Grevillea robusta – Silky Oak Tree Growing Guide

Getting to Know Grevillea robusta

Grevillea robusta is a tall tree with flowing pinnate leaves and beautifully textured deep brown bark. Its bright orange flowers are iconic to the east coast and can be harvested for their nectar, or left on the tree for wildlife. 

It flowers generously for three to four weeks in spring, and can reach eventual heights of 21 metres tall, making them ideal for planting as specimen trees in open lawns.

As well as growing in their tree form, they can be pruned into a shrub or hedge, and will flower quite happily in warmer parts of the country with regular pruning.

Silky Oak Tree's Natural Habitat

Grevillea robusta is native to the Eastern coast of Australia, growing in harsh weather conditions and well adapted to both the long dry summers of the north east, and the humid subtropics further south. 

Salty winds don’t seem to affect it either, and will generally adapt to most other parts of the country, in humid, dry, arid, or cold conditions. Mature trees can cope with temperatures as low as -8°C, but young trees need protection from anything below freezing.

How to Grow Grevillea robusta

Grevillea robusta is a useful tree for anyone without a green thumb. They adapt well to most climates, and are not fussy about soil conditions. They cope with damp soil, free-draining sands, loam, clay, and loose nutrient-poor soil in equal measure. 

Their roots are not invasive, but grow deep to access groundwater when needed, but can adapt to grow close to the surface on moisture-retentive soils. Their deep roots also mean they will not compete with other shallow-rooted plants in the garden, so can be grown well without the need for regular soil improvement.

Planting and Caring for Grevillea robusta

When planting a young Grevillea robusta, it’s worth checking your local winter weather. For the first five years, they should be protected from frost, as it can damage young shoots and even trunks. After that, they will grow quite happily in most conditions.

Soil & Drainage

G. robusta can grow in acid, alkaline or neutral soils, and are not fussy about soil nutrients or moisture retention. Their roots will adapt to most conditions other than completely waterlogged.

Light & Temperature

G. robusta do light full sun. They can compete with more mature trees, and will eventually grow through the canopy of smaller species though, so can be planted in part shade in a warm garden to get them started.

Moisture & Watering Silk Oak Tree

G. robusta should be watered twice a week in their first summer to support their young roots as they establish so they do not have to search too hard for moisture. Stop watering in autumn when rain is more available, and do not water in winter unless the soil dries out completely in the first year.

Mulching 

G. robusta does not need regular mulching, but if there are signs of significant nutrient deficiency (regular onsets of diseases, or wilting and dropping leaves) it’s worth applying a generous layer of leaf mould around the base of your tree to add nutrients and encourage worms to work the organic materials through the soil.

Fertiliser & Feed

G. robusta should only be fed or fertilised if it has a specific deficiency. The only common deficiency is potassium, which should be checked with a soil testing kit before adding nutrients. Signs to indicate potassium deficiency include scorched leaves that do not recover.

If you notice that, and successfully test for potassium deficiency, try RAW Potassium Power to fix the problem.

Pruning Grevillea robusta

For tree-form Grevilleas, including G. robusta, it’s best to never prune them, and leave them to their own devices. They can reach over 20m tall, which may be too much for some gardeners, but it is just breathtaking when they reach maturity in all their conical glory.

How to Propagate Silky Oak Tree

Grevillea robusta can be propagated from cuttings or seed, but cuttings will take far quicker. Grevillea seeds of all species can take up to two years to germinate (though typically germinate in around 6-12 months indoors).

Check out our guide to growing Grevilleas for more information on how to take cuttings from these striking plants, whether it's from low-growing shrubs, or giant trees, it’s all the same; take semi-mature cuttings after flowering; encourage rooting with hormone gel; leave somewhere humid, and bright but slightly shaded; and in a few months, your cutting should take without any issues.

Silky Oak Grevillea Pests and Diseases

Grevillea doesn’t tend to suffer much from pests in Australia. They have learned to adapt to their native predators, and their abundance of sweet nectar helps to attract insects that predate the pest species that would otherwise damage them.

In terms of disease, as an easygoing plant that adapts well to most growing conditions, Grevillea robusta rarely suffers from anything other than nutrient deficiency, or nutrient excess.

Potassium deficiency can be quite common but is easy to resolve,  and as I mentioned earlier, there is a greater risk of over-feeding than under-feeding, particularly with phosphorus which in excess can burn the leaves, leading to leaf scorch.

While leaf scorch is easy to recover from, it may also lead to leaf blight, where fungal infection enters the leaf due to other damage. 

Grevillea robusta Frequently Asked Questions

How hard is Grevillea robusta?

Grevillea robusta shares its name with the common oak tree thanks to its stature but also its timber. The timber of Grevillea robusta is exceptionally hard and makes excellent firewood or building materials.

What is the tallest Grevillea?

Grevillea robusta is the tallest growing species of Grevillea, reaching heights of over 20 m, and recorded in nature at 30 m tall.

What is the lifespan of Grevillea robusta?

Grevillea robusta has a standard lifespan in gardens of around 50-60 years, but some wild specimens are known to be over 100 years old.

Is Grevillea robusta fast-growing?

Grevillea robusta takes about 20 years to reach maturity, growing at a particularly fast rate (even for a grevillea) of a couple of feet per year once established.

Are Grevillea robusta roots invasive?

Grevillea robusta roots are not invasive. In some countries, they are listed as invasive species due to their ability to self-propagate in a wide variety of conditions, but their roots will not cause damage to structures.

How small can you keep Grevillea robusta?

Grevillea robusta can be maintained as a low-growing hedge, but if you prune it regularly, aim to cut back to about 2m each year, and allow it to grow back to 3m before pruning again.

Grevillea robusta is a particularly tall plant and naturally wants to grow as a tree. Over-pruning can cause stress.

Can you grow Grevillea robusta in pots?

Not only can you grow Grevillea robusta in pots, but you can keep it growing as a bonsai for decades. If you do try to grow native bonsai trees like Grevillea robusta, aim to keep them outdoors, and prune regularly, as well as training stems into satisfying shapes while they are supple.

For more Grevillea growing guides, see our list below:

• Grevillea ‘Ned Kelly’
• Grevillea ‘Moonlight’
• Grevillea ‘Honey Gem’
• Grevillea ‘Peaches and Cream’
• Grevillea ‘Robyn Gordon’
• Grevillea ‘Superb’
• Grevillea ‘Orange Marmalade’
• Grevillea ‘Seaspray’
• Grevillea ‘Dorothy Gordon’
• Grevillea ‘John Evans’