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?
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.
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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Grevillea Robusta, an alternative way to grow.
In my last article about Grevillea Robusta, the Silky Oak, I wrote about the fact that seed grown trees often take some years to flower and the possibility that trees propagated from cuttings may actually flower a lot sooner. The other issue with Grevillea Robusta is the fact that most of the flowers are held quite high in the tree and are quite difficult to admire up close.
I’ve often wondered if there was something that could be done to solve this problem, especially since this tree looks so spectacular when in flower. This reminded me of a Grevillea Robusta that I once saw growing under some powerlines that had been pruned quite heavily to limit it’s growth. This tree was about 4 or 5 metres tall and had an wide canopy that actually made the tree look quite attractive. It occurred to me then and there that Grevillea Robusta was indeed quite receptive to heavy pruning.
I then decided to pursue this line of thought that then led me to the obvious idea that instead of waiting until the tree is 3 or 4 metres tall, why not just prune it right from the start and in an attempt to prune it into a bush rather than it’s natural tree habit. Now I must say right from the start some gardeners may be skeptical about the ability to limit the size of a tall tree such as Grevillea Robusta in such a manner. But then again if it is possible to do so by pruning it at 3 or 4 metres to keep it to no more than 5 or 6 metres in overall height then why couldn’t you just prune it at ground level to turn it into a bush that is 3 or 4 metres tall.
Imagine this, Grevillea Robusta grown from a cutting (from a superior specimen). Then successively pruned as required until it branches prolifically to produce a rounded bush. Then when it flowers, it’s covered in it’s trademark orangey, yellow flowers that extend nearly all the way down to ground level. Wouldn’t that just look spectacular?
So what I’ve decided to do is go ahead and do it. Now for me though unfortunately I haven’t got enough room to do it in the ground so I’ve decided to grow one in a pot and here’s what I did.
This wasn't the best specimen of a Grevillea Robusta I could find but it did have lots of branches.
First of all I went to a nursery to purchase a Grevillea Robusta. Normally I would have selected a plant with one straight single trunk but for the purpose of the exercise I decided to select one that had several branches on it already.
Next up was to take it home and then prune it encourage it to branch even more.
Then re pot it into a larger pot, add some water granules so it is always moist and also some low phosphorus native fertilizer so it never goes hungry.
The idea here being to pamper the plant (something I don’t normally like to do) so as to optimize it’s growth potential.
Grevillea Robusta, now pruned and in a larger pot full of water granules and Native plant fertilizer.
The other thing to mention here is that the plant that I’ve selected has most probably been grown from seed so it may take some years to flower. The important thing though is that I’m trying to see how much I can get it to branch into a shrub rather grow into a tree.
As far as flowering goes though, what I intend to do is find a Grevillea Robusta that is mature and does appear to be a good specimen tree so I can try propagating it from cuttings in an attempt to then compare it against the seed grown one. So over a period of time there can be a comparison between the flowering and the growth rates.
This of course will take time but in the meantime my main objective is to just to see how much I can get this Grevillea Robusta to branch while growing in a pot.
So there you have it an alternative method to grow Grevillea Robusta but a while ago I saw something that made me think there may be another way of growing this tree.
As I’ve said before, Grevillea Robusta is quite often used as a rootstock for grafting other less reliable Grevilleas. Not long ago I was in a nursery and saw a grafted Grevillea with a scion that wasn’t growing very well at all. As a result the Grevillea Robusta rootstock had started to grow a shoot just below the graft that was actually doing better than the scion.
This then led me to wonder if you could actually coppice a mature Grevillea Robusta that was struggling as described in the previous article to rejuvenate it and then maybe grow it into a shrub as suggested above. I then went home and researched it and what I discovered was that it wasn’t considered to be a tree that you could coppice. Nevertheless that grafted grevillea had in fact grown a shoot just as you’d expect a coppiced tree to do so. So maybe it is possible.
Now I wouldn’t advocate for a minute that somebody should cut down their Grevillea Robusta to find out. But on the other hand, trees do from time to time get old and do get cut down because of it. This would in fact be a perfect opportunity to leave the stump in the ground and see if it does re shoot. You never know, you may just end up with a new plant because of it.
So there you have it. Whether you’d like to grow a Grevillea Robusta from scratch or if you’d just like to rejuvenate an old one here’s a couple of alternative ways to grow it.
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Grevillea, How to grow.
The Genus Grevillea is one of the most widespread over the continent of Australia. Its distribution ranges from the tropics of northern Australia to the temperate areas of Tasmania, from the deserts of central Australia to the alpine areas of the Australian Great Dividing Range. Different species have adapted to different climatic areas so to answer the question “how do you grow a Grevillea” might not be as straight forward as you might think.
But then again maybe not!
Grevillea is made up of over 300 species and subspecies as well as hybrids that number into the hundreds. Not all of these are in cultivation and most are not readily available in nurseries, but I think it would be safe to say that if you look hard enough you can probably find at least one or two that will suit the conditions in your garden, will be very easy to grow, flower for an extended period and require little or no maintenance.
So here’s what you need to know in a nut shell.
