Australian acacias are in general quite easily cultivated, and some species have in fact become quite weedy both in Australia itself and overseas. They are generally most easily propogated from seed, or more rarely cuttings.
Seeds can be purchased from some native seed houses or nurseries, or collected from plants in cultivation or in the bush, along roadsides etc.
You should check out what the story is with collecting plant material or seeds in your state, and would probably have to get permission to collect seed from someone else's property. If you own your own land or it's along roadsides and other similar places it's fine.
Make sure you get the species you are after, taking a sample to the herbarium for identification doesn't cost anything and can help if you are unsure. Many acacias produce an abundant amount of seed and if you find them at the right time it can be relatively easy to obtain.
Most acacia seeds (see photo left) are covered in a hard outer coat that is impermeable to water. So in order to germinate the seed the hard coat has to be nicked or filed a little bit and soaked in warm water, or alternatively covered with near boiling water and left to soak. Both these processess allow water to penetrate the outer coat and the seed will generally swell up as it takes up water. Be careful not to damage the inside of the seed when filing or nicking the outer coat.
In the wild, quite often heat from fires cause this breaking of the seed coat to allow water in. After bushfires then rains a walk through the bush will quite often reveal many acacia seedlings sprouting. A few species have soft green seeds that can be planted directly, without pretreatment.
The hard coated seeds generally have a long viability time, over years if stored correctly. Some species have been found to regenerate in areas where no mature trees have grown for decades, presumably from stored seed in the ground that was able to germinate and grow back.
Once the seeds have been treated you can plant them in a seed raising mix and treat pretty much like most other seeds. They should start to germinate in about a week to ten days, possibly longer with some species. The first true leaves of acacias are pinnate, then becoming bipinnate, followed then by the development of phyllodes in most of the Australian species.
This whole process is normally quite visible in the seedlings (see photo right), the leaf stalk of the bipinnate leaves expanding and growing to form leaf-like phyllodes, as the bipinnate leaves on the end gradually shrinking until they no are no longer apparent. This transformation can take place in as little as 1 to 3 months, though in some species bipinnate foliage may persist for quite some time, or even retain it for their entire life.
The most common problem with many seedlings is damping off or rot and wilting due to fungus or disease. Well drained soil is generally a must, as although acacias generally like water they don't seem to like boggy conditions. Australian soils in general are poor, and acacias for the most part prefer well drained poor soils, sandy almost gravelly soils they seem to thrive in, even if there is a relatively high rainfall. Soil type is most likely to be the cause of most problems with cultivation, as apart from insect damage acacias are relatively free of diseases. If conditions are right then they may infact become weedy and some species are already considered pests in countries outside Australia where they have become naturalised, the normal insects and biological controls not being found outside Australia.
Another factor may be that in the wild and cultivation Rhizobium bacteria are found in association with Acacias, these bacteria form nodules on the roots and fix nitrogen to the soil. Many mimosaceae and even leguminosae seem to have a symbiotic relationship with these bacteria and they may affect the plants ability to intake nutrients. Soil can be inoculated with Rhizobium culture, or perhaps adding a small amount of soil from around the roots of related healthy mimosa, acacia or at least leguminous plant would probably also add enough bacteria to the soil to establish a culture.
Once they have grown a few sets of phyllodes they can be potted on to bigger pots and planted out once the plant is about 6-9 months old. (See photo to left, acacia seedling ready for planting out)
Some species of acacia can be propagated by cuttings or possibly air layering, with at least some cultivated varieties to retain the features of the parent plant. As the process basically produces clones of the original, whereas seeds will produce some variation from the parent plant, many will grow to be similar though. These processes are however a bit trickier and would require some knowledge, skill and practice of the plant and the technique.
Most acacias should flower after about 3 to 4 years when grown from seed, and from then on produce seed as well, probably building up to a peak over years, depending on conditions and on how long the plant lives for.
Cultivation notes for A. obtusifolia and A. phlebophylla.