Thursday, October 18, 2012

Silky Oak

Grevillea robusta
The silky oaks are just beginning to come into flower.
We can expect to see them continue their spectacular display for much of November.

With a bit of careful planning they can be among our finest landscaping trees. Not suitable for small gardens, they are best used for acreage, highway, and park planting. They also make very good street trees - provided the streets are rather roomy. They co-ordinate wonderfully with the scarlet flowers of the flame trees (Brachychiton acerifolius), another spectacular local native plant that flowers at the same time.

Silky oaks are fast-growing and live for 100 years or more. They are drought hardy, and cope with frost to -4° even when young.
However, their large, shallow roots can be problem,  interfering with built structures, and travelling a long way for water. Ideally, the trees should be should be planted 5 metres away from concrete footpaths and 15 metres from underground pipes.

 They are deciduous in spring, with new leaves appearing as soon as the old ones fall. The impression given is that the younger generation are pushing the oldies off the tree.
The fresh leaves are a pretty shade of green, with silverybacks that show off in the wind.

In this country where deciduous plants are not common, trees which dump all their leaves in one go are regarded by some people as too messy for words. However, they do provide a wonderful “mulch opportunity”, especially as the intricate shape of the leaves means they don’t tend to blow all over the place, like the leaves of some other deciduous trees. Once positioned as mulch they tend to stay put. (I  pick them up from the lawn with a mower, and use the nutrient-rich grass and leaf mix on the garden).

Silky oak flowers are fascinating.

Each flowerhead might have as many as a hundred small flowers. At first glance, each one seems to consist of nothing much but a long style. This is the female part of the flower. The little green knob at the end is its stigma. It’s eventual female role is to catch pollen and pass it along to the ovary, a second little swelling lower down on the style, where the egg cells await fertilisation by the pollen.
New flowers are functionally male. The style is curved in a loop, and the stigma - not yet mature and sticky - is held firmly in a socket formed by the tip of what looks like a single little petal. As the flower begins to mature,  the “petal” breaks up into four tepals, and the four pieces of the opened-up socket are each revealed to have an anther on the inside.  This is the male part of the flower, now mature, producing pollen, and firmly in contact with the immature stigma.
The style straightens out, revealing the pollen-covered green knob of the stigma. At this point the female part of the flower is unable to be fertilised, and simply acts as a pollen presenter. It holds the pollen out there where it will rub off on the feathers of visiting birds which are attracted by the copious flow of nectar that each flower produces at this stage of its maturing process.
As the birds move around feeding on the flowers' sweet bait, the they carry the pollen about with them, much of it reaching stigmae of flowers which are at a later stage of development. The male part of those flowers has retired. It no longer makes pollen, and the flower is now functionally female. The stigma is now sticky and catches pollen from passing birds. Some of it makes its way down a pollen tube to the flower's ovaries.

 This photo shows the three stages of the flower. (Click to enlarge for a close look.) In the first (male) stage, the stigma is head-down among the fertile anthers, being dusted with pollen.  In the middle stage, the style has straightened out, and the stigma is liberally coated with pollen, presented ready for birds to take away.  In the last (female) stage, the stigma has become sticky and is ready to catch pollen and pass it down to the ovary. When  fertilisation has been achieved and the tepals fall off, the style begins to shrivel and the ovary swells to become a seed capsule.

The photo above shows a flowerhead fresh with newly opened flowers. Click to enlarge, to see the generous nectar flow that has suddenly appeared at the base of the styles. The tepals are clearly marked with red nectar guides, advertising its presence to the mobs of shrieking birds which descend on the trees for the feast.
The result is that pollen is scattered about rather wildly. Some pollen grains stay close to home, fertilising more mature flowers from the same tree. Some is spread to other plants. Grevilleas are well-known for their ability to cross-pollinate, even between different species.
In the old days, Aborigines used to make a sugary drink from silky oak flowers, dipping the whole flowerheads into water to wash the nectar off.
Silky oak timber is among our finest of cabinet timbers, with its warm honey colour. It is usually quarter-cut, to show off its silky rays.

Grevillea robusta and Allelopathy
Silky oak seedlings don’t thrive under parent trees, and it may be that they are suppressed by an allelopathic chemical produced by their roots.  I notice that many overseas internet sites claim that Grevillea robusta’s allelopathic effect kills off “the saplings of all other species”. This doesn’t seem to be the case in my garden or in our local rainforests.  In both cases it grows in close harmony with other plant species.
Claims about the allelopathic attributes of various plants are rather fashionable at present. In some cases they are true. In others, they are premature, the scientific research having not been done. Fast-growing trees can also suppress other plants by more efficient use of the available soil nutrients and water, and by shading them out with their canopies or their carpet of mulching leaves. This may be the real cause of some of the sweeping accusations of allelopathy. (An allelopathic plant usually has a different effect on different plants, with some species being suppressed, while others are actually improved.)
It wouldn’t surprise me, however, to hear that silky oak allelopathy suppressed other Proteaceae, and some research showing that it suppresses wheat may also mean that it suppresses grass (though I find that kikuyu grows well around the base of one of my trees).
I would be interested to hear from my readers about their experiences with growing plants near silky oak trees.


