Last Updated on January 20, 2023 by Admin
The grevillea robusta is also commonly known as the silk oak. Although, it comes by many other names as well include silver oak and southern silky oak. While it is named as such, it is not related to the true oak trees.
What makes this grevillea species unique is that it is a flowering tree. Just as importantly, it is very hard to miss this evergreen plant because of its size.
It grows up to 100 feet tall. This makes it perfect if you want to add a shaded area or create a skyline for your yard. As you would expect, the tree is fast growing.
With a lifespan of between 50 to 65 years, the grevillea robusta sports a very rugged look. But, as they say, don’t judge a book by its cover. And, that saying applies to this tree perfectly.
That’s because the wood of this tree is actually very brittle. And, its branches, flowers and leaves often fall off especially where there are winds. As a result, the tree is fairly messy. And, you’ll need to do some cleaning up every so often because of all the debris.
That said, many gardeners believe the extra work is well worth having such a beautiful tree in their backyards.
This is because of its lovely fern-like foliage that grow between 4 to 10 inches long and 4 to 6 inches wide. Similarly, it also produces yellow, orange and red colored flowers.
Last but not least, it is very easy to care and low maintenance.
Grevillea Robusta Plant Care
Grevillea Robusta Light
The grevillea robusta grows best in full sun. Although, no matter where you put it, it will likely be the one getting the most sunlight as well due to its size. Growing up to 100 feet tall it will stand head and shoulders above other smaller trees.
That said, location is still important because some areas don’t get a lot of sunlight, especially during the colder months of the year. Thus, in addition to full sun, it does best in warm climate areas.
However, the silk oak is not for everyone.
Because it grows so big and wide, it is only a good idea to grow this tree if you have a large plot. Additionally, it is a messy tree. That’s because its branches fall down a lot. Thus, you want to give it enough space so that your other plants are clear from these falling debris.
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Grevillea Robusta Temperature & Humidity
As mentioned above, the grevillea robusta likes warm weather. That’s because it is native to dry rainforests and subtropical areas.
This makes it thrive in USDA zones 9 to 11. The tree grows best in regions that experience sunlight the entire year round. While it originates from Australia, the plant has since been grown in the U.S. namely Florida, California, Texas and New Mexico.
Also, because it is used to growing outdoors, it is somewhat hardy to cold temperatures well. While it does like warm weather, it can tolerate climates that go down to 18 degrees or so. However, it is not a fan of strong winds.
One thing worth noting is that if you get a young plant, make sure to shelter it from the cold and frost. During this time, it is more susceptible to lower temperatures.
As such, silk oaks should be grown indoors in containers or somewhere protected from the cold when they are young trees. Once established, the become significantly hardier able to withstand colder conditions.
Grevillea Robusta Watering
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As with many other grevillea varieties, the robusta is quite drought resistant. Thus, you don’t need to water it regularly. In fact, it is happier with irregular irrigation. And, giving it too much water too often results in overwatering fairly quickly.
The plant is likewise susceptible to root rot if you let it sit in water for long periods of time.
To make sure that your tree is does not sit in water for too long a time, it is essential to use well-draining soil. This helps prevent root rot.
You want to be extra careful with younger plants because if you decide to grow them in containers indoors (to protect them from the cold), it is easier to overwater it. Thus, make sure to use a container with a few holes at the bottom.
In general, grevillea robusta are not too picky with soil besides its ability to drain moisture. But, the tree does grow better with rich, moist soil. And, less so in heavy clays or soil with high lime content.
The plant likewise enjoys slightly acidic soil with pH levels ranging between 4.5 to 8. However, be careful of using soil that’s too acidic because it can cause boron deficiency and magnesium toxicity.
Grevillea Robusta Pruning
The grevillea robusta is a fast growing tree, which allows it to achieve its huge size. More importantly, it can also be invasive. Add these two factors plus the fact that it is a messy tree that loses branches, leaves and flowers, means that you want to keep other plants away from it.
Also, it means that you’ll need to trim it every so often to keep it looking neat and tidy.
When it comes to pruning, you can be aggressive with this tree. That’s because it grows better after defoliation. Also, pollarding it, where you cut off the top branches of the tree makes it healthier and grow better as well.
You can then use the wood for fire or other things. And, enjoy a little less shade (for the time being) as the tree regrows.
Grevillea Robusta Propagation
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Grevillea robusta can be propagated via seed or cuttings.
Propagation from cuttings is the best way to go when it comes Grevillea robusta. That’s because it is easier to do, takes less time for your plant to grow, and you don’t have to deal with the problems involved in germination.
To propagate Grevillea robusta from cuttings, you’ll want to take a 3-4 inch wood cutting. You can let it dry then dip it into rooting hormone before planting it again. The rooting hormone powder will speed up its initial growth. Just as importantly, it increases propagation success rate.
Propagation be seed required germinating them. Here, you will want to soak the seeds in cold water for 24 hours before sowing. This will help improve germination.
Plant the seeds in well draining soil. Using a combination of sand and loam work well. The process will take around 2 to 4 weeks so you will need to be patient.
Transplanting & Repotting
The only time you will be repotting this plant is when it is young and you’re growing it in a container. While you can grow it outside in warm conditions, gardeners will keep them in pots indoors if to keep them away from frost and colder nights (to which they are susceptible as young trees).
But, once they grow, they’ll need to go into the ground because of their size.
When repotting the plant, you will want to move up in pot size gradually. While this sounds counterintuitive because of its fast growth, a pot that’s too big will make the plant sit in a lot of soil. Thus, when the soil gets wet, it takes longer to dry causing the tree to sit in water for long periods of time which it hates.
While beautiful and big, you want to be extra careful with this tree. That’s because the contact can cause skin irritation due to its tridecylresorcinol content. But, the sap of the plant contains delicious nectar. Its flowers, seeds and buds are also cyanogenic.
Frequently Asked Questions (FAQ)
Is the silky oak tree an Australian native?
Silky Oaks or Grevillea robusta is native to Australia. It originally was only found in the eastern part of Australia. But today, you can see the tree all over the country. Usually, it adorns backyards, gardens, parks, areas where there is nature and even streets.
What is the scientific name of silver oak?
The scientific name of the Silver Oak is Grevillea robusta. This is a large tree that you’ll see in landscapes including homes, areas or nature and other urban locations.
Is Grevillea robusta native to Victoria?
The Grevillea robusta is native to Eastern Australia. But it has been naturalized across the entire country. As such, the Grevillea robusta has been naturalized to Victoria, Australia as well.
What is the lifespan of grevilleas?
Grevilleas typically live for 15 years. Some can reach 20 to 30 years. Proper care will extend the life of these trees. It is likewise important to give it enough space in your garden or yard to establish itself well. A well-established tree is much stronger and lives longer.
Do grevilleas like sun or shade?
Most Grevilleas enjoy sun, including full sun. They also need well-drained soil to avoid overwatering. That said, there are some Grevillea varieties that prefer slight shade as well. This means it is important to select the location depending of the Grevillea you have.
Do grevilleas need a lot of water?
Grevilleas don’t need lots water. It is drought tolerant. In fact, too much water can cause overwatering. An overwatered Grevillea can experience root rot. Watering the plant once every once to two weeks is enough. And you don’t have to worry about forgetting. Your Grevillea will need more regular watering during hotter summers and much less water during the winter.