Saturday, June 12, 2010

The Perfect Windbreak

Scrub Wilga
Geijera salicifolia
It was so cold on the Darling Downs west of Toowoomba yesterday! The weather report told us the maximum temperature was 14°, and there was a dry, bone-chilling breeze.

However at Irongate Reserve, in the sunny clearings sheltered by the scrub, the weather was delightful. The little birds and butterflies were going about their business as usual, and we felt we could ignore the wind, which was merely providing the musical accompaniment, singing its she-oak song well above our heads.
One of the most effective windbreak trees at Irongate is the scrub wilga.
(Don't confuse it with the very narrow-leaved common wilga, Geijera parviflora, which also occurs at Irongate, but is better known from the plains further west).
The scrub wilga’s great virtue is the density of its dark-green canopy, which it maintains year-round and through the worst droughts. Like so many of our dry rainforest/vine scrub species, scrub wilgas have very deep roots which make use of deep-down groundwater. This lets them be even more resilient in droughts than some of our toughest eucalypts. Over the last few years we have seen Eucalypts dying in paddocks west of Toowoomba, while the nearby “scrub trees” were looking as lush and green as ever.
Here are some roadside specimens growing near Mt Tyson, showing the typical canopy-to-the-ground formation of younger trees, which makes them such good windbreaks. As you can see, they are very happy snuggling up close to the mountain coolibah Eucalyptus orgadophila, making a little clump with obvious bird-appeal.

This garden-grown specimen at Gowrie Junction is known to be 27 years old.

With age, the plants develop into pretty little shade trees. I had thought that pruning by cattle must help with the shape (as pruning by sheep does with the common wilga) but I have been assured by a cattle farmer that cattle won’t eat the leaves of scrub wilgas. They are often left in paddocks as shelter for cattle.
This one has had a hard life, with the soil around it well-compacted by stock.
Paddocks in this district were first cleared about 1880, and it is possible that this tough little tree has been roughly this shape and size for 120 years or so. With many of our vine scrub tree species, growth in tough conditions is very slow indeed, and some very old trees are deceptively small. If this one became part of somebody’s garden, and its soil give a bit of extra care, it would probably start to grow again.
Like many of our local native trees, scrub wilgas are very long-lived. The one at the foot of this article grows at at Gowrie Junction. It has never been dated, but could well be 300 years old.
Don't believe the story - usually told as a justification for planting foreign species - that Australian natives are "short-lived"! (It's partly true, of course. The short-lived species are indeed short lived!!! However, only someone who doesn't know much about Australian natives could think that Australia is more richly endowed with short-lived species than any other part of of the world.)
I imagine, though, that a gardener who wanted a little shade tree, could discourage the early tendency to low branching, by judicious use of the secateurs.
Scrub wilgas have a long flowering season. They were beginning to come out in early April, and are at their best around the district in June and July. They are a mass of flowers at Irongate at present, providing food for insects, which in turn provide useful over-winter food for many little birds. The shiny black seeds which follow will provide a further food source.
Botanists tells us that there two varieties (a broad-leaved one, var. latifolia, and a narrow-leafed one, ver. Salicifolia) however here above the Great Dividing Range we notice that there seems to be a continuum. Close to the range they are broad-leafed, and as we move west they become progressively narrower in the leaf.

This is what the leaves look like at Highfields....

...and this is a photo I took at Jondaryan in April. Note the much narrower leaves....

...while the ones at Irongate are somewhere in between.

This picture shows the deep grey twiglets, which, together with the dark canopy, give the whole tree a rather dark look from the far. This must be part of the reason why the local common name for this tree is “black alley”.

The crushed leaves have a delicious plum-pudding smell. This plant is in the same family as citrus trees, and like them is a host plant for our largest native butterfly, the orchard butterfly (Papilio aegeus).
I am told that the butterflies grow larger on their native host plants than they do on introduced citrus species.
Irongate Reserve is in Wallingford Road, between Pittsworth and Mt Tyson.


Ozibird said...

I was interested to see that Geijera parviflora, or Australian Willow as it is called in the US, recommended as a street tree.

"Makes good street tree; noninvasive roots, casts light shade. Moderate growth rate. Pendulus habit."

Patricia Gardner said...

Yes, I'm interested, too, Ozibird.
The name "Australian Willow" sits oddly with us Australians, doesn't it? When we think of willows here in Australia we tend to think of the weeping willow, Salix babylonica, a widespread invasive weed in our watercourses. It is quite unsuitable as a street tree,with its invasive water-seeking roots that can do so much damage to drains, pipes, and footpaths. And of course it wouldn't survive in the difficult, dry situations where the "common wilga" (G.parviflora) thrives.
I think its taller cousin the "scrub wilga" Geijera salicifolia would be a better street tree for most situations. However the common wilga is even more drought hardy, and would have the advantage of never getting high enough to interfere with overhead wires. It would need quite a bit of space to show its full beauty, though. In its best form it can get to be about 5 metres wide, and makes a very attractive shrub shaped like a rather squat ball of dense green foliage, sitting on the ground. Pruning can do wonders, of course, and on sheep properties it loses the lower half of the foliage. Sheep trim it to a neat, shady umbrella-shape, which those gardeners who love topiary could well imitate for a very pretty, but rather low, shade-tree.

John Mavrouko said...

Native Willow - Geijera parviflora - I have a great specimen in front of my house planted by local council 6-7 yrs ago.
I'm keen to learn how to cultivate Wilga trees from the black seeds and to encourage others 'to plant a tree for life' in the council area.
When and how do we plant the black seeds and is any special treatment required?
John Lesses Unley SA

Patricia Gardner said...

Hello John.
These plants are notoriously difficult to grow from seed. Their dormancy mechanism/s are not properly understood. Some of the best success has come from:
a. keeping the area under a tree well watered, and digging up seedlings as they appear. They must be dug up very early in their lives, as soon as the seed-leaves (dicotyledons) are well established, or perhaps when the first true leaves are just showing, and you need to make sure the soil is damp, and dig deep and carefully so as to not break the fine rootlets. They don't mind being bare-rooted, but don't break anything. Pop them into water as soon as you have them out of the ground, and plant them with care. A bit of mother soil in the pot or around the new planting site may help, as there's a possibility that they grow better in association with certain soil fungi.
b. collect soil and mulch, containing old seed, from under mature plants and keep in a tray which is LIGHTLY watered in an area of low humidity (a shadehouse may be too damp) till seedlings appear.
Other methods which are claimed to work include:
c. Leach seed for 3-10 weeks in muslin bag, tied in cistern of frequently used toilet, to remove substances which cause dormancy. d. Method 1: Leach 3-10 weeks in muslin bag, tied in cistern of frequently used toilet, to remove substances which cause dormancy.
d. Carefully peel off the black seed coat, to remove the germination inhibitor.(This method is used by a successful nurseryperson.)
Best of luck!