Silky Oak, what you really need to know

Grevillea Robusta aka the Silky Oak

The Grevillea Silky Oak is not only one of the most popular Grevilleas in cultivation but it also does have some features that are quite desirable. It does tolerate badly drained soils such as clay and will also tolerate some phosphorus toxicity in the soil as I’ve discovered in my own garden. It’s because of these features that it has become popular as a rootstock for grafting some (but not all) of the more difficult to grow Grevilleas and as such has enabled them to be introduced into cultivation in areas that once would have proven difficult.

The Grevillea Silky Oak also has a reputation for being one of the most adaptable Grevilleas. Despite growing naturally in the sub tropics in deep rainforest soils, near rivers and streams, it seems to have adapted fairly well to the cooler climates of southern Australia. For a rainforest tree it is considered to be drought tolerant and for a tree from the sub tropics it is also considered to be frost tolerant.

So there you have it that’s the Silky Oak in a nutshell, drought tolerant, frost tolerant and very adaptable. Sounds like the perfect tree to plant as a specimen. It’s also a tree that I greatly admire as I’ve seen some absolutely fantastic specimens of it in flower over they years but for me when I started my new garden I just couldn’t bring myself to find a place for it……..and here’s the reason why.

As you drive around you do see lots of Silky Oaks growing as street trees, in parks and in peoples front gardens. You see lots of fantastic looking specimens but for some reason you also see a lot that look like absolute rubbish. Now when I say rubbish I’m taking about the fact that there seems to be lot of Silky Oaks with branches that appear to be dead without any foliage on them……and here’s the strange thing. Quite often you will see a group of them growing in close proximity and they really appear to be a mixed bag. Some look fantastic, some look just average and others look to be quite good specimens……and my question is, why?

Well one reason might be because Silky Oaks do have a reputation as being semi deciduous in cold climates. This is true but some of these trees actually look like this during the warmer months as well. I suppose the important point is that some Silky Oaks are affected by this and others aren’t. So back to my question, why is it so?

The upper branches on the first Silky Oak have lost their leaves while others behind range from "doing quite well" to "struggling" like the one at the front.

These Silky Oaks are growing in Melbourne. The upper branches on the first Silky Oak have lost their leaves while others behind range from "doing quite well" to "struggling", just like the one at the front.

Is it because maybe they’re not as drought tolerant as their reputation suggests. Well my sister lives in central Victoria where the summers are a lot hotter and drier than Melbourne. She has a very mature Silky Oak growing in her front garden that looks to be handling the conditions very well and I also know for a fact that she never waters it so it just survives on the local rainfall. I’d consider this tree to be a very drought tolerant plant. Now the other thing about her locality is that they also get a lot more frosts there over winter so it would be safe to assume that this particular Silky Oak is also frost tolerant.

Now here’s the other interesting thing. Just a few hundred metres up the street there are some Silky Oaks growing as street trees under the same conditions as my sister’s Silky Oak and they are a real mixed bag. Some are doing fine and others have defoliated branches that look quite ugly. So, why does this happen?

The only thing that I can suggest is that the one thing that all of these Silky Oaks have in common is the fact that they are all grown from seed. As any gardener who like to propagate plants knows, when you propagate plants from seed you get variation. It’s just like a litter of puppies and kittens. When you go to pick out a puppy from a litter there will always be one that stands out from the rest, there will be the runt and the rest will be a variation somewhere in between. So it stands to reason that if you are going to propagate Silky Oaks from seed, even if you pick superior trees as the parent you will still get some variation in the ability of the new plants to withstand drought and frost…….and then again, if you were to use seed from inferior trees then the quality of your results will be even less again.

The upper branches on the first Silky Oak have lost their leaves while others behind range from "doing quite well" to "struggling" like the one at the front.

This was the only young Silky Oak I could find.

Now here’s the other thing that I’ve also noticed about Silky Oaks in Melbourne. The other day I drove around my local area looking for Silky Oaks and guess what……..I found heaps of them. They were everywhere. The one thing that they all had in common was that they were all very mature trees that had been planted 20 or 30 years ago. I couldn’t find any that were young of less than say 10 years old except for one solitary Silky Oak growing in a park all by itself. I even looked in new housing estates and couldn’t find any growing there either.

