Native Australian Garden – Acacia
There are over 900 species of Acacia that are native to Australia and, although only a fraction of that number are readily available, selecting suitable species for any given situation rarely presents a problem. Acacias are often regarded as being quick growing but short lived and to a certain extent this is true but they are particularly useful for providing quick growth to cover the stark, empty look of a new garden. Most should live from 12 to 15 years in suitable conditions but many will last much longer.
A few species of Australian acacias have proved to be weed pests both in Australia and in other parts of the world. In South Africa, for example, A.saligna, A.cyclops, A.melanoxylon, A.mearnsii, and A.decurrens cause serious problems and no natural predators exist to keep them under control. Within Australia, A.paradoxa has been reported as being a problem in parts of Victoria and even the popular Cootamundra wattle (A.baileyana), Queensland silver wattle (A.podalyriifolia) and golden wreath wattle (A.saligna) are weeds of the Sydney bush and other areas. These species should not be planted in gardens in the vicinity of natural bushland.
Maintenance of wattles in the garden is fairly straight forward and should not involve much more than a light pruning to shape the plant in its early years and an occasional application of fertilizer, preferably of a slow-release type. Most wattles will not recover from a severe pruning so always ensure there is green foliage below any cut.
The seed of Acacia species is shed annually. When the seed is ripe the pods turn brown and split to release the seeds. By keeping watch on the ripening pods it is fairly easy to collect the seed before it is shed.
Acacia seeds have a hard coat which, in most cases, is impervious to water and germination will normally not occur unless some sort of pretreatment is first carried out. In nature this hard coating is designed to be broken down by the heat of a bushfire to allow the species to re-colonize burnt out areas.
This effect can be replicated in a number of ways but, for most species, the easiest is to pour boiling water over the seeds and allow them to stand overnight. The next day any seeds which have swollen are ready for sowing and can be removed; the remainder of the seeds can be treated with boiling water again and the process repeated for as long as necessary.
In some cases, however, the seeds will not tolerate excessive time in boiling water and respond better to a one minute immersion in boiling water followed by cooling down. Species in this category are likely to be those native to areas where relatively quick grass fires occur which subject the seed to shorter duration heat than would occur in forest environments.
There are also a few Acacia species from inland and tropical areas which do not require treatment prior to sowing and may, in fact, be killed by boiling water.
Another method of pretreatment is to rub the seeds between sheets of sandpaper to reduce the thickness of the outer coating so that moisture can penetrate.
Acacia seed usually germinates well by conventional sowing methods in seed raising mixes. Pre-germination, by sowing into a closed container containing moist vermiculite or a similar material, is also a useful method. Using this method, germination usually occurs in 1-2 weeks and when the root has reached about a centimetre or so in length, the seedling can be placed into a small pot of seed raising mix.
Although not regularly used with acacias, propagation from cuttings is possible and a number of named cultivars are now being routinely produced by this method. Best results are achieved with cuttings of about 75-100mm in length of mature, current season’s growth with the foliage removed from the lower two-thirds of the stem. “Wounding” the lower stem by removing a sliver of bark and treating the lower stem with a “root promoting” hormone both seem to improve the success rate.
