Editor's Note: This article was originally published on March 24, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
Fire is a very important component in the evolution of the flora of Australia. The Acacias, and their close cousins, the Peas and the Cassias have handled the frequency of fire by developing a very hard coat on their seed. This means that the seed under normal conditions will not germinate at all, but a large seed bank can develop in the soil. When a fire does occur, quite probably killing the parent plant, the heat of the fire cracks the outer coating of the seed. The next time it rains, the moisture penetrates the seed and often a quite dense germination of seed will follow.
Silver Wattle Seeds
There are reported cases of fire crossing a cleared area, where no Silver Wattle - Acacia dealbata, has been seen for 50 years and following the fire a whole forest of Silver Wattle started to grow.
It is nevertheless easy to persuade Acacia seed to germinate and seed is the commonest method for propagation of Acacias.
Here are some seed of a Silver Wattle in my garden, collected on 6th September 2005 and sown soon after. It was fortunate that I did, because the parent tree was brought down in a fierce storm shortly after.
The seed germinated well and here are six seedlings on 14th February 2006, perhaps 10 - 15 cm tall. I planted one out this spring and the sapling on the right is one of the 6 in that pot and is now 1.65 metres tall on 1st January 2008. Many wattles are very fast growers.
The sapling is still looking very green, but it will soon develop its silvery foliage.
In order to germinate Acacia seed, one needs to simulate the effect of the fire in cracking the outer shell of the seed. The process is called scarification and some of the techniques used are to either nick each individual seed with a sharp knife, or to abrade the seeds with coarse sandpaper. These methods are effective, but are awkward to apply.
Cootamundra Wattle Seeds
Add Boiling Water
Seedlings after 3 Weeks
The technique which I prefer more closely simulates the effect of a fire by applying a rapid change of temperature to the seed. The method is illustrated here with seed of the Cootamundra Wattle - Acacia baileyana.
1. Place the seed in a cup or small bowl, and pour boiling water over the seed. Some books recommend water that has come off the boil, but I have used boiling water many times on many different species and it has worked every time.
2. Allow the seed to cool in the water. leaving them overnight is quite effective. If left long enough, some of the seed will be seen to have swollen to nearly double the size. With the ten seeds used in the illustrations, four seeds had swollen by the next morning. This can mean that these are seeds taking up water, but in this case two of the swollen seeds were merely empty shells.
3. Discard any obvious duds and sow the remainder in a standard seed growing mix, covering them by about the thickness of the seed.
Germination by this system is quite rapid. Two seedlings appeared after two weeks and were potted up. Three more appeared after three weeks as shown above, and 1 more germinated a week later. That is, 6 of the 8 seeds sown germinated, so maybe the other two that swelled up quickly were also duds!
The first true leaf to appear is usually pinnate, but by the second leaf a bipinnate leaf is produced. In this species, the Cootamundra Wattle, the adult leaf is bipinnate as shown. They are a silvery colour, but this is not apparent in the new seedlings.
This same technique works with all Australian Acacia species. I have personally used it to germinate about 15 different species with consistently good results.
A large number of Australian Acacias, have the bipinnate leaves replaced with leaf-like phyllodes, which are actually flattened leaf stalks. Growing these species from seed is intriguing, because they start with bipinnate leaves like their cousins and probably like their ancestors.
Pictured at the top of this article is a seedling of the Snowy River Wattle - Acacia boormannii. The seedling has a first leaf which is pinnate. The same individual is shown below, a little later, with a bipinnate leaf.
Seedling + bipinnate leaf
As later leaves develop, the leaf stalk, the petiole, will expand slightly and the leaf will reduce and soon disappear.
In most Acacias, the transition to phyllodes is quite rapid and soon leads to the full adult form. The adult phyllodes for the Snowy River Wattle are shown next to the seedling and it is among these phyllodes that the flowers in the next picture will eventually appear.
In a few species, the progression to phyllodes is more gradual, and the Blackwood - Acacia melanoxylon may grow to 1.5 metres with bipinnate leaves, before it begins the transition to the phyllodes borne by the mature tree.
Seeds are a reliable way of propagating Acacias, but they do not guarantee preservation of a special form. In fact seed grown plants can give some variation.
We found a beautiful weeping form of the Flinders Ranges Wattle - Acacia iteaphylla, growing in a garden in Traralgon. There were ripe seed pods overhanging the pavement, so we collected some and grew a plant. We wanted it for its weeping form, but the plant we grew outdid all weeping shrubs I have ever encountered. It grew to about 1 metre tall and had the required weeping form, but to excess perhaps. It grew down to the ground and then set out across the grass. It is still just over a metre tall at its main trunk, and then it spreads out for 4.5 metres from its base.
Sometimes there is a very special form hat wants to be copied and for that the use of cuttings is the only way to make sure that the new plants retain the features of the parent.
A good example of this is the 'Scarlet Blaze' cultivar of the Cinnamon Wattle - Acacia leprosa. This, the only red-flowered Acacia, was found as a single plant growing in the wild, and can only be propagated by cuttings, as seed reverts to the normal Acacia leprosa form.
There are reports that Acacias can be difficult to grow from cuttings, but Marion Simmons from Tasmania reports that at least for species with small phyllodes, cuttings can be grown successfully.
I quote with her permission from her article on Cultivation of Acacias:Acacias may be propagated successfully from cuttings especially from those with smaller phyllodes (leaf like structures). It is usually recommended that cuttings be taken from half hardened wood and be about 5-15 cm long. Material taken from lateral growth and stem material hardening after flowering can be used for cuttings. The cutting should be cut below a node with sharp secateurs and the lower two thirds of the ‘leaves' removed without tearing the bark. Reduce any large ‘leaves' to about half their size or less to reduce water loss and stress. Remove any flowers or buds. Dip the base of the cutting in a rooting hormone, either liquid or powder and place cutting in plastic containers with drainage holes, filled with cutting mix which has been lightly flattened. Make a hole with stick or pencil to take each cutting. Firm, water the pots before placing them under plastic, in a propagator or glass or ‘poly' house. To strike cuttings of bipinnate (fern-like) foliaged Acacias is more difficult as the tiny leaflets tend to hold water and fall very quickly, thus spoiling the cutting. More experiments and trials need to be undertaken with this type of cutting to be consistently successful.
There is one further method of propagation that is very simple to use, but is only available for a small number of species. A few Acacias, produce suckers on their root system. The Blackwood is a very good example of this and a single Blackwood tree can over the years become a thicket and even a small forest. When these suckers first appear, they can be dug up (severing the root of the parent plant) and established in a pot, before being planted out. The endemic Tasmanian species, Wally's Wattle - Acacia pataczekii, also has this habit.
Although potting suckers is a very easy method of propagation, the very habit that makes this possible, tends to make the plant unsuitable for use in a garden, as there is a perennial problem of containing the original plant and removing the multitude of suckers that appear.