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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