Important Facts about Silk Oak

Silky Oak scientifically known as Grevillea robusta is an agroforestry, timber, flowering and honey tree in the Proteaceae (Protea family). 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. Australian silky-oak, he oak, she-oak, silk oak, silky oak, silver oak, southern silky-oak, Fernleaf, Golden pine, silk-oak grevillea, grevillea and river oak are some of the well-known common names of the plant. The genus name Grevillea is named after Charles Francis Greville (1749 – 1809) who helped to establish a society now called the Royal Horticultural Society. The species epithet robusta means large or strong in Latin and it refers to this species’ large growth form. The common name Silky Oak is a reference to the wood which is similar to oak, but has a silk-like texture after being split.

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.

Silk Oak Facts

Plant Description

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.

The plant is found growing in forest edges, coastal forests, disturbed sites, savannah and riparian areas, subtropical rainforest, dry rainforest and wet forests. It adapts to various environmental conditions and is found between 500 and 2000m altitude where annual rainfall ranges from 800 to 1500mm. The plant 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.

Leaves

Leaves are alternate, fernlike, pinnately or almost bipinnately compound, 15-30 cm long and 9–15 cm wide, exstipulate with 11–31 primary lobes. Lobes are entire or divided, ultimate lobes 0.5–5 cm long and 0.2–1 cm wide. Upper surface is shiny dark green and hairless, while lower surface is silky with whitish or ash colored hairs with recurved margins. Petiole is 1.5–7 cm long. They remain on the tree where the dry season is short but fall where it is long or pronounced, leaving the branches mostly bare for a short period, just before flowering.

Leaf arrangement	Alternate
Leaf type	Odd-pinnately compound; made up of 7-19 leaflets
Leaf margin	Parted, revolute
Leaf shape	Lanceolate
Leaf venation	Pinnate
Leaf type and persistence	Evergreen
Leaf blade length	6 to 13 inches; leaflets are 1 to 4 inches
Leaf color	Green on top, silvery white underneath
Fall color	No color change
Fall characteristic	Not showy

 

Flower

Flowers are showy, yellowish, numerous, paired, on long slender stalks 1-2 cm, composed of 4 narrow yellow or orange sepals 12 mm long. Flower clusters are 7.5-15 cm long, unbranched, arising mostly from the trunk, in a terminal or axillary simple or branched raceme, protandrous, petals 4, united into a tube that is mostly recurved under the broadened apex (limb); stamens 4, sessile in the concave limb; disk annular or semiannular, sometimes bilobed; ovary superior, 1-locular with 2 ovules, style curved and protruding from a slit in the perianth tube, the apex free from the limb, eventually straight, persistent. They bloom in perfusion from spring to early summer, with the most striking displays seen on near leafless trees.

Flower color	Yellow orange
Flower characteristics	Showy; emerges on one side of a raceme
Flowering	Spring

 

Fruit

Fertile flower are followed by small, flat, pod like, broad, slightly flattened (boat shaped) seedpods about 1.5-2.cm long, about 1 cm broad. They are initially green turning to dark brown to near black as they mature with long slender stalk and long, threadlike, curved style. Seeds 1-1.5 cm long, 0.5-1 cm broad, broadly winged, thin, ovate, non-endospermic with a brown, ovate central seed body.

Fruit shape	Unknown
Fruit length	½–¾ inch
Fruit covering	Dry or hard
Fruit color	Silvery gray to brown
Fruit characteristics	Does not attract wildlife; not showy; fruit/leaves a litter problem

 

Traditional uses and benefits of Silk 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

• 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.

Other Facts

• The plant yields small quantities of a gum resin.
• 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.
• Wood is used for paneling, joinery, cabinet making etc.
• It is considered a good fuel.
• The leaves are also used as mulch.
• It is very popular in agroforestry systems and often planted to provide shade for tea and coffee plantations.
• 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.
• Wood is used in making railroad ties, plywood, air-freight cases and furniture, parquetry, turnery, boat building, interior trim, cabinet work, parquet flooring, boxes, toys and novelties.
• The golden flowers are attractive to bees, making it an important honey plant.

References:

https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=27783#null

http://www.hear.org/pier/species/grevillea_robusta.htm

https://pfaf.org/user/Plant.aspx?LatinName=Grevillea+robusta

https://www.cabi.org/isc/datasheet/25866

https://www.srs.fs.usda.gov/pubs/misc/ag_654/volume_2/grevillea/robusta.htm

https://plants.sc.egov.usda.gov/home/plantProfile?symbol=GRRO

https://edis.ifas.ufl.edu/publication/ST285

https://gd.eppo.int/taxon/GRERO

http://www.theplantlist.org/tpl/record/kew-2832700

https://indiabiodiversity.org/species/show/229827

https://en.wikipedia.org/wiki/Grevillea_robusta

https://www.iplantz.com/plant/791/grevillea-robusta/

http://apps.worldagroforestry.org/treedb2/AFTPDFS/Grevillea_robusta.PDF

http://www.flowersofindia.net/catalog/slides/Silver%20Oak.html

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