It appears as if what was once a very popular tree has now lost it’s popularity, and understandably so. There are just too many Silky Oaks out there in the gardens of Melbourne (and other places) that actually just look like rubbish, and I reckon it is a real shame. Growing these trees from seed seems to be a little bit too unreliable. So you have to ask, why not just grow them from cuttings?

Well it’s not hard to guess why. Growing Silky Oaks from seed is pretty easy. After flowering they set lots of seed and the seed also germinates fairly easily so from a commercial sense this is probably the most economical way. But as we’ve already discussed this also means that you don’t necessarily get the best trees as a result. The other problem is that a seed grown Silky Oak takes many years to flower. So if you want flowers you have to be patient and then when they finally do flower quite often the flowers are that high in the tree they can only be admired from afar.

On the other hand though if you were propagating cuttings from a superior Silky Oak the advantages would be superior trees, trees that flower at a much earlier age and most importantly the Silky Oak may once again become popular in streets an gardens again.

So there you have it. That’s my take on Grevillea Robusta, the Silky Oak. I think it is a very worthwhile tree to grow, but if you are going to grow it find an old tree that looks like it doing very well and take some cuttings and propagate from it. After all if you are going to spend the 10, 20 or 30 years growing a Silky Oak into a specimen tree then why not take the time to do it properly and just propagate it yourself.

Read more about Grevillea Robusta, the Silky Oak.

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35 Responses to Silky Oak, what you really need to know

  • lozzi says:

    I built my house 5 years ago in a rural area in east Qld (Mareeba) which is such a dry place .I have planted so many silky oak trees that they are now 3 years old and looking great…they give privacy, I hardly ever water them and they just keep growing. I trim them when they are young and now they are quite bushy…definately the drought tolerant tree to plant.

  • Rebel Gardener says:

    Many plants respond well to pruning when young which sets them up to grow into much bushier and attractive plants. Sounds like you’re now reaping the benefits with trees that provide privacy. All the Best.

  • Roger W. Briggs says:

    Is there anything that can be done to minimize the leaf fall from my large silky oak tree. The tree is over 30 yrs old – was about 400mm high when planted – and is easily 25-30 metres high. Its a wonderful shade tree during summer and a picture when in flower in spring but it drops its leaves all over the neighbour’s garden and in his pool and is causing major problems!!

    • Rebel Gardener says:

      Hi Roger. Silky oaks are renowned for this and unfortunately the leaf litter is just part of owning one. One option you have though is to prune the offending branches that hang over your neighbours fence. I’ve seen Silky Oaks that have been pruned quite severely because they’re growing under powerlines. They respond quite well to pruning and these specimens formed a low wide canopy like a large mushroom with low hanging branches which bought the flowers closer to the ground. So if you get to the stage where removal is an option, to maintain good neighbour relations, you may be better off pruning it over time into a shape that doesn’t cause it to drop leaves in you neighbours yard. Remember, if you live in an area with frosty winters autumn may not be a good time to start pruning it.

  • Roger W. Briggs says:

    Thanks R.G. for your reply. I live in Brisbane so frosty winters aren’t a concern. So does this mean that the pruning could take place at anytime in the year or is there a better time?

  • Rebel Gardener says:

    You should be ok in Brisbane to prune now. I’ve got plants here in Melbourne that I pruned a month ago that are showing signs of regrowth. If you’re unsure of how much to take off just be judicious with your silky oak and don’t try to achieve your final result in one go. Cut the offending branches first and then wait for it respond. Over time you should be able to prune it back into an attractive tree that won’t drop leaves in your neighbours yard. Best of luck.

  • Erin says:

    I am new to gardening but really keen on native plants so excitedly planted a silky oak I got for xmas (nursery-bought)around late December 2009 (I live in Brisbane).
    When it was given to me, some of the leaves were dead (perhaps sun-scorched given time of year?), so I pruned those off and then planted it out on my footpath to see how it would go (it’s about 2m tall but fairly sparse, no low branches, cover over only about top third of trunk, with a shoot coming out at ground level).
    It has started sprouting new leaves, but I’ve noticed that the new leaves seem to be being eaten by something (i.e. down to the stems) and others are dying. Is this normal? I can’t see any bugs on it but am keen to get onto it quickly if I need to so I don’t lose it.
    Thank you!

