who was one of the founders of the Royal Horticultural Society, London; because of its hardiness it got the Latin name robusta (robust). The plant 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. Its natural range limited to pockets of subtropical forest on the east coast of the continent. The plant is exotic to China, Eritrea, Ethiopia, India, Indonesia, Jamaica, Kenya, Laos, Malawi, Malaysia, Mauritius, Nepal, Pakistan, South Africa, Sri Lanka, Tanzania, Uganda, USA, Vietnam, Zambia and Zimbabwe. Recently introduced to the Philippines, where it growns as shade tree or ornamental for shape and foliage. Today, the 360 species of grevilleas occur in Indonesia and Australia and are a diverse group. Their colourful, distinctive flowers lack petals and instead consist of a long tube known as a “calyx”, which splits into four “lobes”. Silky oaks have been declared an environmental weed in parts of New South Wales and Victoria where it grows outside its native distribution range. They’re also considered an invasive or invader plant in Hawaii and South Africa. However Grevillea robusta is declining in its natural rainforest/wet forest habitat. In some cities in China, silky oaks have been planted along roadsides with great success. The tree has also gained the Royal Horticultural Society’s Award of Garden Merit for its performance in growing under United Kingdom conditions. That just shows you how one person’s weed is another’s treasure. Some specimens of silky oak can be a bit scraggly in their canopy form. They can benefit enormously from a bit of formative pruning when they are young, and perhaps some structural pruning from a good arborist as they get older. A little attention at the right time will be amply rewarded with a safe and great looking tree that can live for 150 years or more. Silky oak is drought-tolerant. In dry times they often flower a bit later than their usual October blooming, providing a big splash of colour in otherwise drab and difficult years. The trees can be vulnerable to frost when young, but grow well once taller. This makes the silky oak a potential winner as climate change brings warmer, drier weather.
Silk oak is an erect, fast-growing, single-stemmed, and medium-sized to large evergreen tree that normally grows about 35 m (114 ft.) in its natural habitat, though elsewhere it is more typically 15 to 25 m (50 to 80 ft.) tall. Bole is straight, branchless for up to 15 m, up to 80 (max. 120) cm in diameter, usually without buttresses. Crown is conical and symmetrical with major branches spaced at intervals of about 1 m. Bark on the trunk is dark grey and furrowed into a lace-like pattern. Inner bark is reddish-brown. Young branchlets are angular and ridged, sub sericeous to tomentose but glabrous on older growth. Leaves are alternate, fernlike, pinnate, 11 to 21 pairs of pinnae, 4 to 9 centimeters long, and dark green. Leaflets are lanceolate, with entire or lobed margins. The flowers are yellow to orange, numerous, paired, on long slender stalks 1 to 2 centimeters, with 4 narrow yellow or orange sepals 12 millimeters long. Fruits are podlike, broad, slightly flattened, 2 centimeters long, black, with 1 to 2 seeds, 10 to 13 millimeters long.
The tree is often cultivated in the tropics for timber and as a windbreak. It is an ornamental plant, valued particularly for its attractive fern-like foliage and brilliant orange floral display. It is often grown in gardens and as a street tree. The tree flowers freely in subtropical areas, but only poorly in the lowland tropics. In Uganda and East Africa generally, as well as in Brazil, India and Hawaii, it is used for shade in coffee plantations, and for the same purposes in tea plantations in India and Sri Lanka as it protects not only the coffee and tea plants but also the workers. It can grow to heights of between 25 and 40 metres and is very beautiful when in flower. The flowers look like huge furry orange-gold caterpillars crawling across the branches. They are full of nectar and the Aborigines in Australia to which it is native, drink it straight from the flowers, or make a drink from them. The flowers are said to be very rich in vitamin C, and attract honeybees. Because it sheds its leaves and flowers, a thick layer of leaf mulch can build up in the soil around the tree, and as this may go to a depth of 30-40 centimetres, it protects the soil and maintains its temperature. The leaves and twigs ar said to be rich in aluminium.
The wood from Grevillea robusta is used in parts of the world for fuel as it makes good charcoal and firewood, as well as being used to make furniture. It is thought that the gum which exudes from the tree when it is cut could be used for industrial purposes. Yellow and green dyes can be made from the leaves, and the flowers are used for their fragrance.
Some research has already been done on this tree and its properties but there is a lot more to be done before scientists can determine what it can be used for in terms of medicine. So far they have isolated his-resorcinols from it, striatol being the most potent, which may be a potential help for the cardiovascular system. The flowers are rich in vitamin C. Leaves and twigs reported to be high in aluminum. Studies have suggested scavenging activity, leishmanicidal, L-DOPA inhibitory properties.
Bees are attracted to the flowers, but the flowers, fruit and seeds of the tree can cause skin irritation because of the cyanogenic compounds found in them; the leaves can also irritate the skin. Despite this, in Kenya the natives of the Kakamega Forest use the tree for medicinal purposes, which is a little unusual given that it is a non-native species. They use it to cure sore throats, earache, chest problems, flu and toothache, and there are also superstitions regarding it. However in Hawaii where it was also introduced it has come to be seen as invasive.
The flower buds, fruit and seeds are cyanogenic. Contact with leaves may cause contact dermatitis due to tridecylresorcinol, a chemical compound related to the allergen toxicodendron. A report on a case of severe acute dermatitis venenata due to exposure to sawdust of Grevillea robusta. Bracelets made from the wood of Grevellia were shown to be a source of allergic contact dermatitis. The responsible allergen was grevillol, a phenolic with a long side chain resembling the sensitizing uroshiols from poison ivy. For all eye exposures, rinse the eye with water for 15 minutes. Seek medical assistance if irritation persists. Sap and sawdust may cause eye irritation. Anyone who comes across one of the trees on their morning stroll, should, however, keep a safe distance: they also drip hydrogen cyanide. All Grevilleas literally drip nectar, much to the delight of native birds and bees.