Timothy Elsmore said...

I have a sizable silky oak, looking for bee attracting plants to plant underneath it, I was thinking about trying lavender?

Patricia Gardner said...

Hello Timothy.
I am wondering why you particularly want Bee attracting plants. Is it for the honey, or do you just want bees on your silky oak?
I don't know where you are in the world. (Such is the nature of the internet.) But if you are in Australia, your tree will attract birds in the honeyeater group every year when it flowers. These are its main pollinators, so they will help the tree make seeds,

Patricia Gardner said...

To continue from the above: the seeds will then attract a second flush of seed eating birds. The flowers will also attract nectar-eating insects of many kinds, including butterflies and plenty of bees ( provided there are hives around).
Of course if you want lavender, you could give it a try. However silky oaks have rather greedy roots, so some plants don't do well under them. I find small wattle species good. They give a floral display in another season, amd provide bees with useful pollen.
Best of luck, whatever you choose. Perhaps you could let us know sometime, how you go.

John Kaup said...

I have a Large Grevillea Robusta with limbs down to ground level. When is it safe to prune these off without damaging the tree?

Patricia Gardner said...

Hi John.
These are very tough trees, and you should be able to prune them any time.
It's not a good idea to prune a tree in a very humid, rainy period, as this can let fungi in. Not usually a problem in our climate, though. It is only when there are weeks of continuous wet that a problem happens. Similarly, leaving a very slight overhang (cutting on a slant means you are not left with an upward-facing cut surface which might collect water as it heals around the edge. (The healing process results in a raised ring of woody tissue.)
Remember to leave a bit of a collar - just an inch or so of limb - rather than cutting it off flush with the trunk. The very base of the branch contains vigorous tissue which will grow more quickly across the wound. This protruding bit will eventually vanish into the trunk, becoming a neat scar as the tree grows.
And just a thought. Some people like to have a few lower limbs left as a nice bit of casual seating in a shady spot, or to make it a good climbing tree for the grandchildren.

Shannyn Steel said...

Hi there. We have built deep vegetable garden beds. I read you could fill the bottom with branches of trees before layering with cardboard, mulch and soil. Would silky oak branches with seeds cause issues with seeds gerniating later? The branches are 40 cm from what will be the top layer of soil.

Patricia Gardner said...

Hi Shannyn.
The answer is that they wouldn't provided they continue to be 40cm deep from the top (or any gaps in the side).
Seeds don't normally germinate unless they are near the surface.
Even if they did germinate as a result of future turning over of the soil,they are not likely to be a problem as they are very easy to weed out. You might get a few in the future, as the soil layers settle over time, and you turn the soil by pulling out deep-rooted veges, but I don't think you need to worry about them.

QuintaSkye said...

Hi, i have three silkyoaks growing in my yard (Toowoomba/Highfields area). They are each about 6' tall, however I dont like where they are positioned. Would you offer any advice about moving then to other locations?

Patricia Gardner said...

I have no knowledge about how well these trees cope with being moved, However, moving trees once they are past the small seedling stage is a risky thing to do. It is not possible to do it without damaging their roots, and a decision to take the risk may come back to bite you years later. Trees whose roots are not in excellent order may never develop a strong enough root system to do their job of holding up the tree properly.
My advice is to destroy these trees and grow new ones. Why risk having the trees fall and cause damage in future years when it won't really take very many years to replace 6' growth, with this fast-growing species.
The best results come from planting very young trees which are planted from small tubes, and never allowed to become even slightly pot-bound.

Tom said...

I have been doing some landscaping around a very large silky oak in my backyard. Unfortunately, the contractor decided to cut a very large root quite close to the trunk, maybe 1.5 trunk widths away from it. The root is very large and I am worried that the tree is now unsound and in danger of tipping over. Should I be worried. The trunk is perhaps 600mm, and the root perhaps 250mm.

Patricia Gardner said...

Hello Tom.
It is frustrating, isn't it, when people behave as though roots are somehow irrelevant to the health and structural strength of the tree.
I am sorry, but I can't give you advice on whether this has made your tree unsafe. I can comment that the trees seem to have strong roots, which could mean that the tree would manage without one of them - even a big one like that.
However I'm afraid that the decision as to whether the tree is now safe enough for its situation, that is something only you can decide. You might like to take into consideration things like the prevailing wind direction (obviously loss of a root on the upwind side will make it more vulnerable to being pushed over) and which direction the tree is likely to fall if it goes. What would be damaged if it went.
Sorry I can't be more help.