The deciduous handsome Silk Oak, scientifically known as Grevillea robusta (grev-ILL-ee-uh, roh-BUS-tuh) is a part of the family Proteaceae.
It is native to the temperate climate of eastern coastal Australia (Queensland and New South Wales), thriving in riverine, subtropical and dry rainforest environments.
It has become naturalized in many places, including on the Atherton Tableland in Australia and in South Africa, New Zealand, Hawaii, French Polynesia, Norfolk Island (home to Norfolk Island Pine), Jamaica, and Florida.
Within its genus, the Silk Oak is the largest tree.
Its alternatively-arranged fern-like foliage is very stately, making it perfect for ornamental gardening.
You will also find the tree lining streets in its native range, creating a remarkable skyline silhouette.
You may also hear it called by its common names including:
- Silk Oak
- Southern Silky Oak
- Silky Oak
- Silver Oak
- Australian Silver Oak
Grevillea Robusta Care
Size & Growth
Silk Oaks are majestically handsome trees succeeding and flower in temperate or tropical climates.
The particular species is a fast growth rate evergreen tree reaching up to incredible heights between 20’ – 100’ feet.
The trunk is straight, and unbuttressed, and is 31” – 47” inch in diameter.
When planted in tropical areas, the tree can grow at elevations from 40’ feet to 8000 feet and from sea-level to ½ mile in temperate climates.
Flowering and Fragrance
Silk Oaks are evergreen and in leaf throughout the year.
They flower for a few weeks in October and November but only when the growing conditions are right.
In lowland tropics, flowering is poor or does not occur at all.
Silk Oak showy golden orange flowers are one-sided racemes and appear in groups.
They are often branched and 5” – 6” inches long.
These flowers are rich in nectar and attract bees in large quantities.
However, if you’re growing a Silk Oak from dark brown seeds, it will only flower when it is about six years old.
Light & Temperature
Silk Oaks are hardy to USDA hardiness zones 9 to 11.
Once the tree is established, it can tolerate temperatures down to 18° degrees Fahrenheit (-8° C).
However, the plants grow best and prefer warm temperate climates.
The trees love full sun but can grow in semi-shade or light woodland environments.
Choose a location getting at least six hours of sunlight in the day.
Be cautious about prolonged exposure from the western sun as it can cause trunk sun-scald.
Watering and Feeding
Silk Oaks have average watering needs and are suitable for xeriscaping.
However, their roots are susceptible to root rot and other fungal diseases caused by overwatering.
So make sure to use a good-draining soil and don’t drown the roots.
Although the tree is drought resistant and can succeed in dry soils, they do their best and are more likely to flower in moist, slightly acidic soil.
As for feeding, they love fairly high fertility.
Soil & Transplanting
The Silk Oaktree doesn’t respond too well to dry sites.
They thrive in well-draining soil which doesn’t let water stand too long around the roots.
Provided this requirement is fulfilled, the tree will grow in sandy, loamy, and clay soils.
As for the pH, they prefer slightly acidic to neutral soils.
If you are starting seeds inside or in containers, they are transplanted in their permanent locations once they are 8” – 12” inches tall.
This takes probably 6 to 8 months following the seed sowing.
Once transplanted, they will need shade for the first 2 or 3 weeks for it to establish further.
Grooming and Maintenance
Prune the tree as needed as it helps elevate the foliar canopy further.
Pollarding is carried out repeatedly to yield wood and regulate shading.
The Silk Oak is robust and doesn’t need too much maintenance.
Hence, it is often used as a street tree.
The Silk Oak has occasionally caused contact dermatitis following contact with foliage or sawdust.
This is mainly due to the tridecyl resorcinol related to an allergen called Toxicodendron.
The flower buds and fruits produced by the tree are cyanogenic.
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How to Propagate Silk Oak
Silk Oaks is propagated with seeds.
However, germination is delayed and often uneven despite them having no dormancy.
To improve germination, soak the seeds in cold water for at least 24 hours before sowing them.
Afterward, start them in a greenhouse in individual containers or shady seedbeds.
The seeds may then take approximately 2 to 4 weeks to germinate and is transplanted into their permanent location once they’ve reached 8” – 12” inches in height.
Or propagate Silk Oak with cuttings.
Use cuttings of half-ripe wood, about 2” – 3” inches with heel and plant them in the June/July frame.
Silver Oak Pest or Disease Problems
When planted in poorly drained soils and overwatered, the Silk Oak is susceptible to root damage.
The species is known to be particularly resistant to honey fungus and wood rot.
When planted in lowland conditions, especially in the Caribbean, Silk Oak is affected by the scale insect known as Asterolecanium pustulans.
Termites are a problem specifically if planted in dry conditions in Africa.
Suggested Silky Oak Uses
Interestingly, the Silky Oak flowers are one of the richest sources of nectar.
The nectar is edible and is extracted directly from the flowers.
Also, the nectar falls in showers when you shake the flowers.
The tree also yields small amounts of gum resin used for industrial applications.
Most commonly, the Silky Oak species have been used in agroforestry, especially for reforestation in Nepal.
It is not excellent for colonizing disturbed land but also used as a shade tree providing shade to other crops.
Other industrial uses of Silk Oak include the extraction of yellow and green dyes from the leaves.
It is frequently used as a windbreak.
In the past, before the advent of aluminum, Silky Oak wood was used to make external window joinery and other furniture.
Larrivée, a guitar manufacturer, used the wood to make the side and backwoods of their instruments.
However, due to the declining populations of the species, felling has been discouraged and restricted in several areas.