The University of Melbourne’s Dr Gregory Moore wrote in a piece for The Conversation, with Indigenous Australians once enjoying “the sweet nectar straight from the plant or mixed with water, the original lolly water”. “Like other grevilleas the Silky Oak also contains tridecyl resorcinol, which causes an allergic reaction leading to contact dermatitis. The chemical is similar to toxicodendron in poison ivy”, Dr Moore said. He advised anyone working with silky oaks “to wear gloves, a face mask, protective eye wear (or face shield) and long sleeved clothing”. “Washing hands and showering at the end of the day is also recommended”, Dr Moore added.
A 2007 study "Cytotoxic 5-Alkylresorcinol Metabolites from the Leaves of Grevillea robusta" published in J. Nat. Prod. by Ta-Hsien Chuang and Pei-Lin Wu isolated 6 new 5-alkylresorcinols and 8 known compounds, all of which showed marginal toxicity against cancer cell lines MCF-7, NCI-H460 and SF-268 ki cell lines.
A 1999 study "Investigation of plant-derived phenolic compounds as plasma membrane Ca2+-ATPase inhibitors with potential cardiovascular activity" published in Drug Development Research vol. 46, no 3-4 where several his-resorcinols were isolated from Grevellia robusta, the most potent, striatol, exhibiting inhibitory activity on the Ca-ATPase system suggesting a potential for cardiovascular activity.
A 2014 study "Pharmacological Activities of Grevillea robusta, a Medicinal Plant of Bangladesh" published in Bangladesh Pharmaceutical Journal by Md. Sharif Ullah, Md. Al Amin Sikder, Tasnuva Sharmin and Mohammad A. Rashid evaluated a crude methanolic extract of leaf of Grevillea robusta and various soluble partitionates for cytotoxic, thrombolytic, membrane stabilizing and antimicrobial activities. The crude ME showed highest cytotoxic activity in brine shrimp lethality bioassay with LC50 values of 1.50±0.45 µg/ml. Extractives showed 69.95±0.11% clot lysis. A chloroform soluble fraction inhibited hemolysis of RBC induced by heat and hypotonic solution. A chloroform partitionate showed highest zone of inhibition against Salmonella typhi.
Traditional Uses of Silky Oak:
- In Kenya, natives of the Kakamega Forest use the plant to treat sore throats, earache, chest problems, flu and toothache.
- In North Garo Hills, Meghalaya, NE India, bark and leaves used for headaches and dizziness.
Culinary Uses of Silky Oak:
- The flowers are one of the richest sources of nectar. This can be sucked directly from the flowers, shaken into a bowl or washed out in a small quantity of water. The nectar falls in showers when the flowers are shaken.
- Aborigines in Australia reported to drink the nectar straight from the flowers.
- Indigenous people used to make a sugary drink from silky oak flowers by dipping the whole flower heads into water to wash the nectar off.
Other Uses of Silky Oak:
- Leaves consist of rutin, though quantities are not specified.
- Intense yellow and green dyes are obtained from the leaves.
- This tree is one of the most important re-afforestation trees in Nepal.
- It is sometimes used as a rootstock for the more susceptible species.
- The leaves are also used as mulch.
- Trees usually begin to flower at about 10 years.
- There are reported to be 64,000 to 154,000 seeds per kilogram (29,000 to 70,000/lb).
- It is regarded as a weed in parts of New South Wales and Victoria, as invasive in Hawaii and as an invader in South Africa.
- Silk Oak timber was extensively used for external window joinery, as it is resistant to wood rot.
- It has been used in the manufacture of furniture, cabinetry, and fences.
- In the UK, Silk oak has gained the Royal Horticultural Society’s Award of Garden Merit.
- It is grown in plantations in South Africa, and can also be grown alongside maize in agroforestry systems.
- Dye: Yellow and green dye from the leaves; used for dyeing silk.
- Wood: Used for joinery, cabinetry and paneling. Wood is used in making railroad ties, plywood, air-freight cases and furniture, parquetry, turnery, boat building, interior trim, cabinet work, parquet flooring, paneling, joinery, boxes, toys and novelties.
- Fuel: Wood makes a good charcoal. Used to fuel locomotives and river steamers, power boilers and small industries. Sapwood has calorific value of 4800 kcal/kg; the heartwood yields 4950 kcal/kg.
- Gum or resin: The plant yields small quantities of a gum resin. Natural gum has potential as adhesive. By virtue of their solubility, viscosity and relatively high resistance to hydrolysis, the gums may have some industrial applications. A natural gum from the plant has been studied and analysis showed that with suitable modifications, the gum from Silky Oak can be used as a wood adhesive.
- Ornamental: Grown for its attractive foliage. Leaves used in flower arrangements.
- Apiculture: Golden flowers are an attractant for bees, making it an important honey plant.
- Poison: Flower buds, fruit and seeds.
Agroforestry Uses of Silky Oak:
- This tree is one of the most important reforestation trees in Nepal.
- It is a pioneering colonizer of disturbed sites.
- An excellent agroforestry species as it interferes little with crops. Best growth is obtained when it is planted in rows or intermixed with crops
- It is regarded as more compatible with crops on small farms than most other tree species.
- The leaves are used as a mulch.
- The Silky Oak provides abundant quantities of leaf mulch, which may accumulate to a depth of 30 - 40 cm. This thick layer protects the soil and maintains soil temperature.
- It is very popular in agroforestry systems and often planted to provide shade for tea and coffee plantations.