Thursday, October 18, 2012

Silky Oak

Grevillea robusta
Family: PROTEACEAE
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 sticky stigma. It’s 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.
In a new flower, the style is curved and the stigma is held firmly in a socket formed by the tip of what looks like a single little petal. As the flower matures, however, the “petal” breaks up into four tepals, and the four pieces of the socket are each revealed to have an anther on the inside.  This is the male part of the flower, and produces the pollen. In the last stage, fertilisation has been achieved and the tepals have fallen off. The ovary is ready to swell and become a mature seed capsule.



 This photo shows the three stages of the flower. In the middle stage, the style has straightened out, and the stigma is liberally coated with pollen (clearly visible in the photo. Click to enlarge.). 
Here is an example of a flower which has evolved to eliminate the middleman. There's no need for a pollinator, a bird or insect to carry the pollen from anther to stigma.  Self-pollination is common in grevilleas - but it's very obvious, from the flower's appearance and behaviour, that this is not the whole story, because at this stage the flower suddenly produces copious amounts of nectar.


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.
There is so much nectar that a lot of it gets spilled. Also, the rainbow lorikeets look beautiful, but... Well to put it mildly, under a silky oak tree is not the place to hang your washing to dry!
The result is that pollen is also spread effectively from flower to flower. 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.

5 comments:

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.
Cheers
Trish

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.
Trish