1. Never fertilize your Grevillea. Grevilleas are a member of the Proteaceae family of plants and have evolved a very sophisticated root system that is very efficient at finding the nutrients it needs in very impoverished soils that are low in phosphorus. High phosphorus fertilizers will kill your Grevillea. In fact I don’t recommended fertilizing your Grevilleas at all, even if it’s low phosphorus and designed for natives. The only exception being if you were growing your Grevilleas in pots or containers or if you had a problem with phosphorus in your soil as I do. Therefore to summarise, don’t fertilize them and don’t plant them near plants (or lawns) that you do fertilize.
2. Don’t plant them in an area where the water tends to sit or doesn’t drain away. Plant them either on a slope, in a raised garden bed or in reasonably well draining soil. Most Grevilleas don’t like too much water around their roots. Of course though there are exceptions. I’ll list some of these shortly.
3. Don’t cultivate around their roots. Grevilleas have lots of surface roots and if you do, you will probably damage the roots and maybe kill the plant. If you do this by accident just water the affected area with a solution of Seasol. This may help.
4. Plant the right type of Grevillea for your climate and soil conditions.
5. Most Grevilleas are very drought tolerant so water for the first summer to establish then only when they really, really need it after that. Therefore don’t plant them near plants that need lots of water to survive.
6. Prune your Grevillea after flowering. Now pruning isn’t essential but you will find that most Grevilleas respond very well to pruning and if you do prune your plant it will look a lot neater and tidier, flower more profusely next year and will probably live a lot longer.
Grevilleas tolerant of poorly drained soil. There are many grevilleas that will tolerate poor drainage. Most are lesser known and not readily available at nurseries, the exception being Grevillea Robusta aka the Silky Oak. This Grevillea grows into a large tree but the great thing about it is that is can be used as the rootstock to graft lots of other more difficult species and hybrids, therefore making these Grevilleas a lot more adaptable. So if you have a space where the water doesn’t drain away very well, then a grafted grevillea could be a good option. Some examples that I’ve grown with this rootstock are Grevillea “Billy Bonkers”, “Pink Ice” and “Lollypop”, but there are many, many more. Just finally, I have found that Grevillea Robusta does appear to have some tolerance of phosphorus in the soil.
Grevilleas that are easy to grow. These grevilleas are adaptable to a variety of soils, climates and are reasonably easy to source. There are so many to choose from. Here are just a few that are common or easy to find in nurseries.
Grevillea Robyn Gordon, Superb, Ned Kelly, Peaches and Cream, Coconut Ice, Strawberry Sundae (all Grevillea Banksii x bipinnatifida hybrids).
Grevillea Juniperina
Grevillea Lanigera
Grevillea Long John
Grevillea Rhyolitica, Deua Flame.
Grevillea Sea Spray
Grevillea Speciosa
Grevillea Rosmarinifolia
Grevillea Thelemanniana
Grevilleas for tropical areas. This could be a case of where do you start here. There are so many spectacular Grevillea hybrids that come from the tropics. Most of these Grevilleas won’t tolerate frost but I have found that Grevillea Moonlight seems to be the most tolerant of cold conditions. Here’s just a few.
Grevillea Formosa
Grevillea Honey Gem
Grevillea Misty Pink
Grevillea Moonlight
Grevillea Sylvia
And the list goes on and on
Grevilleas for arid/desert areas. Most of these are more suited to well drained sandy/gravelly soils. They are very drought tolerant and some have some very spectacular flowers. Most aren’t readily available in most nurseries though, but can be found in specialist nurseries. The other great thing is that most can be grafted onto more reliable rootstocks, so these grafted grevilleas do make them much more adaptable.
Grevillea Eriostachya
Grevillea Excelsior
Grevillea Juncifolia
Grevillea Petrophiloides
Grevilleas tolerant of frost. The news here is that most Grevilleas will tolerate at least some frost, especially since many desert areas have temperatures that drop below freezing at night during the winter. Most of the tropical grevilleas are the exception here but if you get light frost once or twice a year they may still be worth trying. As plants get more established their frost tolerance does increase and Seasol is also meant to help with a plants frost tolerance.
Grevillea Eriostachya
Grevillea Excelsior
Grevillea Juncifolia
Grevillea Juniperina
Grevillea Lanigera
Grevillea Petrophiloides
Grevillea Rosmarinifolia
Grevillea Speciosa
Grevillea Thelemanniana
So just as the size, colour and shape of the Grevilleas leaves and flowers varies so does the range of climatic and soil conditions. There are grevilleas that suit any gardener’s tastes and Grevilleas that will suit any garden or position in the garden you wish to plant them.
The important thing to remember when you’re selecting a grevillea at your nursery is that just because it’s in your nursery doesn’t mean that it will grow in your area or in your soil. Ask the nursery person first otherwise just take the risk, give it a go and you may actually discover that the grevillea that isn’t meant to grow in your area or in your soil actually does. That’s what I often do. You may make a new discovery and learn something new.
Anyway next time you’re at your plant nursery give a Grevillea a try, follow the 6 tips I’ve listed above and let me know what you think.
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- Grevillea Robusta, an alternative way to grow.