    • Rebel Gardener says:

      The best solution would be to take one of the chewed leaves to your local garden centre and ask them there as they would be more qualified to give give advice relevant to your local conditions. If you can’t see any bugs during the day you may be able to check it of an evening after dark. I have a silky oak growing in a pot here in Melbourne and the new growth got eaten by caterpillars in spring, which were difficult to see on the new stems as they do camouflage really well but there’s no guarantee that they will be the same culprits in Brisbane.

  • Di says:

    I planted two silky oaks arround 4 yrs ago 4 metres apart.They both have grown to the size of 8 metres high both just beautiful, then one lost all its leaves last fall and has been like that since.The other is full of foliage.not quite sure what to do,and have been awaiting any sign of green.I am on the mid north coast and there are lots of beautiful silky oaks here all doing well.

    • Rebel Gardener says:

      I’m sorry but if it hasn’t had any foliage since last fall (I take it you mean autumn?) it sounds like it might have died. You may need to consult with an arbourist.

  • mary williams says:

    Can anyone give me advice on the tree roots of a silky oak? We have a tree thats about 40 feet tall which the local bird life (and us) adore. The problem is though that the trunk is about two feet away from our house – we’re concerned this may cause a problem as the tree continues to grow. Does it have to go?

  • Anthony says:

    I have a large Silky Oak in my back yard which was there when we purchased. It’s quite an attractive tree but a disaster being rather close to our house. When it’s not dropping thousands of frond like leaves, it’s the yellow flowers and when it’s not that it’s the brown seed pods – all over the back yard or on the roof and in gutters. Forget pruning away from neighouring properties – unless they’re upwind.
    I would not recommend planting one of these – unless you would like an almost full time job cleaning up the mess below it.

    • Rebel Gardener says:

      Hi Anthony, thanks for your comment and you are spot on. Way too many people do this with way too many trees. Eucalypts are another prime example. I visited a house just the other day that was surrounded by Gum Trees (eucalypts) and the gutters were over flowing with leaf litter. In fact it was so blocked up you can see where dirt and debris has caused weeds and grass to grow in the gutter that could be clearly seem from the ground. There were also sticks and small branches littered all over the roof and ground. All these trees are beautiful and they certainly have their place but they should be kept well away from buildings. At the end of the day though it all comes down to common sense. Remember plants only get a bad reputation where planted in an inappropriate position therefore do you believe that it is the plant that is at fault or maybe the gardener that planted it???????

  • Juanita Chadwick says:

    Hi everyone I have numerous Silky Oaks planted down our driveway (semi rural property). We noticed that one of our trees which is about 3 years old and stands 8 metres tall still looks healthy and flowered this year is leaking large blobs of sap. Does any one have any idea what this is all about.???

    • Rebel Gardener says:

      It may be under attack by some sort of pest. Best to ask at your local nursery and if they cannot give a satisfactory answer then you may have to get a professional in to look at it.

  • Bambi says:

    We live on acreage on the Sunshine Coast, QLD and have quite a few silky oaks planted on the farm. We think we may have a Tip Moth invasion but I am struggling to find out more information on how to get rid of them. They are eating the new shoots on the tip of the juvenile trees and stunting growth, making them quite bushy rather than tall and straight. The trees are only 1.5m tall at present.
    What can you recommend? This is what the grubs look like –
    I prefer not to use chemicals on the farm.


    • admin says:

      Hi Your best solution would be to consult a local Arbourist as this will require some specialised knowledge. Having said that though, the damage may have already been done. On a brighter side you may not end up with the desired form you were after but the bushy plants, when in flower, will produce a much more spectacular and prolific display. One of the traits of tall silky oaks is their flowers are often held high in the tree. Whereas a bushier plant will display them closer to the ground where can more closely be admired.

