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 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.
