Australian Silky Oak
Grevillea robusta
NEMBA category
English: Australian Silky Oak, Silver Oak, Silky Oak, He-oak
Afrikaans: Australiese Silwereik
Grevillea robusta, commonly known as the Australian Silky Oak or Silver Oak, is a tall, graceful tree with a somewhat fern-like appearance, sporting striking golden-orange flowers that bloom in brush-like sprays.
It's a fast-growing, evergreen tree with deeply fissured bark and distinctive leaves that are dark green above and silvery-white or rusty-gold underneath. Its eye-catching flower display makes it a popular ornamental, but don't be fooled by its beauty; it's a known invader in South Africa, often spreading by its winged seeds.
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Origin:
Native to eastern coastal regions of Australia, specifically southern Queensland and northern New South Wales.
Distinguishing features:
Fern-like leaves: Large, deeply divided (pinnate) leaves with a distinct fern-like appearance, dark green on top and silvery or rusty underneath.
Golden-orange flower spikes: Bright, showy, golden-orange flowers arranged in one-sided, bottle-brush-like sprays.
Deeply fissured bark: The bark on mature trees is grey or dark brown and deeply furrowed.
Winged seeds: Fruits are leathery, brown-black capsules containing one or two flat, winged seeds.
Overall appearance:
A large, fast-growing, evergreen tree with a straight trunk and a moderately spreading, often conical or oval crown. It has a distinctive fine texture due to its highly dissected leaves.
Height:
Typically grows to between 18 and 30 metres tall, but can reach up to 40 metres in its native habitat.
Width:
The crown can spread between 5 and 20 metres wide.
Leaves:
Alternate, large (15-30 cm long), and deeply divided (pinnately compound) into numerous narrow, fern-like segments (lobes). The upper surface is dark green, while the underside is densely covered in fine, silvery-white or rusty-coloured hairs, giving it a characteristic silky sheen. Young foliage can be particularly silvery.
Flowers:
Bright golden-orange to orange-yellow, arranged in dense, one-sided, bottle-brush-like sprays (racemes) that are typically 10 to 15 centimetres long. These sprays occur terminally (at the ends of branches) or in the leaf axils. The flowers are rich in nectar and are attractive to birds. The plant is hermaphroditic, with both male and female parts in each flower.
Flowering season:
In South Africa, it primarily flowers from late winter to spring (around September to November), but sporadic flowering can occur at other times depending on local climatic conditions.
Fruits:
The fruit is a brownish-black, leathery, egg-shaped or ellipsoid capsule (follicle), approximately 1.5 to 2 centimetres long, with a persistent style (a small beak-like projection). Each fruit typically contains one or two flat, winged seeds.
Seeds:
The seeds are flat, oval, and surrounded by a thin, membranous wing, measuring about 1.5 to 2.5 centimetres long (including the wing). These wings aid in wind dispersal.
Bark:
The bark on mature trees is distinctive: grey or dark brown, deeply fissured, and furrowed, giving it a rough, cork-like appearance. Young stems are often tomentose (hairy) and can be orangish-brown near the tip.
Thorns:
This species does not have thorns.
Wood:
The wood is distinctive with a prominent grain, giving it a "silky" appearance when quartersawn, hence the common name "Silky Oak." It is a pale yellowish or reddish-brown, moderately hard, and durable, used for furniture, cabinetry, and veneer.
Roots:
Develops a strong, extensive root system. It can also produce "proteoid roots" in low-fertility soils, which are specialised cluster roots that enhance nutrient uptake. Mature trees can have considerable surface roots that may damage nearby infrastructure.
Soil type:
Tolerant of a wide range of well-drained soils, from sandy loams to clay loams, but prefers moderately fertile conditions. It can grow in acidic to slightly alkaline soils (pH 4.5 to 7.5). It is drought-tolerant once established.
Aspect:
Prefers full sun for optimal growth and flowering. It can tolerate some partial shade but will not grow as robustly.
Ediblity:
Not typically considered edible. While the flowers produce abundant nectar that some animals and people have historically consumed (e.g., by shaking flowers to collect it), other parts of the plant are not consumed.
Toxicity:
The sap, sawdust, and fresh timber can cause contact dermatitis (skin irritation, itching, rash) in sensitive individuals due to the presence of allergenic compounds like tridecylresorcinol. This reaction can be delayed by several days. While not orally toxic in the conventional sense, direct contact with the plant should be minimised if you have sensitive skin.
Uses:
Widely cultivated globally as an ornamental tree for its attractive foliage and striking flowers, used in parks, large gardens, and as a street tree. It is also extensively used in agroforestry systems, particularly in tea and coffee plantations, to provide shade for crops. Its timber is valued for furniture, cabinetry, and veneers. It is also used for firewood and poles.
Ecology:
As an alien invasive plant in South Africa, Grevillea robusta can escape cultivation and establish itself in natural areas. It spreads primarily by its numerous winged seeds, which are efficiently dispersed by wind over considerable distances (up to 364 metres from the parent tree). It invades disturbed sites, forest margins, savannahs, and particularly riparian (riverbank) areas. Once established, it can form dense stands, outcompeting indigenous plant species for resources (light, water, nutrients) and potentially altering the local ecosystem structure. While its flowers attract nectar-feeding birds and insects, its invasive capacity outweighs these benefits in natural environments.
Distribution and habitat:
Widely planted as an ornamental and shade tree throughout South Africa. It has naturalised and is considered an invasive species in many provinces, including the Western Cape, Eastern Cape (where it invades forest edges and riparian areas, e.g., around Kenton-on-Sea), KwaZulu-Natal, Gauteng, Limpopo, and Mpumalanga. It thrives in areas with sufficient rainfall and prefers well-drained soils, often found along watercourses and in disturbed woodlands or grasslands.
Derivation of name:
The genus name Grevillea honours Charles Francis Greville (1749-1809), a co-founder of the Royal Horticultural Society in England.
The species epithet robusta is Latin for "robust" or "strong," referring to its large and sturdy growth habit.
The common name "Silky Oak" refers to the silky texture of its leaves and the oak-like appearance of its timber when cut.
Historical aspects:
Grevillea robusta was introduced to South Africa and many other parts of the world in the mid to late 19th century, largely for its value as a fast-growing timber tree and for providing shade in coffee and tea plantations. Its ornamental appeal also contributed to its widespread planting in gardens. Over time, its ability to produce abundant, wind-dispersed seeds and its vigorous growth led to its escape from cultivation and naturalisation, resulting in its classification as an invasive species in many countries, including South Africa.
NEMBA status:
Category 1b invasive species in South Africa. This means it must be controlled or eradicated, especially in riparian zones, and it is illegal to plant, propagate, import, or otherwise possess this plant without a permit. Previously, it was often classified as Category 3, but its invasive potential in riparian areas has led to its upgrade for these specific habitats.
Interesting facts:
This tree is often called "Silky Oak" not because it's a real oak (it's not!), but because its timber, when cut, has a beautiful, wavy grain that looks as smooth as silk!
Its stunning golden-orange flowers are packed with so much nectar that you can sometimes see birds flocking to them for a sugary drink, and some people have even collected the dripping nectar to taste!
Even though it's a tree, its leaves are so deeply divided and feathery that they look just like giant ferns, especially when they're young and have a shimmering silver underside!
If you're working with this tree, be careful: its sap and sawdust can sometimes cause a rash, almost like a natural allergic reaction, so it's best to wear gloves and eye protection!
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