  • Marc says:

    I have a massive silky oak gravillia in backyard up in Lismore and the neighbors across the roads kids remember that big tree while they were growing up befor my house was even here. That was 30 years ago. How old could this tree be? How can I tell? Should I saw it down and count the rings

    • admin says:

      Marc, Silky Oaks can grow to an old age. If you want to find out how old it is it would be best to consult with a local tree professional but really cutting it down to count the rings would be a real shame wouldn’t it? Surely you wouldn’t do that?

  • Tony says:

    I have a large Silky Oak in my back yard in Greensborough, Melbourne. It is about 40 years old, the trunk is 2 metres in circumference at the base and it is probably 20 to 30 metres high. It is close our house and causes the neighbour some concern with the amount of debris it drops. I love the tree and do not want to remove it. I am thinking of having it cut back (topped) to about half its present size and severely trimmed. What do you think about this course of action? Would the tree survive?

    • admin says:

      Tony, it may survive. I’ve seen quite mature silky oaks growing under power lines that have been pruned severely and have grown out into a wide canopy. The best way would be, to consult with an experienced arborist. Just to make sure it survives, it might be a case of prune half and wait for regrowth before completing the job, but an arborist will advise the best course of action. Best of luck.

  • Brad says:

    I have a mature Silky oak in the backyard which has developed a streak/ line of shedding bark extending up the main trunk. the trunk appears to be cracking slightly behind the broken bark. The tree has also lost a lot of its leaves. What would the cause of this be?

  • Andy says:

    Silky Oaks are amazing trees, however they’re actually considered to be environmental weeds in some parts of Victoria and New South Wales, for example they’re on the Boroondara weed list in VIC. So it may be worth double checking with your local council before planting to avoid compounding a potential problem.

  • Gilly47 says:

    I love silky oaks,
    I am planting and propagating them every year from seed.
    I am selecting the best for planting.
    Just as a litter of pups are individually different so are the trees

    Currently have about 100 seed that have struck
    Just wondering if anyone has hedged these trees
    Currently using photinias for hedging because they grow so fast
    If any one has had success with hedging silky oak I would be pleased to hear from them

  • Do Silky Oaks make good windbreak trees? We do get a lot of wind here and this year we have had a couple of large storms and lost some trees from the middle of the windbreak. Would Silky Oaks be good trees to plant?
    Thank You

  • Jo-Anne says:

    I have 2 self sown silky oaks (curteosy of the street planted one :)) that have seeded themselves along side my rainwater tanks. I do like the look of them and would love them to shade the tanks. (in 2 years – they are as high as my 2700ltr tanks) Unfortunately, I can’t find anything about the root system and whether my newish tanks are in danger of being ruptured underneath. Any help would be greatly appreciated.

    Jo-Anne. (north central victoria)

    • mikeb says:

      If your tanks are plastic I think they should be fairly safe. For my way of thinking though the Silky Oak does grow into quite a large tree so I wouldn’t plant it close to an structure. They also drop quite a lot of leaves as well. If you have an open area that would be more suitable? they are probably still small enough to shift easily.

      • Jo-Anne says:

        Thank you Mikeb. My tanks are plastic and I came to the same conclusion after posting, that they really are too close and I need to remove them.

        As much as I hate killing trees, I don’t have enough land to move it elsewhere – especially as I want a medicinal and edible garden with lots of fruit trees 🙂 . I can enjoy the silky oaks that are planted on my nature strip and the magnificent specimen across the road. I’ve found quite a few self sown ones now, so I may try potting them up and selling them at the gate 🙂 🙂

        Thanks again for your reply 🙂

      • zahra jamal says:

        can i use the wood of silver oak for make furniture or wooden doors .it is usefull for furniture.

  • George Jaeger says:

    I live in Scottsdale AZ and have an old silky oak next to my house. I found a stump that looks like another one was cut down. Has anyone fertilized one of these trees. My nursery guy sold me some iron and normal fertilizer to help the remaining old and stragly silky oak get healthier. And he said that like water so I was going to add some deep watering?. Yes?

    • Administrator says:

      Silky Oaks grow naturally in the sub tropics of Queensland. I’m guessing Arizona is quite a dry climate so the deep watering would help. If you decide to fertilise I’d use a low phosphorus fetliser.

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